Move related content validation to the model

We were manually checking validation rules (like both relationable
objects are present, or they're both the same object) in the controller
and then using the `save!` method.

However, we usually use the `save` method (which checks all validations)
in a condition, and proceed depending on the result.

Now we're taking the same approach here. This means introducing a new
validation rule in the model to check whether both relationable objects
are the same, which is more robust than checking a condition in the
controller.

app/controllers/related_contents_controller.rb

@@ -4,18 +4,20 @@ class RelatedContentsController < ApplicationController
   respond_to :html, :js
 
   def create
-    if relationable_object && related_object
+    related_content = RelatedContent.new(
-      if relationable_object != related_object
+      parent_relationable: relationable_object,
-        RelatedContent.create!(parent_relationable: @relationable, child_relationable: @related, author: current_user)
+      child_relationable: related_object,
+      author: current_user
+    )
 
+    if related_content.save
       flash[:success] = t("related_content.success")
-      else
+    elsif related_content.same_parent_and_child?
       flash[:error] = t("related_content.error_itself")
-      end
     else
       flash[:error] = t("related_content.error", url: Setting["url"])
     end
-    redirect_to polymorphic_path(@relationable)
+    redirect_to polymorphic_path(relationable_object)
   end
 
   def score_positive
@@ -40,7 +42,7 @@ class RelatedContentsController < ApplicationController
     end
 
     def relationable_object
-      @relationable = params[:relationable_klass].singularize.camelize.constantize.find_by(id: params[:relationable_id])
+      @relationable ||= params[:relationable_klass].singularize.camelize.constantize.find_by(id: params[:relationable_id])
     end
 
     def related_object
@@ -51,7 +53,7 @@ class RelatedContentsController < ApplicationController
                            .flatten.map { |i| i.to_s.singularize.camelize }.join("::")
         related_id = url.match(/\/(\d+)(?!.*\/\d)/)[1]
 
-        @related = related_klass.singularize.camelize.constantize.find_by(id: related_id)
+        related_klass.singularize.camelize.constantize.find_by(id: related_id)
       end
     rescue
       nil

app/models/related_content.rb

@@ -16,6 +16,7 @@ class RelatedContent < ApplicationRecord
   validates :child_relationable_id, presence: true
   validates :child_relationable_type, presence: true
   validates :parent_relationable_id, uniqueness: { scope: [:parent_relationable_type, :child_relationable_id, :child_relationable_type] }
+  validate :different_parent_and_child
 
   after_create :create_opposite_related_content, unless: proc { opposite_related_content.present? }
   after_create :create_author_score
@@ -34,8 +35,18 @@ class RelatedContent < ApplicationRecord
     related_content_scores.exists?(user: user)
   end
 
+  def same_parent_and_child?
+    parent_relationable == child_relationable
+  end
+
   private
 
+    def different_parent_and_child
+      if same_parent_and_child?
+        errors.add(:parent_relationable, :itself)
+      end
+    end
+
     def create_opposite_related_content
       related_content = RelatedContent.create!(opposite_related_content: self, parent_relationable: child_relationable,
                                                child_relationable: parent_relationable, author: author)

spec/models/related_content_spec.rb

@@ -25,6 +25,14 @@ describe RelatedContent do
     expect(new_related_content).not_to be_valid
   end
 
+  it "does not allow relating a content to itself" do
+    related_content = build(:related_content,
+                            parent_relationable: parent_relationable,
+                            child_relationable: parent_relationable)
+
+    expect(related_content).not_to be_valid
+  end
+
   describe "create_opposite_related_content" do
     let(:parent_relationable) { create(:proposal) }
     let(:child_relationable) { create(:debate) }