testear relación has_and_belongs_to_many

un artículo puede pertenecer y tener muchos artículos (una relación
poliamorosa de muchos a muchos)

también utilizamos asignación para aprovechar `value=`.

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

test/models/metadata_has_and_belongs_to_many_test.rb

@@ -0,0 +1,58 @@
+# 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
+  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 = []
+    ENV['HOLA'] = 'hola'
+    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
+  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
+  end
+end