added forign key and specs

app/models/user.rb

@@ -139,6 +139,7 @@ class User < ActiveRecord::Base
   has_many :issue_assignees
   has_many :assigned_issues, class_name: "Issue", through: :issue_assignees, source: :issue
   has_many :assigned_merge_requests,  dependent: :nullify, foreign_key: :assignee_id, class_name: "MergeRequest" # rubocop:disable Cop/ActiveRecordDependent
+  has_many :closed_issues, class_name: 'Issue', dependent: :nullify, foreign_key: :closed_by_id
 
   has_many :custom_attributes, class_name: 'UserCustomAttribute'
   has_many :callouts, class_name: 'UserCallout'

db/migrate/20180209165249_add_closed_by_to_issues.rb

@@ -4,10 +4,17 @@
 class AddClosedByToIssues < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
+  disable_ddl_transaction!
   # Set this constant to true if this migration requires downtime.
   DOWNTIME = false
 
-  def change
+  def up
     add_column :issues, :closed_by_id, :integer
+    add_concurrent_foreign_key :issues, :users, column: :closed_by_id, on_delete: :nullify
+  end
+
+  def down
+    remove_foreign_key :issues, column: :closed_by_id
+    remove_column :issues, :closed_by_id
   end
 end

db/schema.rb

@@ -1293,6 +1293,7 @@ ActiveRecord::Schema.define(version: 20180327101207) do
     t.integer "last_edited_by_id"
     t.boolean "discussion_locked"
     t.datetime_with_timezone "closed_at"
+    t.integer "closed_by_id"
   end
 
   add_index "issues", ["author_id"], name: "index_issues_on_author_id", using: :btree
@@ -2695,6 +2696,7 @@ ActiveRecord::Schema.define(version: 20180327101207) do
   add_foreign_key "issues", "milestones", name: "fk_96b1dd429c", on_delete: :nullify
   add_foreign_key "issues", "projects", name: "fk_899c8f3231", on_delete: :cascade
   add_foreign_key "issues", "users", column: "author_id", name: "fk_05f1e72feb", on_delete: :nullify
+  add_foreign_key "issues", "users", column: "closed_by_id", name: "fk_c63cbf6c25", on_delete: :nullify
   add_foreign_key "issues", "users", column: "updated_by_id", name: "fk_ffed080f01", on_delete: :nullify
   add_foreign_key "label_priorities", "labels", on_delete: :cascade
   add_foreign_key "label_priorities", "projects", on_delete: :cascade

spec/services/issues/close_service_spec.rb

@@ -67,6 +67,10 @@ describe Issues::CloseService do
         expect(issue).to be_closed
       end
 
+      it 'records closed user' do
+        expect(issue.closed_by_id).to be(user.id)
+      end
+
       it 'sends email to user2 about assign of new issue' do
         email = ActionMailer::Base.deliveries.last
         expect(email.to.first).to eq(user2.email)