Switched over to DB reset strategy in tests and removed DatabaseCleaner because it can't guarantee a clean DB state for all test scenarios.

Gemfile

@@ -176,10 +176,6 @@ group :development, :test do
   # record and replay TCP/HTTP transactions
   gem 'tcr'
   gem 'vcr'
-
-  # database cleanup when transactions are not possible
-  # and DB initialization before running RSpec suite
-  gem 'database_cleaner'
 end
 
 # Want to extend Zammad with additional gems?

Gemfile.lock

@@ -113,7 +113,6 @@ GEM
     crass (1.0.4)
     daemons (1.2.5)
     dalli (2.7.6)
-    database_cleaner (1.7.0)
     debug_inspector (0.0.3)
     delayed_job (4.1.3)
       activesupport (>= 3.0, < 5.2)
@@ -518,7 +517,6 @@ DEPENDENCIES
   coveralls
   daemons
   dalli
-  database_cleaner
   delayed_job_active_record
   diffy
   doorkeeper

lib/tasks/zammad/db/reset.rake

@@ -2,10 +2,25 @@ namespace :zammad do
 
   namespace :db do
 
-    desc 'Truncates and reseeds the database, clears the Cache and reloads the Settings'
+    desc 'Drops, recreates and seeds the database, clears the Cache and reloads the Settings'
     task reset: :environment do
-      DatabaseCleaner.clean_with(:truncation)
-      Rails.application.load_seed
+
+      # we loop over each dependent task to be able to
+      # execute them and their prerequisites multiple times (in tests)
+      # there is no way in rake to achive that
+      %w[db:drop:_unsafe db:create db:schema:load db:seed].each do |task|
+
+        begin
+          $stdout = StringIO.new if task == 'db:schema:load'.freeze
+
+          Rake::Task[task].reenable
+          Rake::Task[task].invoke
+        ensure
+          $stdout = STDOUT
+        end
+      end
+
+      ActiveRecord::Base.connection.reconnect!
       Cache.clear
       Setting.reload
     end

spec/db/migrate/issue_1977_remove_invalid_user_foreign_keys_spec.rb

@@ -8,7 +8,7 @@ RSpec.describe Issue1977RemoveInvalidUserForeignKeys, type: :db_migration do
 
     context 'invalid User foreign key columns' do
 
-      it 'cleans up OnlineNotification#user_id', db_strategy: :truncation do
+      it 'cleans up OnlineNotification#user_id', db_strategy: :reset do
         witout_foreign_key(:online_notifications, column: :user_id)
 
         create(:online_notification, user_id: 1337)
@@ -21,7 +21,7 @@ RSpec.describe Issue1977RemoveInvalidUserForeignKeys, type: :db_migration do
         }.by(-1)
       end
 
-      it 'cleans up RecentView#created_by_id', db_strategy: :truncation do
+      it 'cleans up RecentView#created_by_id', db_strategy: :reset do
         witout_foreign_key(:online_notifications, column: :user_id)
         witout_foreign_key(:recent_views, column: :created_by_id)
 
@@ -35,7 +35,7 @@ RSpec.describe Issue1977RemoveInvalidUserForeignKeys, type: :db_migration do
         }.by(-1)
       end
 
-      it 'cleans up Avatar#o_id', db_strategy: :truncation do
+      it 'cleans up Avatar#o_id', db_strategy: :reset do
         witout_foreign_key(:online_notifications, column: :user_id)
 
         create(:avatar, object_lookup_id: ObjectLookup.by_name('User'), o_id: 1337)

spec/support/db_strategies.rb

@@ -1,8 +1,9 @@
 RSpec.configure do |config|
 
-  config.around(:each, db_strategy: :truncation) do |example|
+  config.around(:each, db_strategy: :reset) do |example|
     self.use_transactional_tests = false
     example.run
-    Rake::Task['zammad:db:reset'].execute
+    Rake::Task['zammad:db:reset'].reenable
+    Rake::Task['zammad:db:reset'].invoke
   end
 end