diff --git a/app/controllers/sites_controller.rb b/app/controllers/sites_controller.rb
index d221628..20ce5ba 100644
--- a/app/controllers/sites_controller.rb
+++ b/app/controllers/sites_controller.rb
@@ -72,7 +72,8 @@ class SitesController < ApplicationController
     authorize site
 
     # XXX: Convertir en una máquina de estados?
-    DeployJob.perform_async site.id if site.enqueue!
+    site.enqueue!
+    DeployJob.perform_async site.id
 
     redirect_to site_posts_path(site, locale: site.default_locale)
   end
diff --git a/app/jobs/deploy_job.rb b/app/jobs/deploy_job.rb
index 98e474a..f1ceca9 100644
--- a/app/jobs/deploy_job.rb
+++ b/app/jobs/deploy_job.rb
@@ -8,13 +8,20 @@ class DeployJob < ApplicationJob
   def perform(site, notify = true)
     ActiveRecord::Base.connection_pool.with_connection do
       @site = Site.find(site)
-      @site.update_attribute :status, 'building'
+
+      # Si ya hay una tarea corriendo, aplazar esta
+      if @site.building?
+        DeployJob.perform_in(60, site, notify)
+        return
+      end
+
+      @site.update status: 'building'
       # Asegurarse que DeployLocal sea el primero!
       @deployed = { deploy_local: deploy_locally }
 
       # No es opcional
       unless @deployed[:deploy_local]
-        @site.update_attribute :status, 'waiting'
+        @site.update status: 'waiting'
         notify_usuaries if notify
 
         # Hacer fallar la tarea
@@ -23,7 +30,7 @@ class DeployJob < ApplicationJob
 
       deploy_others
       notify_usuaries if notify
-      @site.update_attribute :status, 'waiting'
+      @site.update status: 'waiting'
     end
   end
   # rubocop:enable Metrics/MethodLength
diff --git a/app/models/metadata_belongs_to.rb b/app/models/metadata_belongs_to.rb
index 0626ba0..ee182a5 100644
--- a/app/models/metadata_belongs_to.rb
+++ b/app/models/metadata_belongs_to.rb
@@ -3,13 +3,6 @@
 # Almacena el UUID de otro Post y actualiza el valor en el Post
 # relacionado.
 class MetadataBelongsTo < MetadataRelatedPosts
-  def value_was=(new_value)
-    @belongs_to = nil
-    @belonged_to = nil
-
-    super(new_value)
-  end
-
   # TODO: Convertir algunos tipos de valores en módulos para poder
   # implementar varios tipos de campo sin repetir código
   #
@@ -39,10 +32,14 @@ class MetadataBelongsTo < MetadataRelatedPosts
 
     # Si estamos cambiando la relación, tenemos que eliminar la relación
     # anterior
-    belonged_to[inverse].value.delete post.uuid.value if changed? && belonged_to.present?
+    if belonged_to.present?
+      belonged_to[inverse].value = belonged_to[inverse].value.reject do |rej|
+        rej == post.uuid.value
+      end
+    end
 
     # No duplicar las relaciones
-    belongs_to[inverse].value << post.uuid.value unless belongs_to.blank? || included?
+    belongs_to[inverse].value = (belongs_to[inverse].value.dup << post.uuid.value) unless belongs_to.blank? || included?
 
     true
   end
@@ -63,20 +60,13 @@ class MetadataBelongsTo < MetadataRelatedPosts
   end
 
   # El Post relacionado con este artículo
-  #
-  # XXX: Memoizamos usando el valor para tener el valor siempre
-  # actualizado.
   def belongs_to
-    return if value.blank?
-
-    @belongs_to ||= posts.find(value, uuid: true)
+    posts.find(value, uuid: true) if value.present?
   end
 
   # El artículo relacionado anterior
   def belonged_to
-    return if value_was.blank?
-
-    @belonged_to ||= posts.find(value_was, uuid: true)
+    posts.find(value_was, uuid: true) if value_was.present?
   end
 
   def related_posts?
diff --git a/app/models/metadata_document_date.rb b/app/models/metadata_document_date.rb
index 39e6873..7512cbf 100644
--- a/app/models/metadata_document_date.rb
+++ b/app/models/metadata_document_date.rb
@@ -8,16 +8,52 @@ class MetadataDocumentDate < MetadataTemplate
   end
 
   def value_from_document
+    return nil if post.new?
+
     document.date
   end
 
+  # Siempre es obligatorio
+  def required
+    true
+  end
+
+  def validate
+    super
+
+    errors << I18n.t('metadata.date.invalid_format') unless valid_format?
+
+    errors.empty?
+  end
+
   # El valor puede ser un Date, Time o una String en el formato
   # "yyyy-mm-dd"
+  #
+  # XXX: Date.iso8601 acepta fechas en el futuro lejano, como 20000,
+  # pero Jekyll las limita a cuatro cifras, así que vamos a mantener
+  # eso.
+  #
+  # @see {https://github.com/jekyll/jekyll/blob/master/lib/jekyll/document.rb#L15}
   def value
-    return (self[:value] = value_from_document || default_value) if self[:value].nil?
+    self[:value] =
+      case self[:value]
+      when String
+        begin
+          Date.iso8601(self[:value]).to_time
+        rescue Date::Error
+          value_from_document || default_value
+        end
+      else
+        self[:value] || value_from_document || default_value
+      end
+  end
 
-    self[:value] = Date.iso8601(self[:value]).to_time if self[:value].is_a? String
+  private
 
-    self[:value]
+  def valid_format?
+    return true if self[:value].is_a?(Time)
+
+    @valid_format_re ||= /\A\d{2,4}-\d{1,2}-\d{1,2}\z/
+    @valid_format_re =~ self[:value].to_s
   end
 end
diff --git a/app/models/metadata_has_and_belongs_to_many.rb b/app/models/metadata_has_and_belongs_to_many.rb
index f14827e..2c4f3d4 100644
--- a/app/models/metadata_has_and_belongs_to_many.rb
+++ b/app/models/metadata_has_and_belongs_to_many.rb
@@ -18,6 +18,7 @@ class MetadataHasAndBelongsToMany < MetadataHasMany
   #
   # Buscamos en belongs_to la relación local, si se eliminó hay que
   # quitarla de la relación remota, sino hay que agregarla.
+  #
   def save
     # XXX: No usamos super
     self[:value] = sanitize value
@@ -25,27 +26,21 @@ class MetadataHasAndBelongsToMany < MetadataHasMany
     return true unless changed?
     return true unless inverse?
 
+    # XXX: Usamos asignación para aprovechar value= que setea el valor
+    # anterior en @value_was
     (had_many - has_many).each do |remove|
-      remove[inverse]&.value&.delete post.uuid.value
+      remove[inverse].value = remove[inverse].value.reject do |rej|
+        rej == post.uuid.value
+      end
     end
 
     (has_many - had_many).each do |add|
       next unless add[inverse]
       next if add[inverse].value.include? post.uuid.value
 
-      add[inverse].value << post.uuid.value
+      add[inverse].value = (add[inverse].value.dup << post.uuid.value)
     end
 
     true
   end
-
-  private
-
-  # Igual que en MetadataRelatedPosts
-  # TODO: Mover a un módulo
-  def sanitize(uuid)
-    super(uuid.map do |u|
-      u.to_s.gsub(/[^a-f0-9\-]/i, '')
-    end)
-  end
 end
diff --git a/app/models/metadata_has_many.rb b/app/models/metadata_has_many.rb
index 6135401..a24a5f1 100644
--- a/app/models/metadata_has_many.rb
+++ b/app/models/metadata_has_many.rb
@@ -6,14 +6,6 @@
 # Localmente tenemos un Array de UUIDs.  Remotamente tenemos una String
 # apuntando a un Post, que se mantiene actualizado como el actual.
 class MetadataHasMany < MetadataRelatedPosts
-  # Invalidar la relación anterior
-  def value_was=(new_value)
-    @had_many = nil
-    @has_many = nil
-
-    super(new_value)
-  end
-
   def validate
     super
 
@@ -24,14 +16,16 @@ class MetadataHasMany < MetadataRelatedPosts
 
   # Todos los Post relacionados
   def has_many
-    @has_many ||= posts.where(uuid: value)
+    return default_value if value.blank?
+
+    posts.where(uuid: value)
   end
 
   # La relación anterior
   def had_many
-    return [] if value_was.blank?
+    return default_value if value_was.blank?
 
-    @had_many ||= posts.where(uuid: value_was)
+    posts.where(uuid: value_was)
   end
 
   def inverse?
diff --git a/app/models/metadata_related_posts.rb b/app/models/metadata_related_posts.rb
index 9a73dea..855ea73 100644
--- a/app/models/metadata_related_posts.rb
+++ b/app/models/metadata_related_posts.rb
@@ -22,7 +22,7 @@ class MetadataRelatedPosts < MetadataArray
 
   # Obtiene todos los posts y opcionalmente los filtra
   def posts
-    @posts ||= site.posts(lang: lang).where(**filter)
+    site.posts(lang: lang).where(**filter)
   end
 
   def title(post)
diff --git a/app/models/post.rb b/app/models/post.rb
index 461733f..31a269e 100644
--- a/app/models/post.rb
+++ b/app/models/post.rb
@@ -49,9 +49,7 @@ class Post
       public_send(attr)&.value = args[attr] if args.key?(attr)
     end
 
-    # XXX: No usamos Post#read porque a esta altura todavía no sabemos
-    # nada del Document
-    document.read! if File.exist? document.path
+    document.read! unless new?
   end
 
   def inspect
@@ -230,12 +228,14 @@ class Post
       template = public_send attr
 
       unless template.front_matter?
-        body += "\n\n"
+        body += "\n\n" if body.present?
         body += template.value
         next
       end
 
-      next if template.empty?
+      # Queremos mantener los Array en el resultado final para que
+      # siempre respondan a {% for %} en Liquid.
+      next if template.empty? && !template.value.is_a?(Array)
 
       [attr.to_s, template.value]
     end.compact.to_h
@@ -285,6 +285,7 @@ class Post
     return false unless write
 
     # Vuelve a leer el post para tomar los cambios
+    document.reset
     read
 
     written?
diff --git a/app/models/post_relation.rb b/app/models/post_relation.rb
index 850a83d..531d3cc 100644
--- a/app/models/post_relation.rb
+++ b/app/models/post_relation.rb
@@ -93,8 +93,7 @@ class PostRelation < Array
   def where(**args)
     return self if args.empty?
 
-    @where ||= {}
-    @where[args.hash.to_s] ||= begin
+    begin
       PostRelation.new(site: site, lang: lang).concat(select do |post|
         result = args.map do |attr, value|
           next unless post.attribute?(attr)
diff --git a/app/models/site.rb b/app/models/site.rb
index 901d70c..a4349f5 100644
--- a/app/models/site.rb
+++ b/app/models/site.rb
@@ -303,14 +303,25 @@ class Site < ApplicationRecord
 
   # Poner en la cola de compilación
   def enqueue!
-    !enqueued? && update_attribute(:status, 'enqueued')
+    update(status: 'enqueued') if waiting?
   end
 
   # Está en la cola de compilación?
+  #
+  # TODO: definir todos estos métodos dinámicamente, aunque todavía no
+  # tenemos una máquina de estados propiamente dicha.
   def enqueued?
     status == 'enqueued'
   end
 
+  def waiting?
+    status == 'waiting'
+  end
+
+  def building?
+    status == 'building'
+  end
+
   # Cargar el sitio Jekyll
   #
   # TODO: En lugar de leer todo junto de una vez, extraer la carga de
diff --git a/config/initializers/core_extensions.rb b/config/initializers/core_extensions.rb
index 8bbcbe7..d43a511 100644
--- a/config/initializers/core_extensions.rb
+++ b/config/initializers/core_extensions.rb
@@ -52,7 +52,7 @@ module Jekyll
 
     # Solo lee los datos
     def read_data
-      @site.data = DataReader.new(site).read(site.config["data_dir"])
+      @site.data = DataReader.new(site).read(site.config['data_dir'])
     end
 
     # Lee todos los artículos del sitio
@@ -73,6 +73,15 @@ module Jekyll
   Document.class_eval do
     alias_method :read!, :read
     def read; end
+
+    # Permitir restablecer el documento sin crear uno nuevo
+    def reset
+      @path = @extname = @has_yaml_header = @relative_path = nil
+      @basename_without_ext = @data = @basename = nil
+      @renderer = @url_placeholders = @url = nil
+      @to_liquid = @write_p = @excerpt_separator = @id = nil
+      @related_posts = @cleaned_relative_path = self.content = nil
+    end
   end
 
   # Prevenir la instanciación de Time
diff --git a/test/controllers/sites_controller_test.rb b/test/controllers/sites_controller_test.rb
index 5f67092..a7e2f68 100644
--- a/test/controllers/sites_controller_test.rb
+++ b/test/controllers/sites_controller_test.rb
@@ -102,6 +102,13 @@ class SitesControllerTest < ActionDispatch::IntegrationTest
                                  'index.html'))
   end
 
+  test 'no se pueden encolar varias veces seguidas' do
+    assert_enqueued_jobs 2 do
+      post site_enqueue_url(@site), headers: @authorization
+      post site_enqueue_url(@site), headers: @authorization
+    end
+  end
+
   test 'se pueden actualizar' do
     name = SecureRandom.hex
     design = Design.all.where.not(id: @site.design_id).sample
diff --git a/test/fixtures/site_with_relationships/README.md b/test/fixtures/site_with_relationships/README.md
new file mode 100644
index 0000000..89b5b89
--- /dev/null
+++ b/test/fixtures/site_with_relationships/README.md
@@ -0,0 +1,2 @@
+This is site where posts can have many authors and viceversa and posts
+can be replies to others.
diff --git a/test/fixtures/site_with_relationships/_config.yml b/test/fixtures/site_with_relationships/_config.yml
new file mode 100644
index 0000000..da2d25c
--- /dev/null
+++ b/test/fixtures/site_with_relationships/_config.yml
@@ -0,0 +1,2 @@
+locales:
+- en
diff --git a/test/fixtures/site_with_relationships/_data/layouts/author.yml b/test/fixtures/site_with_relationships/_data/layouts/author.yml
new file mode 100644
index 0000000..afe620e
--- /dev/null
+++ b/test/fixtures/site_with_relationships/_data/layouts/author.yml
@@ -0,0 +1,9 @@
+---
+title:
+  type: 'string'
+  required: true
+posts:
+  type: 'has_and_belongs_to_many'
+  inverse: 'authors'
+  filter:
+    layout: 'post'
diff --git a/test/fixtures/site_with_relationships/_data/layouts/post.yml b/test/fixtures/site_with_relationships/_data/layouts/post.yml
new file mode 100644
index 0000000..c98baf7
--- /dev/null
+++ b/test/fixtures/site_with_relationships/_data/layouts/post.yml
@@ -0,0 +1,23 @@
+---
+title:
+  type: 'string'
+  required: true
+authors:
+  type: 'has_and_belongs_to_many'
+  inverse: 'posts'
+  filter:
+    layout: 'author'
+posts:
+  type: 'has_many'
+  inverse: 'in_reply_to'
+  filter:
+    layout: 'post'
+in_reply_to:
+  type: 'belongs_to'
+  inverse: 'posts'
+  filter:
+    layout: 'post'
+recommended_posts:
+  type: 'related_posts'
+  filter:
+    layout: 'post'
diff --git a/test/fixtures/site_with_relationships/_en/.keep b/test/fixtures/site_with_relationships/_en/.keep
new file mode 100644
index 0000000..e69de29
diff --git a/test/fixtures/site_with_relationships/_posts b/test/fixtures/site_with_relationships/_posts
new file mode 120000
index 0000000..3da1d67
--- /dev/null
+++ b/test/fixtures/site_with_relationships/_posts
@@ -0,0 +1 @@
+_en
\ No newline at end of file
diff --git a/test/models/metadata_belongs_to_test.rb b/test/models/metadata_belongs_to_test.rb
new file mode 100644
index 0000000..09b9714
--- /dev/null
+++ b/test/models/metadata_belongs_to_test.rb
@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+require_relative 'metadata_test'
+
+class MetadataBelongsToTest < ActiveSupport::TestCase
+  include MetadataTest
+
+  test 'se pueden relacionar artículos' do
+    post = @site.posts.create(layout: :post, title: SecureRandom.hex)
+    reply = @site.posts.create(layout: :post, title: SecureRandom.hex, in_reply_to: post.uuid.value)
+
+    assert_equal post, reply.in_reply_to.belongs_to
+    assert_includes post.posts.has_many, reply
+
+    assert post.save
+
+    assert_equal reply.document.data['in_reply_to'], post.document.data['uuid']
+    assert_includes post.document.data['posts'], reply.document.data['uuid']
+  end
+
+  test 'se puede eliminar la relación' do
+    post = @site.posts.create(layout: :post, title: SecureRandom.hex)
+    reply = @site.posts.create(layout: :post, title: SecureRandom.hex, in_reply_to: post.uuid.value)
+
+    reply.in_reply_to.value = ''
+    assert reply.save
+
+    assert_not_equal post, reply.in_reply_to.belongs_to
+    assert_equal post, reply.in_reply_to.belonged_to
+    assert_nil reply.in_reply_to.belongs_to
+    assert_not_includes post.posts.has_many, reply
+
+    assert post.save
+
+    assert_nil reply.document.data['in_reply_to']
+    assert_not_includes post.document.data['posts'], reply.document.data['uuid']
+  end
+
+  test 'se puede cambiar la relación' do
+    post1 = @site.posts.create(layout: :post, title: SecureRandom.hex)
+    post2 = @site.posts.create(layout: :post, title: SecureRandom.hex)
+    reply = @site.posts.create(layout: :post, title: SecureRandom.hex, in_reply_to: post1.uuid.value)
+
+    reply.in_reply_to.value = post2.uuid.value
+    assert reply.save
+
+    assert_not_equal post1, reply.in_reply_to.belongs_to
+    assert_equal post1, reply.in_reply_to.belonged_to
+    assert_not_includes post1.posts.has_many, reply
+
+    assert_equal post2, reply.in_reply_to.belongs_to
+    assert_includes post2.posts.has_many, reply
+
+    assert post1.save
+    assert post2.save
+
+    assert_equal post2.document.data['uuid'], reply.document.data['in_reply_to']
+    assert_includes post2.document.data['posts'], reply.document.data['uuid']
+    assert_not_includes post1.document.data['posts'], reply.document.data['uuid']
+  end
+end
diff --git a/test/models/metadata_has_and_belongs_to_many_test.rb b/test/models/metadata_has_and_belongs_to_many_test.rb
new file mode 100644
index 0000000..4887a96
--- /dev/null
+++ b/test/models/metadata_has_and_belongs_to_many_test.rb
@@ -0,0 +1,76 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+require_relative 'metadata_test'
+
+class MetadataHasAndBelongsManyTest < ActiveSupport::TestCase
+  include MetadataTest
+
+  test 'se pueden relacionar artículos' do
+    author = @site.posts.create(layout: :author, title: SecureRandom.hex)
+    post = @site.posts.create(layout: :post, title: SecureRandom.hex)
+
+    post.authors.value = [author.uuid.value]
+    assert post.save
+
+    assert_includes author.posts.has_many, post
+    assert_includes post.authors.has_many, author
+
+    assert author.save
+
+    assert_includes author.document.data['posts'], post.document.data['uuid']
+    assert_includes post.document.data['authors'], author.document.data['uuid']
+  end
+
+  test 'se puede eliminar la relación' do
+    author = @site.posts.create(layout: :author, title: SecureRandom.hex)
+    post = @site.posts.create(layout: :post, title: SecureRandom.hex, authors: [author.uuid.value])
+
+    assert_includes post.authors.value, author.uuid.value
+    assert_includes author.posts.value, post.uuid.value
+
+    post.authors.value = []
+    assert post.save
+
+    assert_not_includes author.posts.has_many, post
+    assert_not_includes post.authors.has_many, author
+
+    assert_includes author.posts.had_many, post
+    assert_includes post.authors.had_many, author
+
+    assert author.save
+
+    assert_not_includes author.document.data['posts'], post.document.data['uuid']
+    assert_not_includes post.document.data['authors'], author.document.data['uuid']
+  end
+
+  test 'se puede cambiar la relación' do
+    author = @site.posts.create(layout: :author, title: SecureRandom.hex)
+    post1 = @site.posts.create(layout: :post, title: SecureRandom.hex, authors: [author.uuid.value])
+    post2 = @site.posts.create(layout: :post, title: SecureRandom.hex)
+
+    author.posts.value = [post2.uuid.value]
+    assert author.save
+
+    assert_not_includes author.posts.has_many, post1
+    assert_not_includes post1.authors.has_many, author
+
+    assert_includes author.posts.had_many, post1
+    assert_includes post1.authors.had_many, author
+
+    assert_not_includes author.posts.had_many, post2
+    assert_not_includes post2.authors.had_many, author
+
+    assert_includes author.posts.has_many, post2
+    assert_includes post2.authors.has_many, author
+
+    assert post1.save
+    assert post2.save
+
+    assert_not_includes author.document.data['posts'], post1.document.data['uuid']
+    assert_not_includes post1.document.data['authors'], author.document.data['uuid']
+
+    assert_includes author.document.data['posts'], post2.document.data['uuid']
+    assert_includes post2.document.data['authors'], author.document.data['uuid']
+  end
+end
diff --git a/test/models/metadata_has_many_test.rb b/test/models/metadata_has_many_test.rb
new file mode 100644
index 0000000..38b4d46
--- /dev/null
+++ b/test/models/metadata_has_many_test.rb
@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+require_relative 'metadata_test'
+
+class MetadataHasManyTest < ActiveSupport::TestCase
+  include MetadataTest
+
+  test 'se pueden relacionar artículos' do
+    reply = @site.posts.create(layout: :post, title: SecureRandom.hex)
+    post = @site.posts.create(layout: :post, title: SecureRandom.hex, posts: [reply.uuid.value])
+
+    assert_equal post, reply.in_reply_to.belongs_to
+    assert_includes post.posts.has_many, reply
+
+    assert reply.save
+
+    assert_equal reply.document.data['in_reply_to'], post.document.data['uuid']
+    assert_includes post.document.data['posts'], reply.document.data['uuid']
+  end
+
+  test 'se puede eliminar la relación' do
+    reply = @site.posts.create(layout: :post, title: SecureRandom.hex)
+    post = @site.posts.create(layout: :post, title: SecureRandom.hex, posts: [reply.uuid.value])
+
+    post.posts.value = []
+    assert post.save
+
+    assert_not_equal post, reply.in_reply_to.belongs_to
+    assert_equal post, reply.in_reply_to.belonged_to
+    assert_nil reply.in_reply_to.belongs_to
+    assert_not_includes post.posts.has_many, reply
+
+    assert reply.save
+
+    assert_nil reply.document.data['in_reply_to']
+    assert_not_includes post.document.data['posts'], reply.document.data['uuid']
+  end
+
+  test 'se puede cambiar la relación' do
+    reply = @site.posts.create(layout: :post, title: SecureRandom.hex)
+    post1 = @site.posts.create(layout: :post, title: SecureRandom.hex, posts: [reply.uuid.value])
+    post2 = @site.posts.create(layout: :post, title: SecureRandom.hex)
+
+    reply.in_reply_to.value = post2.uuid.value
+    assert reply.save
+
+    assert_not_equal post1, reply.in_reply_to.belongs_to
+    assert_equal post1, reply.in_reply_to.belonged_to
+    assert_not_includes post1.posts.has_many, reply
+
+    assert_equal post2, reply.in_reply_to.belongs_to
+    assert_includes post2.posts.has_many, reply
+
+    assert post1.save
+    assert post2.save
+
+    assert_equal post2.document.data['uuid'], reply.document.data['in_reply_to']
+    assert_includes post2.document.data['posts'], reply.document.data['uuid']
+    assert_not_includes post1.document.data['posts'], reply.document.data['uuid']
+  end
+end
diff --git a/test/models/metadata_test.rb b/test/models/metadata_test.rb
new file mode 100644
index 0000000..24d955a
--- /dev/null
+++ b/test/models/metadata_test.rb
@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+module MetadataTest
+  extend ActiveSupport::Concern
+
+  included do
+    setup do
+      name = SecureRandom.hex
+      # TODO: Poder cambiar el nombre
+      FileUtils.cp_r(Rails.root.join('test', 'fixtures', 'site_with_relationships'), Rails.root.join('_sites', name))
+
+      @site = create :site, name: name
+    end
+
+    teardown do
+      @site&.destroy
+    end
+  end
+end