Create refereces model

app/models/reference.rb

+# frozen_string_literal: true
+
+class Reference < ApplicationRecord
+  belongs_to :source, polymorphic: true
+  belongs_to :target, polymorphic: true
+  validates :target_id, uniqueness: {scope: %i[target_type source_id source_type]}
+end

db/migrate/20170920214158_create_references_table.rb

+# frozen_string_literal: true
+
+class CreateReferencesTable < ActiveRecord::Migration[5.1]
+  def change
+    create_table :references do |t|
+      t.integer  :source_id,              null: false
+      t.string   :source_type, limit: 60, null: false
+      t.integer  :target_id,              null: false
+      t.string   :target_type, limit: 60, null: false
+    end
+
+    add_index :references, %i[source_id source_type target_id target_type],
+              name: :index_references_on_source_and_target, unique: true
+    add_index :references, %i[source_id source_type], name: :index_references_on_source_id_and_source_type
+  end
+end

spec/factories.rb

@@ -258,6 +258,11 @@ FactoryGirl.define do
     end
   end
 
+  factory :reference do
+    association :source, factory: :status_message
+    association :target, factory: :status_message
+  end
+
   factory(:notification, class: Notifications::AlsoCommented) do
     association :recipient, :factory => :user
     association :target, :factory => :comment

spec/models/reference_spec.rb

+# frozen_string_literal: true
+
+describe Reference, type: :model do
+  context "validation" do
+    it "validates a valid reference" do
+      expect(FactoryGirl.build(:reference)).to be_valid
+    end
+
+    it "requires a source" do
+      expect(FactoryGirl.build(:reference, source: nil)).not_to be_valid
+    end
+
+    it "requires a target" do
+      expect(FactoryGirl.build(:reference, target: nil)).not_to be_valid
+    end
+
+    it "disallows to link the same target twice from one source" do
+      reference = FactoryGirl.create(:reference)
+      expect(FactoryGirl.build(:reference, source: reference.source, target: reference.target)).not_to be_valid
+    end
+  end
+end