Removed id column from milestone_releases

Added migration to remove id column from table.
Removed uniqueness validation on MilestoneRelease
since the index over the two foreign_key plays
the same role.

app/models/milestone_release.rb

@@ -4,7 +4,6 @@ class MilestoneRelease < ApplicationRecord
   belongs_to :milestone
   belongs_to :release
 
-  validates :milestone_id, uniqueness: { scope: [:release_id] }
   validate :same_project_between_milestone_and_release
 
   private

db/migrate/20190904205212_remove_id_column_from_intermediate_release_milestones.rb

+# frozen_string_literal: true
+
+class RemoveIdColumnFromIntermediateReleaseMilestones < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    remove_column :milestone_releases, :id
+  end
+end

db/schema.rb

@@ -2196,7 +2196,7 @@ ActiveRecord::Schema.define(version: 2019_09_12_061145) do
     t.index ["user_id"], name: "index_merge_trains_on_user_id"
   end
 
-  create_table "milestone_releases", force: :cascade do |t|
+  create_table "milestone_releases", id: false, force: :cascade do |t|
     t.bigint "milestone_id", null: false
     t.bigint "release_id", null: false
     t.index ["milestone_id", "release_id"], name: "index_miletone_releases_on_milestone_and_release", unique: true

spec/models/milestone_release_spec.rb

@@ -15,7 +15,14 @@ describe MilestoneRelease do
   end
 
   describe 'validations' do
-    it { is_expected.to validate_uniqueness_of(:milestone_id).scoped_to(:release_id) }
+    context 'when trying to create the same record in milestone_releases twice' do
+      it 'is not committing on the second time' do
+        described_class.create!(milestone_id: milestone.id, release_id: release.id)
+        expect do
+          described_class.create!(milestone_id: milestone.id, release_id: release.id)
+        end.to raise_error(ActiveRecord::RecordNotUnique)
+      end
+    end
 
     context 'when milestone and release do not have the same project' do
       it 'is not valid' do