Avoid a brakeman warning in related contents

Although it wasn't a real security concern because we were only calling
a `find_by` method based on the user input, it's a good practice to
avoid using constants based on user parameters.

Since we don't use the `find_by` method anymore but we still need to
check the associated record exists, we're changing the validations in
the `RelatedContent` model to do exactly that.

app/controllers/related_contents_controller.rb

@@ -4,10 +4,10 @@ class RelatedContentsController < ApplicationController
   respond_to :html, :js
 
   def create
-    related_content = RelatedContent.new(
-      parent_relationable: relationable_object,
-      child_relationable: related_object,
-      author: current_user
+    related_content = current_user.related_contents.new(
+      parent_relationable_id: params[:relationable_id],
+      parent_relationable_type: params[:relationable_klass],
+      child_relationable: related_object
     )
 
     if related_content.save
@@ -17,7 +17,7 @@ class RelatedContentsController < ApplicationController
     else
       flash[:error] = t("related_content.error", url: Setting["url"])
     end
-    redirect_to polymorphic_path(relationable_object)
+    redirect_to polymorphic_path(related_content.parent_relationable)
   end
 
   def score_positive
@@ -41,10 +41,6 @@ class RelatedContentsController < ApplicationController
       params[:url].start_with?(Setting["url"])
     end
 
-    def relationable_object
-      @relationable ||= params[:relationable_klass].singularize.camelize.constantize.find_by(id: params[:relationable_id])
-    end
-
     def related_object
       if valid_url?
         url = params[:url]

app/models/related_content.rb

@@ -6,15 +6,11 @@ class RelatedContent < ApplicationRecord
   include ActsAsParanoidAliases
 
   belongs_to :author, class_name: "User"
-  belongs_to :parent_relationable, polymorphic: true, touch: true
-  belongs_to :child_relationable, polymorphic: true, touch: true
+  belongs_to :parent_relationable, polymorphic: true, optional: false, touch: true
+  belongs_to :child_relationable, polymorphic: true, optional: false, touch: true
   has_one :opposite_related_content, class_name: self.name, foreign_key: :related_content_id
   has_many :related_content_scores
 
-  validates :parent_relationable_id, presence: true
-  validates :parent_relationable_type, presence: true
-  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
 

app/models/user.rb

@@ -74,6 +74,7 @@ class User < ApplicationRecord
     class_name:  "Poll::Recount",
     foreign_key: :author_id,
     inverse_of:  :author
+  has_many :related_contents, foreign_key: :author_id, inverse_of: :author, dependent: nil
   has_many :topics, foreign_key: :author_id, inverse_of: :author
   belongs_to :geozone
 

spec/models/related_content_spec.rb

@@ -3,9 +3,10 @@ require "rails_helper"
 describe RelatedContent do
   let(:parent_relationable) { create([:proposal, :debate].sample) }
   let(:child_relationable) { create([:proposal, :debate].sample) }
+  let(:related_content) { build(:related_content) }
 
   it "is valid" do
-    expect(build(:related_content)).to be_valid
+    expect(related_content).to be_valid
   end
 
   it "allows relationables from various classes" do
@@ -33,6 +34,28 @@ describe RelatedContent do
     expect(related_content).not_to be_valid
   end
 
+  it "is not valid with an invalid parent relationable type" do
+    related_content.parent_relationable_type = "NotARealModel"
+
+    expect { related_content.valid? }.to raise_exception "uninitialized constant NotARealModel"
+  end
+
+  it "is not valid with parent relationable ID of a non-existent record" do
+    related_content.parent_relationable_id = related_content.parent_relationable.class.last.id + 1
+
+    expect(related_content).not_to be_valid
+  end
+
+  it "is not valid with an invalid child relationable type" do
+    related_content.child_relationable_type = "NotARealModel"
+
+    expect { related_content.valid? }.to raise_exception "uninitialized constant NotARealModel"
+  end
+
+  it "is not valid with child relationable ID of a non-existent record" do
+    related_content.child_relationable_id = related_content.child_relationable.class.last.id + 1
+  end
+
   describe "create_opposite_related_content" do
     let(:parent_relationable) { create(:proposal) }
     let(:child_relationable) { create(:debate) }