Add closed_at field to issues

app/models/issue.rb

@@ -55,6 +55,14 @@ class Issue < ActiveRecord::Base
     state :opened
     state :reopened
     state :closed
+
+    before_transition any => :closed do |issue|
+      issue.closed_at = Time.zone.now
+    end
+
+    before_transition closed: any do |issue|
+      issue.closed_at = nil
+    end
   end
 
   def hook_attrs

changelogs/unreleased/issue_27212.yml

+---
+title: Add closed_at field to issues
+merge_request:
+author:

db/migrate/20170315194013_add_closed_at_to_issues.rb

+class AddClosedAtToIssues < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def change
+    add_column :issues, :closed_at, :datetime
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170315174634) do
+ActiveRecord::Schema.define(version: 20170315194013) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -531,6 +531,7 @@ ActiveRecord::Schema.define(version: 20170315174634) do
     t.text "description_html"
     t.integer "time_estimate"
     t.integer "relative_position"
+    t.datetime "closed_at"
   end
 
   add_index "issues", ["assignee_id"], name: "index_issues_on_assignee_id", using: :btree

spec/lib/gitlab/import_export/safe_model_attributes.yml

@@ -15,6 +15,7 @@ Issue:
 - updated_by_id
 - confidential
 - deleted_at
+- closed_at
 - due_date
 - moved_to_id
 - lock_version

spec/models/issue_spec.rb

@@ -37,6 +37,30 @@ describe Issue, models: true do
     end
   end
 
+  describe '#closed_at' do
+    after do
+      Timecop.return
+    end
+
+    let!(:now) { Timecop.freeze(Time.now) }
+
+    it 'sets closed_at to Time.now when issue is closed' do
+      issue = create(:issue, state: 'opened')
+
+      issue.close
+
+      expect(issue.closed_at).to eq(now)
+    end
+
+    it 'sets closed_at to nil when issue is reopened' do
+      issue = create(:issue, state: 'closed')
+
+      issue.reopen
+
+      expect(issue.closed_at).to be_nil
+    end
+  end
+
   describe '#to_reference' do
     let(:namespace) { build(:namespace, path: 'sample-namespace') }
     let(:project)   { build(:empty_project, name: 'sample-project', namespace: namespace) }