diff --git a/app/models/email_address.rb b/app/models/email_address.rb
index b2918ebbc..6c36238e9 100644
--- a/app/models/email_address.rb
+++ b/app/models/email_address.rb
@@ -12,7 +12,7 @@ class EmailAddress < ApplicationModel
   before_update   :check_if_channel_exists_set_inactive
   after_create    :update_email_address_id
   after_update    :update_email_address_id
-  after_destroy   :delete_group_reference
+  before_destroy  :delete_group_reference
 
 =begin
 
@@ -59,7 +59,7 @@ check and if channel not exists reset configured channels for email addresses
   # delete group.email_address_id reference if email address get's deleted
   def delete_group_reference
     Group.where(email_address_id: id).each { |group|
-      group.email_address_id = nil
+      group.update_attributes!(email_address_id: nil)
     }
   end
 
diff --git a/app/models/history.rb b/app/models/history.rb
index 6b7c72046..80ef771c6 100644
--- a/app/models/history.rb
+++ b/app/models/history.rb
@@ -5,17 +5,9 @@ class History < ApplicationModel
   include History::Assets
 
   self.table_name = 'histories'
-  belongs_to :history_type,             class_name: 'History::Type'
-  belongs_to :history_object,           class_name: 'History::Object'
-  belongs_to :history_attribute,        class_name: 'History::Attribute'
-  #  before_validation :check_type, :check_object
-  #  attr_writer :history_type, :history_object
-
-  # rubocop:disable Style/ClassVars
-  @@cache_type = {}
-  @@cache_object = {}
-  @@cache_attribute = {}
-# rubocop:enable Style/ClassVars
+  belongs_to :history_type,      class_name: 'History::Type'
+  belongs_to :history_object,    class_name: 'History::Object'
+  belongs_to :history_attribute, class_name: 'History::Attribute'
 
 =begin
 
@@ -216,96 +208,54 @@ returns
   end
 
   def self.type_lookup_id(id)
-
-    # use cache
-    return @@cache_type[ id ] if @@cache_type[ id ]
-
-    # lookup
-    history_type = History::Type.lookup(id: id)
-    @@cache_type[ id ] = history_type
-    history_type
+    History::Type.lookup(id: id)
   end
 
   def self.type_lookup(name)
-
-    # use cache
-    return @@cache_type[ name ] if @@cache_type[ name ]
-
     # lookup
     history_type = History::Type.lookup(name: name)
     if history_type
-      @@cache_type[ name ] = history_type
       return history_type
     end
 
     # create
-    history_type = History::Type.create(
+    History::Type.create(
       name: name
     )
-    @@cache_type[ name ] = history_type
-    history_type
   end
 
   def self.object_lookup_id(id)
-
-    # use cache
-    return @@cache_object[ id ] if @@cache_object[ id ]
-
-    # lookup
-    history_object = History::Object.lookup(id: id)
-    @@cache_object[ id ] = history_object
-    history_object
+    History::Object.lookup(id: id)
   end
 
   def self.object_lookup(name)
-
-    # use cache
-    return @@cache_object[ name ] if @@cache_object[ name ]
-
     # lookup
     history_object = History::Object.lookup(name: name)
     if history_object
-      @@cache_object[ name ] = history_object
       return history_object
     end
 
     # create
-    history_object = History::Object.create(
+    History::Object.create(
       name: name
     )
-    @@cache_object[ name ] = history_object
-    history_object
   end
 
   def self.attribute_lookup_id(id)
-
-    # use cache
-    return @@cache_attribute[ id ] if @@cache_attribute[ id ]
-
-    # lookup
-    history_attribute = History::Attribute.lookup(id: id)
-    @@cache_attribute[ id ] = history_attribute
-    history_attribute
+    History::Attribute.lookup(id: id)
   end
 
   def self.attribute_lookup(name)
-
-    # use cache
-    return @@cache_attribute[ name ] if @@cache_attribute[ name ]
-
     # lookup
     history_attribute = History::Attribute.lookup(name: name)
     if history_attribute
-      @@cache_attribute[ name ] = history_attribute
       return history_attribute
     end
 
     # create
-    history_attribute = History::Attribute.create(
+    History::Attribute.create(
       name: name
     )
-    @@cache_attribute[ name ] = history_attribute
-    history_attribute
   end
 
   class Object < ApplicationModel
diff --git a/app/models/object_lookup.rb b/app/models/object_lookup.rb
index a20194dba..5ada6c174 100644
--- a/app/models/object_lookup.rb
+++ b/app/models/object_lookup.rb
@@ -1,35 +1,23 @@
 # Copyright (C) 2012-2016 Zammad Foundation, http://zammad-foundation.org/
 
 class ObjectLookup < ApplicationModel
-  @@cache_object = {} # rubocop:disable Style/ClassVars
 
   def self.by_id(id)
-
-    # use cache
-    return @@cache_object[ id ] if @@cache_object[ id ]
-
     # lookup
     lookup = self.lookup(id: id)
     return if !lookup
-    @@cache_object[ id ] = lookup.name
     lookup.name
   end
 
   def self.by_name(name)
-
-    # use cache
-    return @@cache_object[ name ] if @@cache_object[ name ]
-
     # lookup
     lookup = self.lookup(name: name)
     if lookup
-      @@cache_object[ name ] = lookup.id
       return lookup.id
     end
 
     # create
     lookup = create(name: name)
-    @@cache_object[ name ] = lookup.id
     lookup.id
   end
 
diff --git a/app/models/store.rb b/app/models/store.rb
index d423b90bc..51a84ed5b 100644
--- a/app/models/store.rb
+++ b/app/models/store.rb
@@ -118,25 +118,22 @@ returns
 
 remove one attachment from storage
 
-  result = Store.remove_item(store_id)
-
-returns
-
-  result = true
+  Store.remove_item(store_id)
 
 =end
 
   def self.remove_item(store_id)
 
-    # check backend for references
-    store = Store.find(store_id)
-    files = Store.where(store_file_id: store.store_file_id)
-    if files.count == 1 && files.first.id == store.id
-      Store::File.find(store.store_file_id).destroy
-    end
-
+    store   = Store.find(store_id)
+    file_id = store.store_file_id
     store.destroy
-    true
+
+    # check backend for references
+    files = Store.where(store_file_id: file_id)
+    return if files.count != 1
+    return if files.first.id != store.id
+
+    Store::File.find(file_id).destroy
   end
 
 =begin
diff --git a/app/models/type_lookup.rb b/app/models/type_lookup.rb
index 095db3762..5934498d6 100644
--- a/app/models/type_lookup.rb
+++ b/app/models/type_lookup.rb
@@ -1,29 +1,17 @@
 # Copyright (C) 2012-2016 Zammad Foundation, http://zammad-foundation.org/
 
 class TypeLookup < ApplicationModel
-  @@cache_object = {} # rubocop:disable Style/ClassVars
 
   def self.by_id( id )
-
-    # use cache
-    return @@cache_object[ id ] if @@cache_object[ id ]
-
-    # lookup
     lookup = self.lookup( id: id )
     return if !lookup
-    @@cache_object[ id ] = lookup.name
     lookup.name
   end
 
   def self.by_name( name )
-
-    # use cache
-    return @@cache_object[ name ] if @@cache_object[ name ]
-
     # lookup
     lookup = self.lookup( name: name )
     if lookup
-      @@cache_object[ name ] = lookup.id
       return lookup.id
     end
 
@@ -31,8 +19,6 @@ class TypeLookup < ApplicationModel
     lookup = create(
       name: name
     )
-    @@cache_object[ name ] = lookup.id
     lookup.id
   end
-
 end
diff --git a/db/migrate/20120101000001_create_base.rb b/db/migrate/20120101000001_create_base.rb
index 722e5c517..bc746804a 100644
--- a/db/migrate/20120101000001_create_base.rb
+++ b/db/migrate/20120101000001_create_base.rb
@@ -56,6 +56,8 @@ class CreateBase < ActiveRecord::Migration
     add_index :users, [:mobile]
     add_index :users, [:source]
     add_index :users, [:created_by_id]
+    add_foreign_key :users, :users, column: :created_by_id
+    add_foreign_key :users, :users, column: :updated_by_id
 
     create_table :signatures do |t|
       t.string :name,                 limit: 100,  null: false
@@ -67,6 +69,8 @@ class CreateBase < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :signatures, [:name], unique: true
+    add_foreign_key :signatures, :users, column: :created_by_id
+    add_foreign_key :signatures, :users, column: :updated_by_id
 
     create_table :email_addresses do |t|
       t.integer :channel_id,                        null: true
@@ -80,6 +84,8 @@ class CreateBase < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :email_addresses, [:email], unique: true
+    add_foreign_key :email_addresses, :users, column: :created_by_id
+    add_foreign_key :email_addresses, :users, column: :updated_by_id
 
     create_table :groups do |t|
       t.references :signature,                      null: true
@@ -95,6 +101,10 @@ class CreateBase < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :groups, [:name], unique: true
+    add_foreign_key :groups, :signatures
+    add_foreign_key :groups, :email_addresses
+    add_foreign_key :groups, :users, column: :created_by_id
+    add_foreign_key :groups, :users, column: :updated_by_id
 
     create_table :roles do |t|
       t.string :name,                   limit: 100, null: false
@@ -107,6 +117,8 @@ class CreateBase < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :roles, [:name], unique: true
+    add_foreign_key :roles, :users, column: :created_by_id
+    add_foreign_key :roles, :users, column: :updated_by_id
 
     create_table :permissions do |t|
       t.string :name,                   limit: 255, null: false
@@ -135,27 +147,36 @@ class CreateBase < ActiveRecord::Migration
     end
     add_index :organizations, [:name], unique: true
     add_index :organizations, [:domain]
+    add_foreign_key :users, :organizations
+    add_foreign_key :organizations, :users, column: :created_by_id
+    add_foreign_key :organizations, :users, column: :updated_by_id
 
     create_table :roles_users, id: false do |t|
-      t.integer :user_id
-      t.integer :role_id
+      t.references :user
+      t.references :role
     end
     add_index :roles_users, [:user_id]
     add_index :roles_users, [:role_id]
+    add_foreign_key :roles_users, :users
+    add_foreign_key :roles_users, :roles
 
     create_table :groups_users, id: false do |t|
-      t.integer :user_id
-      t.integer :group_id
+      t.references :user
+      t.references :group
     end
     add_index :groups_users, [:user_id]
     add_index :groups_users, [:group_id]
+    add_foreign_key :groups_users, :users
+    add_foreign_key :groups_users, :groups
 
     create_table :organizations_users, id: false do |t|
-      t.integer :user_id
-      t.integer :organization_id
+      t.references :user
+      t.references :organization
     end
     add_index :organizations_users, [:user_id]
     add_index :organizations_users, [:organization_id]
+    add_foreign_key :organizations_users, :users
+    add_foreign_key :organizations_users, :organizations
 
     create_table :authorizations do |t|
       t.string :provider,             limit: 250, null: false
@@ -169,6 +190,7 @@ class CreateBase < ActiveRecord::Migration
     add_index :authorizations, [:uid, :provider]
     add_index :authorizations, [:user_id]
     add_index :authorizations, [:username]
+    add_foreign_key :authorizations, :users
 
     create_table :locales do |t|
       t.string  :locale,              limit: 20,  null: false
@@ -192,6 +214,8 @@ class CreateBase < ActiveRecord::Migration
     end
     add_index :translations, [:source], length: 255
     add_index :translations, [:locale]
+    add_foreign_key :translations, :users, column: :created_by_id
+    add_foreign_key :translations, :users, column: :updated_by_id
 
     create_table :object_lookups do |t|
       t.string :name,                 limit: 250, null: false
@@ -220,6 +244,7 @@ class CreateBase < ActiveRecord::Migration
     add_index :tokens, [:name, :action], unique: true
     add_index :tokens, :created_at
     add_index :tokens, :persistent
+    add_foreign_key :tokens, :users
 
     create_table :packages do |t|
       t.string :name,                 limit: 250, null: false
@@ -230,6 +255,9 @@ class CreateBase < ActiveRecord::Migration
       t.integer :created_by_id,                   null: false
       t.timestamps limit: 3, null: false
     end
+    add_foreign_key :packages, :users, column: :created_by_id
+    add_foreign_key :packages, :users, column: :updated_by_id
+
     create_table :package_migrations do |t|
       t.string :name,                 limit: 250, null: false
       t.string :version,              limit: 250, null: false
@@ -237,7 +265,7 @@ class CreateBase < ActiveRecord::Migration
     end
 
     create_table :taskbars do |t|
-      t.integer :user_id,                           null: false
+      t.references :user,                           null: false
       t.datetime :last_contact,                     null: false
       t.string :client_id,                          null: false
       t.string :key,                   limit: 100,  null: false
@@ -253,16 +281,7 @@ class CreateBase < ActiveRecord::Migration
     add_index :taskbars, [:user_id]
     add_index :taskbars, [:client_id]
     add_index :taskbars, [:key]
-
-    create_table :tags do |t|
-      t.references :tag_item,                       null: false
-      t.references :tag_object,                     null: false
-      t.integer :o_id,                              null: false
-      t.integer :created_by_id,                     null: false
-      t.timestamps limit: 3, null: false
-    end
-    add_index :tags, [:o_id]
-    add_index :tags, [:tag_object_id]
+    add_foreign_key :taskbars, :users
 
     create_table :tag_objects do |t|
       t.string :name,                   limit: 250, null: false
@@ -277,6 +296,19 @@ class CreateBase < ActiveRecord::Migration
     end
     add_index :tag_items, [:name_downcase]
 
+    create_table :tags do |t|
+      t.references :tag_item,                       null: false
+      t.references :tag_object,                     null: false
+      t.integer :o_id,                              null: false
+      t.integer :created_by_id,                     null: false
+      t.timestamps limit: 3, null: false
+    end
+    add_index :tags, [:o_id]
+    add_index :tags, [:tag_object_id]
+    add_foreign_key :tags, :tag_items
+    add_foreign_key :tags, :tag_objects
+    add_foreign_key :tags, :users, column: :created_by_id
+
     create_table :recent_views do |t|
       t.references :recent_view_object,             null: false
       t.integer :o_id,                              null: false
@@ -287,6 +319,8 @@ class CreateBase < ActiveRecord::Migration
     add_index :recent_views, [:created_by_id]
     add_index :recent_views, [:created_at]
     add_index :recent_views, [:recent_view_object_id]
+    add_foreign_key :recent_views, :object_lookups, column: :recent_view_object_id
+    add_foreign_key :recent_views, :users, column: :created_by_id
 
     create_table :activity_streams do |t|
       t.references :activity_stream_type,           null: false
@@ -304,6 +338,30 @@ class CreateBase < ActiveRecord::Migration
     add_index :activity_streams, [:created_at]
     add_index :activity_streams, [:activity_stream_object_id]
     add_index :activity_streams, [:activity_stream_type_id]
+    add_foreign_key :activity_streams, :type_lookups, column: :activity_stream_type_id
+    add_foreign_key :activity_streams, :object_lookups, column: :activity_stream_object_id
+    add_foreign_key :activity_streams, :permissions
+    add_foreign_key :activity_streams, :groups
+    add_foreign_key :activity_streams, :users, column: :created_by_id
+
+    create_table :history_types do |t|
+      t.string :name,                   limit: 250, null: false
+      t.timestamps limit: 3, null: false
+    end
+    add_index :history_types, [:name], unique: true
+
+    create_table :history_objects do |t|
+      t.string :name,                   limit: 250, null: false
+      t.string :note,                   limit: 250, null: true
+      t.timestamps limit: 3, null: false
+    end
+    add_index :history_objects, [:name], unique: true
+
+    create_table :history_attributes do |t|
+      t.string :name,                   limit: 250, null: false
+      t.timestamps limit: 3, null: false
+    end
+    add_index :history_attributes, [:name], unique: true
 
     create_table :histories do |t|
       t.references :history_type,                   null: false
@@ -329,25 +387,10 @@ class CreateBase < ActiveRecord::Migration
     add_index :histories, [:id_from]
     add_index :histories, [:value_from], length: 255
     add_index :histories, [:value_to], length: 255
-
-    create_table :history_types do |t|
-      t.string :name,                   limit: 250, null: false
-      t.timestamps limit: 3, null: false
-    end
-    add_index :history_types, [:name], unique: true
-
-    create_table :history_objects do |t|
-      t.string :name,                   limit: 250, null: false
-      t.string :note,                   limit: 250, null: true
-      t.timestamps limit: 3, null: false
-    end
-    add_index :history_objects, [:name], unique: true
-
-    create_table :history_attributes do |t|
-      t.string :name,                   limit: 250, null: false
-      t.timestamps limit: 3, null: false
-    end
-    add_index :history_attributes, [:name], unique: true
+    add_foreign_key :histories, :history_types
+    add_foreign_key :histories, :history_objects
+    add_foreign_key :histories, :history_attributes
+    add_foreign_key :histories, :users, column: :created_by_id
 
     create_table :settings do |t|
       t.string :title,                  limit: 200,  null: false
@@ -365,18 +408,6 @@ class CreateBase < ActiveRecord::Migration
     add_index :settings, [:area]
     add_index :settings, [:frontend]
 
-    create_table :stores do |t|
-      t.references :store_object,               null: false
-      t.references :store_file,                 null: false
-      t.integer :o_id,              limit: 8,   null: false
-      t.string :preferences,        limit: 2500, null: true
-      t.string :size,               limit: 50,  null: true
-      t.string :filename,           limit: 250, null: false
-      t.integer :created_by_id,                 null: false
-      t.timestamps limit: 3, null: false
-    end
-    add_index :stores, [:store_object_id, :o_id]
-
     create_table :store_objects do |t|
       t.string :name,               limit: 250, null: false
       t.string :note,               limit: 250, null: true
@@ -392,6 +423,21 @@ class CreateBase < ActiveRecord::Migration
     add_index :store_files, [:sha], unique: true
     add_index :store_files, [:provider]
 
+    create_table :stores do |t|
+      t.references :store_object,               null: false
+      t.references :store_file,                 null: false
+      t.integer :o_id,              limit: 8,   null: false
+      t.string :preferences,        limit: 2500, null: true
+      t.string :size,               limit: 50,  null: true
+      t.string :filename,           limit: 250, null: false
+      t.integer :created_by_id,                 null: false
+      t.timestamps limit: 3, null: false
+    end
+    add_index :stores, [:store_object_id, :o_id]
+    add_foreign_key :stores, :store_objects
+    add_foreign_key :stores, :store_files
+    add_foreign_key :stores, :users, column: :created_by_id
+
     create_table :store_provider_dbs do |t|
       t.string :sha,                limit: 128,            null: false
       t.binary :data,               limit: 200.megabytes,  null: true
@@ -418,6 +464,8 @@ class CreateBase < ActiveRecord::Migration
     add_index :avatars, [:store_hash]
     add_index :avatars, [:source]
     add_index :avatars, [:default]
+    add_foreign_key :avatars, :users, column: :created_by_id
+    add_foreign_key :avatars, :users, column: :updated_by_id
 
     create_table :online_notifications do |t|
       t.integer :o_id,                          null: false
@@ -433,6 +481,8 @@ class CreateBase < ActiveRecord::Migration
     add_index :online_notifications, [:seen]
     add_index :online_notifications, [:created_at]
     add_index :online_notifications, [:updated_at]
+    add_foreign_key :online_notifications, :users, column: :created_by_id
+    add_foreign_key :online_notifications, :users, column: :updated_by_id
 
     create_table :schedulers do |t|
       t.string :name,                     limit: 250,   null: false
@@ -449,6 +499,8 @@ class CreateBase < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :schedulers, [:name], unique: true
+    add_foreign_key :schedulers, :users, column: :created_by_id
+    add_foreign_key :schedulers, :users, column: :updated_by_id
 
     create_table :calendars do |t|
       t.string  :name,                   limit: 250,  null: true
@@ -464,6 +516,8 @@ class CreateBase < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :calendars, [:name], unique: true
+    add_foreign_key :calendars, :users, column: :created_by_id
+    add_foreign_key :calendars, :users, column: :updated_by_id
 
     create_table :user_devices do |t|
       t.references :user,             null: false
@@ -483,6 +537,7 @@ class CreateBase < ActiveRecord::Migration
     add_index :user_devices, [:fingerprint]
     add_index :user_devices, [:updated_at]
     add_index :user_devices, [:created_at]
+    add_foreign_key :user_devices, :users
 
     create_table :external_credentials do |t|
       t.string :name
@@ -511,6 +566,9 @@ class CreateBase < ActiveRecord::Migration
     end
     add_index :object_manager_attributes, [:object_lookup_id, :name],   unique: true
     add_index :object_manager_attributes, [:object_lookup_id]
+    add_foreign_key :object_manager_attributes, :object_lookups
+    add_foreign_key :object_manager_attributes, :users, column: :created_by_id
+    add_foreign_key :object_manager_attributes, :users, column: :updated_by_id
 
     create_table :delayed_jobs, force: true do |t|
       t.integer  :priority, default: 0      # Allows some jobs to jump to the front of the queue
@@ -573,19 +631,20 @@ class CreateBase < ActiveRecord::Migration
     add_index :cti_logs, [:from]
 
     create_table :cti_caller_ids do |t|
-      t.string  :caller_id,              limit: 100, null: false
-      t.string  :comment,                limit: 500, null: true
-      t.string  :level,                  limit: 100, null: false
-      t.string  :object,                 limit: 100, null: false
-      t.integer :o_id,                               null: false
-      t.integer :user_id,                            null: true
-      t.text    :preferences,            limit: 500.kilobytes + 1, null: true
+      t.string     :caller_id,              limit: 100, null: false
+      t.string     :comment,                limit: 500, null: true
+      t.string     :level,                  limit: 100, null: false
+      t.string     :object,                 limit: 100, null: false
+      t.integer    :o_id,                               null: false
+      t.references :user,                            null: true
+      t.text       :preferences,            limit: 500.kilobytes + 1, null: true
       t.timestamps limit: 3, null: false
     end
     add_index :cti_caller_ids, [:caller_id]
     add_index :cti_caller_ids, [:caller_id, :level]
     add_index :cti_caller_ids, [:caller_id, :user_id]
     add_index :cti_caller_ids, [:object, :o_id]
+    add_foreign_key :cti_caller_ids, :users
 
     create_table :stats_stores do |t|
       t.references :stats_store_object,             null: false
@@ -602,6 +661,7 @@ class CreateBase < ActiveRecord::Migration
     add_index :stats_stores, [:stats_store_object_id]
     add_index :stats_stores, [:created_by_id]
     add_index :stats_stores, [:created_at]
+    add_foreign_key :stats_stores, :users, column: :created_by_id
 
     create_table :http_logs do |t|
       t.column :direction,            :string, limit: 20,    null: false
@@ -619,6 +679,7 @@ class CreateBase < ActiveRecord::Migration
     add_index :http_logs, [:facility]
     add_index :http_logs, [:created_by_id]
     add_index :http_logs, [:created_at]
-
+    add_foreign_key :http_logs, :users, column: :created_by_id
+    add_foreign_key :http_logs, :users, column: :updated_by_id
   end
 end
diff --git a/db/migrate/20120101000010_create_ticket.rb b/db/migrate/20120101000010_create_ticket.rb
index bbf3b3723..586b5cca7 100644
--- a/db/migrate/20120101000010_create_ticket.rb
+++ b/db/migrate/20120101000010_create_ticket.rb
@@ -8,6 +8,8 @@ class CreateTicket < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :ticket_state_types, [:name], unique: true
+    add_foreign_key :ticket_state_types, :users, column: :created_by_id
+    add_foreign_key :ticket_state_types, :users, column: :updated_by_id
 
     create_table :ticket_states do |t|
       t.references :state_type, null: false
@@ -25,6 +27,9 @@ class CreateTicket < ActiveRecord::Migration
     add_index :ticket_states, [:name], unique: true
     add_index :ticket_states, [:default_create]
     add_index :ticket_states, [:default_follow_up]
+    add_foreign_key :ticket_states, :ticket_state_types, column: :state_type_id
+    add_foreign_key :ticket_states, :users, column: :created_by_id
+    add_foreign_key :ticket_states, :users, column: :updated_by_id
 
     create_table :ticket_priorities do |t|
       t.column :name,                 :string, limit: 250, null: false
@@ -37,6 +42,8 @@ class CreateTicket < ActiveRecord::Migration
     end
     add_index :ticket_priorities, [:name], unique: true
     add_index :ticket_priorities, [:default_create]
+    add_foreign_key :ticket_priorities, :users, column: :created_by_id
+    add_foreign_key :ticket_priorities, :users, column: :updated_by_id
 
     create_table :tickets do |t|
       t.references :group,                                                null: false
@@ -102,30 +109,28 @@ class CreateTicket < ActiveRecord::Migration
     add_index :tickets, [:pending_time]
     add_index :tickets, [:type]
     add_index :tickets, [:time_unit]
+    add_foreign_key :tickets, :groups
+    add_foreign_key :tickets, :users, column: :owner_id
+    add_foreign_key :tickets, :users, column: :customer_id
+    add_foreign_key :tickets, :ticket_priorities, column: :priority_id
+    add_foreign_key :tickets, :ticket_states, column: :state_id
+    add_foreign_key :tickets, :organizations
+    add_foreign_key :tickets, :users, column: :created_by_id
+    add_foreign_key :tickets, :users, column: :updated_by_id
 
     create_table :ticket_flags do |t|
-      t.references :tickets,                         null: false
+      t.references :ticket,                          null: false
       t.column :key,            :string, limit: 50,  null: false
       t.column :value,          :string, limit: 50,  null: true
       t.column :created_by_id,  :integer,            null: false
       t.timestamps limit: 3, null: false
     end
-    add_index :ticket_flags, [:tickets_id, :created_by_id]
-    add_index :ticket_flags, [:tickets_id, :key]
-    add_index :ticket_flags, [:tickets_id]
+    add_index :ticket_flags, [:ticket_id, :created_by_id]
+    add_index :ticket_flags, [:ticket_id, :key]
+    add_index :ticket_flags, [:ticket_id]
     add_index :ticket_flags, [:created_by_id]
-
-    create_table :ticket_time_accountings do |t|
-      t.references :ticket,                                       null: false
-      t.references :ticket_article,                               null: true
-      t.column :time_unit,      :decimal, precision: 6, scale: 2, null: false
-      t.column :created_by_id,  :integer,                         null: false
-      t.timestamps limit: 3, null: false
-    end
-    add_index :ticket_time_accountings, [:ticket_id]
-    add_index :ticket_time_accountings, [:ticket_article_id]
-    add_index :ticket_time_accountings, [:created_by_id]
-    add_index :ticket_time_accountings, [:time_unit]
+    add_foreign_key :ticket_flags, :tickets, column: :ticket_id
+    add_foreign_key :ticket_flags, :users, column: :created_by_id
 
     create_table :ticket_article_types do |t|
       t.column :name,                 :string, limit: 250, null: false
@@ -137,6 +142,8 @@ class CreateTicket < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :ticket_article_types, [:name], unique: true
+    add_foreign_key :ticket_article_types, :users, column: :created_by_id
+    add_foreign_key :ticket_article_types, :users, column: :updated_by_id
 
     create_table :ticket_article_senders do |t|
       t.column :name,                 :string, limit: 250, null: false
@@ -146,6 +153,8 @@ class CreateTicket < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :ticket_article_senders, [:name], unique: true
+    add_foreign_key :ticket_article_senders, :users, column: :created_by_id
+    add_foreign_key :ticket_article_senders, :users, column: :updated_by_id
 
     create_table :ticket_articles do |t|
       t.references :ticket,                                    null: false
@@ -177,18 +186,41 @@ class CreateTicket < ActiveRecord::Migration
     add_index :ticket_articles, [:internal]
     add_index :ticket_articles, [:type_id]
     add_index :ticket_articles, [:sender_id]
+    add_foreign_key :ticket_articles, :tickets
+    add_foreign_key :ticket_articles, :ticket_article_types, column: :type_id
+    add_foreign_key :ticket_articles, :ticket_article_senders, column: :sender_id
+    add_foreign_key :ticket_articles, :users, column: :created_by_id
+    add_foreign_key :ticket_articles, :users, column: :updated_by_id
+    add_foreign_key :ticket_articles, :users, column: :origin_by_id
 
     create_table :ticket_article_flags do |t|
-      t.references :ticket_articles,                     null: false
+      t.references :ticket_article,                      null: false
       t.column :key,                 :string, limit: 50, null: false
       t.column :value,               :string, limit: 50, null: true
       t.column :created_by_id,       :integer,           null: false
       t.timestamps limit: 3,  null: false
     end
-    add_index :ticket_article_flags, [:ticket_articles_id, :created_by_id], name: 'index_ticket_article_flags_on_articles_id_and_created_by_id'
-    add_index :ticket_article_flags, [:ticket_articles_id, :key]
-    add_index :ticket_article_flags, [:ticket_articles_id]
+    add_index :ticket_article_flags, [:ticket_article_id, :created_by_id], name: 'index_ticket_article_flags_on_articles_id_and_created_by_id'
+    add_index :ticket_article_flags, [:ticket_article_id, :key]
+    add_index :ticket_article_flags, [:ticket_article_id]
     add_index :ticket_article_flags, [:created_by_id]
+    add_foreign_key :ticket_article_flags, :ticket_articles, column: :ticket_article_id
+    add_foreign_key :ticket_article_flags, :users, column: :created_by_id
+
+    create_table :ticket_time_accountings do |t|
+      t.references :ticket,                                       null: false
+      t.references :ticket_article,                               null: true
+      t.column :time_unit,      :decimal, precision: 6, scale: 2, null: false
+      t.column :created_by_id,  :integer,                         null: false
+      t.timestamps limit: 3, null: false
+    end
+    add_index :ticket_time_accountings, [:ticket_id]
+    add_index :ticket_time_accountings, [:ticket_article_id]
+    add_index :ticket_time_accountings, [:created_by_id]
+    add_index :ticket_time_accountings, [:time_unit]
+    add_foreign_key :ticket_time_accountings, :tickets
+    add_foreign_key :ticket_time_accountings, :ticket_articles
+    add_foreign_key :ticket_time_accountings, :users, column: :created_by_id
 
     create_table :ticket_counters do |t|
       t.column :content,              :string, limit: 100, null: false
@@ -211,27 +243,35 @@ class CreateTicket < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :overviews, [:name]
+    add_foreign_key :overviews, :users, column: :created_by_id
+    add_foreign_key :overviews, :users, column: :updated_by_id
 
     create_table :overviews_roles, id: false do |t|
-      t.integer :overview_id
-      t.integer :role_id
+      t.references :overview
+      t.references :role
     end
     add_index :overviews_roles, [:overview_id]
     add_index :overviews_roles, [:role_id]
+    add_foreign_key :overviews_roles, :overviews
+    add_foreign_key :overviews_roles, :roles
 
     create_table :overviews_users, id: false do |t|
-      t.integer :overview_id
-      t.integer :user_id
+      t.references :overview
+      t.references :user
     end
     add_index :overviews_users, [:overview_id]
     add_index :overviews_users, [:user_id]
+    add_foreign_key :overviews_users, :overviews
+    add_foreign_key :overviews_users, :users
 
     create_table :overviews_groups, id: false do |t|
-      t.integer :overview_id
-      t.integer :group_id
+      t.references :overview
+      t.references :group
     end
     add_index :overviews_groups, [:overview_id]
     add_index :overviews_groups, [:group_id]
+    add_foreign_key :overviews_groups, :overviews
+    add_foreign_key :overviews_groups, :groups
 
     create_table :triggers do |t|
       t.column :name,                 :string, limit: 250,    null: false
@@ -245,6 +285,8 @@ class CreateTicket < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :triggers, [:name], unique: true
+    add_foreign_key :triggers, :users, column: :created_by_id
+    add_foreign_key :triggers, :users, column: :updated_by_id
 
     create_table :jobs do |t|
       t.column :name,                 :string,  limit: 250,    null: false
@@ -265,6 +307,8 @@ class CreateTicket < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :jobs, [:name], unique: true
+    add_foreign_key :jobs, :users, column: :created_by_id
+    add_foreign_key :jobs, :users, column: :updated_by_id
 
     create_table :notifications do |t|
       t.column :subject,      :string, limit: 250,   null: false
@@ -281,7 +325,7 @@ class CreateTicket < ActiveRecord::Migration
       t.column :active,       :boolean,              null: false, default: true
       t.timestamps limit: 3, null: false
     end
-    add_index :link_types, [:name],     unique: true
+    add_index :link_types, [:name], unique: true
 
     create_table :link_objects do |t|
       t.column :name,         :string, limit: 250,   null: false
@@ -300,6 +344,7 @@ class CreateTicket < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :links, [:link_object_source_id, :link_object_source_value, :link_object_target_id, :link_object_target_value, :link_type_id], unique: true, name: 'links_uniq_total'
+    add_foreign_key :links, :link_types
 
     create_table :postmaster_filters do |t|
       t.column :name,           :string, limit: 250,    null: false
@@ -313,6 +358,8 @@ class CreateTicket < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :postmaster_filters, [:channel]
+    add_foreign_key :postmaster_filters, :users, column: :created_by_id
+    add_foreign_key :postmaster_filters, :users, column: :updated_by_id
 
     create_table :text_modules do |t|
       t.references :user,                                    null: true
@@ -328,13 +375,18 @@ class CreateTicket < ActiveRecord::Migration
     end
     add_index :text_modules, [:user_id]
     add_index :text_modules, [:name]
+    add_foreign_key :text_modules, :users
+    add_foreign_key :text_modules, :users, column: :created_by_id
+    add_foreign_key :text_modules, :users, column: :updated_by_id
 
     create_table :text_modules_groups, id: false do |t|
-      t.integer :text_module_id
-      t.integer :group_id
+      t.references :text_module
+      t.references :group
     end
     add_index :text_modules_groups, [:text_module_id]
     add_index :text_modules_groups, [:group_id]
+    add_foreign_key :text_modules_groups, :text_modules
+    add_foreign_key :text_modules_groups, :groups
 
     create_table :templates do |t|
       t.references :user,                                    null: true
@@ -346,13 +398,18 @@ class CreateTicket < ActiveRecord::Migration
     end
     add_index :templates, [:user_id]
     add_index :templates, [:name]
+    add_foreign_key :templates, :users
+    add_foreign_key :templates, :users, column: :created_by_id
+    add_foreign_key :templates, :users, column: :updated_by_id
 
     create_table :templates_groups, id: false do |t|
-      t.integer :template_id
-      t.integer :group_id
+      t.references :template
+      t.references :group
     end
     add_index :templates_groups, [:template_id]
     add_index :templates_groups, [:group_id]
+    add_foreign_key :templates_groups, :templates
+    add_foreign_key :templates_groups, :groups
 
     create_table :channels do |t|
       t.references :group,                             null: true
@@ -369,10 +426,13 @@ class CreateTicket < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :channels, [:area]
+    add_foreign_key :channels, :groups
+    add_foreign_key :channels, :users, column: :created_by_id
+    add_foreign_key :channels, :users, column: :updated_by_id
 
     create_table :slas do |t|
+      t.references :calendar,                                   null: false
       t.column :name,                 :string, limit: 150,      null: true
-      t.column :calendar_id,          :integer,                 null: false
       t.column :first_response_time,  :integer,                 null: true
       t.column :update_time,          :integer,                 null: true
       t.column :solution_time,        :integer,                 null: true
@@ -382,6 +442,8 @@ class CreateTicket < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :slas, [:name], unique: true
+    add_foreign_key :slas, :users, column: :created_by_id
+    add_foreign_key :slas, :users, column: :updated_by_id
 
     create_table :macros do |t|
       t.string  :name,                   limit: 250,    null: true
@@ -393,6 +455,8 @@ class CreateTicket < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :macros, [:name], unique: true
+    add_foreign_key :macros, :users, column: :created_by_id
+    add_foreign_key :macros, :users, column: :updated_by_id
 
     create_table :chats do |t|
       t.string  :name,                   limit: 250,  null: true
@@ -406,6 +470,8 @@ class CreateTicket < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :chats, [:name], unique: true
+    add_foreign_key :chats, :users, column: :created_by_id
+    add_foreign_key :chats, :users, column: :updated_by_id
 
     create_table :chat_topics do |t|
       t.integer :chat_id,                             null: false
@@ -416,13 +482,15 @@ class CreateTicket < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :chat_topics, [:name], unique: true
+    add_foreign_key :chat_topics, :users, column: :created_by_id
+    add_foreign_key :chat_topics, :users, column: :updated_by_id
 
     create_table :chat_sessions do |t|
-      t.integer :chat_id,                             null: false
+      t.references :chat,                             null: false
       t.string  :session_id,                          null: false
       t.string  :name,                   limit: 250,  null: true
       t.string  :state,                  limit:  50,  null: false, default: 'waiting' # running, closed
-      t.integer :user_id,                             null: true
+      t.references :user,                             null: true
       t.text    :preferences,            limit: 100.kilobytes + 1, null: true
       t.integer :updated_by_id,                       null: true
       t.integer :created_by_id,                       null: true
@@ -432,14 +500,20 @@ class CreateTicket < ActiveRecord::Migration
     add_index :chat_sessions, [:state]
     add_index :chat_sessions, [:user_id]
     add_index :chat_sessions, [:chat_id]
+    add_foreign_key :chat_sessions, :chats
+    add_foreign_key :chat_sessions, :users
+    add_foreign_key :chat_sessions, :users, column: :created_by_id
+    add_foreign_key :chat_sessions, :users, column: :updated_by_id
 
     create_table :chat_messages do |t|
-      t.integer :chat_session_id,                     null: false
+      t.references :chat_session,                     null: false
       t.text    :content,    limit: 20.megabytes + 1, null: false
       t.integer :created_by_id,                       null: true
       t.timestamps limit: 3, null: false
     end
     add_index :chat_messages, [:chat_session_id]
+    add_foreign_key :chat_messages, :chat_sessions
+    add_foreign_key :chat_messages, :users, column: :created_by_id
 
     create_table :chat_agents do |t|
       t.boolean :active,                              null: false, default: true
@@ -451,6 +525,8 @@ class CreateTicket < ActiveRecord::Migration
     add_index :chat_agents, [:active]
     add_index :chat_agents, [:updated_by_id], unique: true
     add_index :chat_agents, [:created_by_id], unique: true
+    add_foreign_key :chat_agents, :users, column: :created_by_id
+    add_foreign_key :chat_agents, :users, column: :updated_by_id
 
     create_table :report_profiles do |t|
       t.column :name,           :string, limit: 150,    null: true
@@ -461,14 +537,17 @@ class CreateTicket < ActiveRecord::Migration
       t.timestamps limit: 3, null: false
     end
     add_index :report_profiles, [:name], unique: true
+    add_foreign_key :report_profiles, :users, column: :created_by_id
+    add_foreign_key :report_profiles, :users, column: :updated_by_id
 
     create_table :karma_users do |t|
-      t.integer :user_id,                           null: false
+      t.references :user,                           null: false
       t.integer :score,                             null: false
       t.string  :level,               limit: 200,   null: false
       t.timestamps limit: 3, null: false
     end
     add_index :karma_users, [:user_id], unique: true
+    add_foreign_key :karma_users, :users
 
     create_table :karma_activities do |t|
       t.string  :name,                limit: 200,    null: false
@@ -482,7 +561,7 @@ class CreateTicket < ActiveRecord::Migration
     create_table :karma_activity_logs do |t|
       t.integer :o_id,                          null: false
       t.integer :object_lookup_id,              null: false
-      t.integer :user_id,                       null: false
+      t.references :user,                       null: false
       t.integer :activity_id,                   null: false
       t.integer :score,                         null: false
       t.integer :score_total,                   null: false
@@ -491,7 +570,8 @@ class CreateTicket < ActiveRecord::Migration
     add_index :karma_activity_logs, [:user_id]
     add_index :karma_activity_logs, [:created_at]
     add_index :karma_activity_logs, [:o_id, :object_lookup_id]
-
+    add_foreign_key :karma_activity_logs, :users
+    add_foreign_key :karma_activity_logs, :karma_activities, column: :activity_id
   end
 
   def self.down
diff --git a/db/migrate/20170531144425_foreign_keys.rb b/db/migrate/20170531144425_foreign_keys.rb
new file mode 100644
index 000000000..b38e3ab71
--- /dev/null
+++ b/db/migrate/20170531144425_foreign_keys.rb
@@ -0,0 +1,227 @@
+class ForeignKeys < ActiveRecord::Migration
+  def change
+
+    # return if it's a new setup
+    return if !Setting.find_by(name: 'system_init_done')
+
+    # remove wrong plural of ID columns
+    ActiveRecord::Migration.rename_column :ticket_flags, :tickets_id, :ticket_id
+    ActiveRecord::Migration.rename_column :ticket_article_flags, :ticket_articles_id, :ticket_article_id
+
+    # add missing foreign keys
+    foreign_keys = [
+      # Base
+      [:users, :organizations],
+      [:users, :users, column: :created_by_id],
+      [:users, :users, column: :updated_by_id],
+
+      [:signatures, :users, column: :created_by_id],
+      [:signatures, :users, column: :updated_by_id],
+
+      [:email_addresses, :users, column: :created_by_id],
+      [:email_addresses, :users, column: :updated_by_id],
+
+      [:groups, :signatures],
+      [:groups, :email_addresses],
+      [:groups, :users, column: :created_by_id],
+      [:groups, :users, column: :updated_by_id],
+
+      [:roles, :users, column: :created_by_id],
+      [:roles, :users, column: :updated_by_id],
+
+      [:organizations, :users, column: :created_by_id],
+      [:organizations, :users, column: :updated_by_id],
+
+      [:roles_users, :users],
+      [:roles_users, :roles],
+
+      [:groups_users, :users],
+      [:groups_users, :groups],
+
+      [:organizations_users, :users],
+      [:organizations_users, :organizations],
+
+      [:authorizations, :users],
+
+      [:translations, :users, column: :created_by_id],
+      [:translations, :users, column: :updated_by_id],
+
+      [:tokens, :users],
+
+      [:packages, :users, column: :created_by_id],
+      [:packages, :users, column: :updated_by_id],
+
+      [:taskbars, :users],
+
+      [:tags, :tag_items],
+      [:tags, :tag_objects],
+      [:tags, :users, column: :created_by_id],
+
+      [:recent_views, :object_lookups, column: :recent_view_object_id],
+      [:recent_views, :users, column: :created_by_id],
+
+      [:activity_streams, :type_lookups, column: :activity_stream_type_id],
+      [:activity_streams, :object_lookups, column: :activity_stream_object_id],
+      [:activity_streams, :permissions],
+      [:activity_streams, :groups],
+      [:activity_streams, :users, column: :created_by_id],
+
+      [:histories, :history_types],
+      [:histories, :history_objects],
+      [:histories, :history_attributes],
+      [:histories, :users, column: :created_by_id],
+
+      [:stores, :store_objects],
+      [:stores, :store_files],
+      [:stores, :users, column: :created_by_id],
+
+      [:avatars, :users, column: :created_by_id],
+      [:avatars, :users, column: :updated_by_id],
+
+      [:online_notifications, :users, column: :created_by_id],
+      [:online_notifications, :users, column: :updated_by_id],
+
+      [:schedulers, :users, column: :created_by_id],
+      [:schedulers, :users, column: :updated_by_id],
+
+      [:calendars, :users, column: :created_by_id],
+      [:calendars, :users, column: :updated_by_id],
+
+      [:user_devices, :users],
+
+      [:object_manager_attributes, :object_lookups],
+      [:object_manager_attributes, :users, column: :created_by_id],
+      [:object_manager_attributes, :users, column: :updated_by_id],
+
+      [:cti_caller_ids, :users],
+
+      [:stats_stores, :users, column: :created_by_id],
+
+      [:http_logs, :users, column: :created_by_id],
+      [:http_logs, :users, column: :updated_by_id],
+
+      # Ticket
+      [:ticket_state_types, :users, column: :created_by_id],
+      [:ticket_state_types, :users, column: :updated_by_id],
+
+      [:ticket_states, :ticket_state_types, column: :state_type_id],
+      [:ticket_states, :users, column: :created_by_id],
+      [:ticket_states, :users, column: :updated_by_id],
+
+      [:ticket_priorities, :users, column: :created_by_id],
+      [:ticket_priorities, :users, column: :updated_by_id],
+
+      [:tickets, :groups],
+      [:tickets, :users, column: :owner_id],
+      [:tickets, :users, column: :customer_id],
+      [:tickets, :ticket_priorities, column: :priority_id],
+      [:tickets, :ticket_states, column: :state_id],
+      [:tickets, :organizations],
+      [:tickets, :users, column: :created_by_id],
+      [:tickets, :users, column: :updated_by_id],
+
+      [:ticket_flags, :tickets, column: :ticket_id],
+      [:ticket_flags, :users, column: :created_by_id],
+
+      [:ticket_article_types, :users, column: :created_by_id],
+      [:ticket_article_types, :users, column: :updated_by_id],
+
+      [:ticket_article_senders, :users, column: :created_by_id],
+      [:ticket_article_senders, :users, column: :updated_by_id],
+
+      [:ticket_articles, :tickets],
+      [:ticket_articles, :ticket_article_types, column: :type_id],
+      [:ticket_articles, :ticket_article_senders, column: :sender_id],
+      [:ticket_articles, :users, column: :created_by_id],
+      [:ticket_articles, :users, column: :updated_by_id],
+      [:ticket_articles, :users, column: :origin_by_id],
+
+      [:ticket_article_flags, :ticket_articles, column: :ticket_article_id],
+      [:ticket_article_flags, :users, column: :created_by_id],
+
+      [:ticket_time_accountings, :tickets],
+      [:ticket_time_accountings, :ticket_articles],
+      [:ticket_time_accountings, :users, column: :created_by_id],
+
+      [:overviews, :users, column: :created_by_id],
+      [:overviews, :users, column: :updated_by_id],
+
+      [:overviews_roles, :overviews],
+      [:overviews_roles, :roles],
+
+      [:overviews_users, :overviews],
+      [:overviews_users, :users],
+
+      [:overviews_groups, :overviews],
+      [:overviews_groups, :groups],
+
+      [:triggers, :users, column: :created_by_id],
+      [:triggers, :users, column: :updated_by_id],
+
+      [:jobs, :users, column: :created_by_id],
+      [:jobs, :users, column: :updated_by_id],
+
+      [:links, :link_types],
+
+      [:postmaster_filters, :users, column: :created_by_id],
+      [:postmaster_filters, :users, column: :updated_by_id],
+
+      [:text_modules, :users],
+      [:text_modules, :users, column: :created_by_id],
+      [:text_modules, :users, column: :updated_by_id],
+
+      [:text_modules_groups, :text_modules],
+      [:text_modules_groups, :groups],
+
+      [:templates, :users],
+      [:templates, :users, column: :created_by_id],
+      [:templates, :users, column: :updated_by_id],
+
+      [:templates_groups, :templates],
+      [:templates_groups, :groups],
+
+      [:channels, :groups],
+      [:channels, :users, column: :created_by_id],
+      [:channels, :users, column: :updated_by_id],
+
+      [:slas, :users, column: :created_by_id],
+      [:slas, :users, column: :updated_by_id],
+
+      [:macros, :users, column: :created_by_id],
+      [:macros, :users, column: :updated_by_id],
+
+      [:chats, :users, column: :created_by_id],
+      [:chats, :users, column: :updated_by_id],
+
+      [:chat_topics, :users, column: :created_by_id],
+      [:chat_topics, :users, column: :updated_by_id],
+
+      [:chat_sessions, :chats],
+      [:chat_sessions, :users],
+      [:chat_sessions, :users, column: :created_by_id],
+      [:chat_sessions, :users, column: :updated_by_id],
+
+      [:chat_messages, :chat_sessions],
+      [:chat_messages, :users, column: :created_by_id],
+
+      [:chat_agents, :users, column: :created_by_id],
+      [:chat_agents, :users, column: :updated_by_id],
+
+      [:report_profiles, :users, column: :created_by_id],
+      [:report_profiles, :users, column: :updated_by_id],
+
+      [:karma_users, :users],
+
+      [:karma_activity_logs, :users],
+      [:karma_activity_logs, :karma_activities, column: :activity_id],
+    ]
+
+    foreign_keys.each do |foreign_key|
+      begin
+        ActiveRecord::Migration.add_foreign_key(*foreign_key)
+      rescue => e
+        Rails.logger.error "Inconsistent data status detected while adding foreign key '#{foreign_key.inspect}': #{e.message}"
+      end
+    end
+  end
+end
diff --git a/spec/support/cache.rb b/spec/support/cache.rb
new file mode 100644
index 000000000..4a6b30119
--- /dev/null
+++ b/spec/support/cache.rb
@@ -0,0 +1,8 @@
+RSpec.configure do |config|
+  config.before(:all) do
+    # clear the cache otherwise it won't
+    # be able to recognize the rollbacks
+    # done by RSpec
+    Cache.clear
+  end
+end
diff --git a/test/unit/assets_test.rb b/test/unit/assets_test.rb
index 6f042ab9e..95ed76cfa 100644
--- a/test/unit/assets_test.rb
+++ b/test/unit/assets_test.rb
@@ -132,9 +132,9 @@ class AssetsTest < ActiveSupport::TestCase
     assert( diff(attributes, assets[:User][user3.id]), 'check assets' )
     travel_back
 
-    user1.destroy
-    user2.destroy
     user3.destroy
+    user2.destroy
+    user1.destroy
     org1.destroy
     org2.destroy
 
@@ -270,9 +270,9 @@ class AssetsTest < ActiveSupport::TestCase
     assert( diff(attributes, assets[:User][user_new_2.id]), 'check assets' )
     travel_back
 
-    user1.destroy
-    user2.destroy
     user3.destroy
+    user2.destroy
+    user1.destroy
     org.destroy
     org_new.destroy
 
diff --git a/test/unit/cti_caller_id_test.rb b/test/unit/cti_caller_id_test.rb
index a5dea0a81..8619fd9b5 100644
--- a/test/unit/cti_caller_id_test.rb
+++ b/test/unit/cti_caller_id_test.rb
@@ -210,10 +210,12 @@ Mob: +49 333 1112222",
     assert_equal(2, caller_ids[0].user_id)
     assert_nil(caller_ids[0].comment)
 
+    user_id = User.find_by(login: 'ticket-caller_id-customer1@example.com').id
+
     Cti::CallerId.maybe_add(
       caller_id: '4912345678901',
       level: 'maybe',
-      user_id: 3,
+      user_id: user_id,
       object: 'Ticket',
       o_id: 2,
     )
@@ -221,7 +223,7 @@ Mob: +49 333 1112222",
     caller_ids = Cti::CallerId.lookup('4912345678901')
     assert_equal(2, caller_ids.length)
     assert_equal('maybe', caller_ids[0].level)
-    assert_equal(3, caller_ids[0].user_id)
+    assert_equal(user_id, caller_ids[0].user_id)
     assert_nil(caller_ids[0].comment)
     assert_equal('maybe', caller_ids[1].level)
     assert_equal(2, caller_ids[1].user_id)
@@ -230,7 +232,7 @@ Mob: +49 333 1112222",
     Cti::CallerId.maybe_add(
       caller_id: '4912345678901',
       level: 'known',
-      user_id: 3,
+      user_id: user_id,
       object: 'User',
       o_id: 2,
     )
@@ -238,7 +240,7 @@ Mob: +49 333 1112222",
     caller_ids = Cti::CallerId.lookup('4912345678901')
     assert_equal(1, caller_ids.length)
     assert_equal('known', caller_ids[0].level)
-    assert_equal(3, caller_ids[0].user_id)
+    assert_equal(user_id, caller_ids[0].user_id)
     assert_nil(caller_ids[0].comment)
 
   end
diff --git a/test/unit/email_address_test.rb b/test/unit/email_address_test.rb
index 9741b00d2..261cd033e 100644
--- a/test/unit/email_address_test.rb
+++ b/test/unit/email_address_test.rb
@@ -40,7 +40,7 @@ class EmailAddressTest < ActiveSupport::TestCase
     email_address1.destroy
 
     group1 = Group.find(group1.id)
-    assert(group1.email_address_id)
+    assert_nil(group1.email_address_id, 'References to groups are deleted')
   end
 
   test 'channel tests' do
diff --git a/test/unit/user_test.rb b/test/unit/user_test.rb
index 31919b5d0..799294515 100644
--- a/test/unit/user_test.rb
+++ b/test/unit/user_test.rb
@@ -550,7 +550,15 @@ class UserTest < ActiveSupport::TestCase
   end
 
   test 'min admin permission check' do
-    User.with_permissions('admin').each(&:destroy)
+    # workaround:
+    # - We need to get rid of all admin users but can't delete them
+    #   because we have foreign keys pointing at them since the tests are not isolated yet :(
+    # - We can't just remove the roles since then our check would take place
+    # So we need to merge them with the User Nr 1 and destroy them afterwards
+    User.with_permissions('admin').each do |user|
+      Models.merge('User', 1, user.id)
+      user.destroy
+    end
 
     # store current admin count
     admin_count_inital = User.with_permissions('admin').count