Refactor code based on MR comments

db/migrate/20171107130813_create_epic_issues_table.rb

 class CreateEpicIssuesTable < ActiveRecord::Migration
-  include Gitlab::Database::MigrationHelpers
-
   DOWNTIME = false
 
   disable_ddl_transaction!

db/schema.rb

@@ -735,8 +735,6 @@ ActiveRecord::Schema.define(version: 20171107144726) do
   create_table "epic_issues", force: :cascade do |t|
     t.integer "epic_id", null: false
     t.integer "issue_id", null: false
-    t.datetime_with_timezone "created_at"
-    t.datetime_with_timezone "updated_at"
   end
 
   add_index "epic_issues", ["epic_id"], name: "index_epic_issues_on_epic_id", using: :btree
@@ -2403,8 +2401,8 @@ ActiveRecord::Schema.define(version: 20171107144726) do
   add_foreign_key "deploy_keys_projects", "projects", name: "fk_58a901ca7e", on_delete: :cascade
   add_foreign_key "deployments", "projects", name: "fk_b9a3851b82", on_delete: :cascade
   add_foreign_key "environments", "projects", name: "fk_d1c8c1da6a", on_delete: :cascade
-  add_foreign_key "epic_issues", "epics"
-  add_foreign_key "epic_issues", "issues"
+  add_foreign_key "epic_issues", "epics", on_delete: :cascade
+  add_foreign_key "epic_issues", "issues", on_delete: :cascade
   add_foreign_key "epic_metrics", "epics", on_delete: :cascade
   add_foreign_key "epics", "milestones", on_delete: :nullify
   add_foreign_key "epics", "namespaces", column: "group_id", name: "fk_f081aa4489", on_delete: :cascade

ee/app/services/epic_issues/create_service.rb

@@ -17,7 +17,7 @@ module EpicIssues
     end
 
     def linkable_issues(issues)
-      return [] unless  can?(current_user, :admin_epic, issuable.group)
+      return [] unless can?(current_user, :admin_epic, issuable.group)
 
       issues.select { |issue| issue.project.group == issuable.group }
     end

spec/ee/spec/controllers/groups/epic_issues_controller_spec.rb

@@ -46,7 +46,7 @@ describe Groups::EpicIssuesController do
       post :create, group_id: group, epic_id: epic.to_param, issue_references: reference
     end
 
-    context 'when user has permissions to create requested associtaion' do
+    context 'when user has permissions to create requested association' do
       before do
         group.add_developer(user)
       end
@@ -64,7 +64,7 @@ describe Groups::EpicIssuesController do
       end
     end
 
-    context 'when user does not have permissions to create requested associtaion' do
+    context 'when user does not have permissions to create requested association' do
       it 'returns correct response for the correct issue reference' do
         subject
 
@@ -134,7 +134,7 @@ describe Groups::EpicIssuesController do
       end
     end
 
-    context 'when the epic_issue record does not exixst' do
+    context 'when the epic_issue record does not exists' do
       it 'returns status 404' do
         delete :destroy, group_id: group, epic_id: epic.to_param, id: 9999
 

spec/ee/spec/models/epic_spec.rb

@@ -40,22 +40,22 @@ describe Epic do
       ]
     end
 
-    subject { epic.issues(user) }
+    let(:result) { epic.issues(user) }
 
     it 'returns all issues if a user has access to them' do
       group.add_developer(user)
 
-      expect(subject.count).to eq(2)
-      expect(subject.map(&:id)).to match_array([issue.id, other_issue.id])
-      expect(subject.map(&:epic_issue_id)).to match_array(epic_issues.map(&:id))
+      expect(result.count).to eq(2)
+      expect(result.map(&:id)).to match_array([issue.id, other_issue.id])
+      expect(result.map(&:epic_issue_id)).to match_array(epic_issues.map(&:id))
     end
 
     it 'does not return issues user can not see' do
       project.add_developer(user)
 
-      expect(subject.count).to eq(1)
-      expect(subject.map(&:id)).to match_array([issue.id])
-      expect(subject.map(&:epic_issue_id)).to match_array([epic_issues.first.id])
+      expect(result.count).to eq(1)
+      expect(result.map(&:id)).to match_array([issue.id])
+      expect(result.map(&:epic_issue_id)).to match_array([epic_issues.first.id])
     end
   end
 end

spec/ee/spec/services/epic_issues/create_service_spec.rb

@@ -59,6 +59,22 @@ describe EpicIssues::CreateService do
           subject { assign_issue([valid_reference]) }
 
           include_examples 'returns success'
+
+          it 'does not perofrm N + 1 queries' do
+            params = { issue_references: [valid_reference] }
+            control_count = ActiveRecord::QueryRecorder.new { described_class.new(epic, user, params).execute }.count
+
+            user = create(:user)
+            group = create(:group)
+            project = create(:project, group: group)
+            issues = create_list(:issue, 5, project: project)
+            epic = create(:epic, group: group)
+            group.add_developer(user)
+
+            params = { issue_references: issues.map { |i| i.to_reference(full: true) } }
+
+            expect { described_class.new(epic, user, params).execute }.not_to exceed_query_limit(control_count)
+          end
         end
 
         context 'when an issue links is given' do