Manage and display labels from epic graphql

- add labels to epic_type
- Add and remove labels from UpdateEpic mutation

changelogs/unreleased/31912-epic-labels.yml

+---
+title: Manage and display labels from epic in the GraphQL API
+merge_request: 19642
+author:
+type: added

doc/api/graphql/reference/gitlab_schema.graphql

@@ -216,6 +216,11 @@ type CreateDiffNotePayload {
 Autogenerated input type of CreateEpic
 """
 input CreateEpicInput {
+  """
+  The IDs of labels to be added to the epic.
+  """
+  addLabelIds: [ID!]
+
   """
   A unique identifier for the client performing the mutation.
   """
@@ -241,6 +246,11 @@ input CreateEpicInput {
   """
   groupPath: ID!
 
+  """
+  The IDs of labels to be removed from the epic.
+  """
+  removeLabelIds: [ID!]
+
   """
   The start date of the epic
   """
@@ -1171,6 +1181,31 @@ type Epic implements Noteable {
     last: Int
   ): EpicIssueConnection
 
+  """
+  Labels assigned to the epic
+  """
+  labels(
+    """
+    Returns the elements in the list that come after the specified cursor.
+    """
+    after: String
+
+    """
+    Returns the elements in the list that come before the specified cursor.
+    """
+    before: String
+
+    """
+    Returns the first _n_ elements from the list.
+    """
+    first: Int
+
+    """
+    Returns the last _n_ elements from the list.
+    """
+    last: Int
+  ): LabelConnection
+
   """
   All notes on this noteable
   """
@@ -5025,6 +5060,11 @@ type TreeEntryEdge {
 Autogenerated input type of UpdateEpic
 """
 input UpdateEpicInput {
+  """
+  The IDs of labels to be added to the epic.
+  """
+  addLabelIds: [ID!]
+
   """
   A unique identifier for the client performing the mutation.
   """
@@ -5055,6 +5095,11 @@ input UpdateEpicInput {
   """
   iid: String!
 
+  """
+  The IDs of labels to be removed from the epic.
+  """
+  removeLabelIds: [ID!]
+
   """
   The start date of the epic
   """

ee/app/graphql/mutations/concerns/mutations/shared_epic_arguments.rb

@@ -38,6 +38,14 @@ module Mutations
                 GraphQL::BOOLEAN_TYPE,
                 required: false,
                 description: 'Indicates end date should be sourced from due_date_fixed field not the issue milestones'
+      argument :add_label_ids,
+               [GraphQL::ID_TYPE],
+               required: false,
+               description: 'The IDs of labels to be added to the epic.'
+      argument :remove_label_ids,
+               [GraphQL::ID_TYPE],
+               required: false,
+               description: 'The IDs of labels to be removed from the epic.'
     end
 
     def validate_arguments!(args)

ee/app/graphql/types/epic_type.rb

@@ -46,6 +46,8 @@ module Types
           ::Types::EpicType.connection_type,
           null: true,
           resolver: ::Resolvers::EpicResolver
+    field :labels, Types::LabelType.connection_type, null: true,
+          description: 'Labels assigned to the epic'
 
     field :has_children, GraphQL::BOOLEAN_TYPE, null: false, method: :has_children? # rubocop:disable Graphql/Descriptions
     field :has_issues, GraphQL::BOOLEAN_TYPE, null: false, method: :has_issues? # rubocop:disable Graphql/Descriptions

ee/spec/graphql/types/epic_type_spec.rb

@@ -5,7 +5,7 @@ require 'spec_helper'
 describe GitlabSchema.types['Epic'] do
   let(:fields) do
     %i[
-      id iid title description state group parent author
+      id iid title description state group parent author labels
       start_date start_date_is_fixed start_date_fixed start_date_from_milestones
       due_date due_date_is_fixed due_date_fixed due_date_from_milestones
       closed_at created_at updated_at children has_children has_issues

ee/spec/requests/api/graphql/mutations/epics/update_spec.rb

@@ -7,7 +7,10 @@ describe 'Updating an Epic' do
 
   let_it_be(:current_user) { create(:user) }
   let_it_be(:group) { create(:group) }
-  let(:epic) { create(:epic, group: group, title: 'original title') }
+  let(:label_1) { create(:group_label, group: group) }
+  let(:label_2) { create(:group_label, group: group) }
+  let(:label_3) { create(:group_label, group: group) }
+  let(:epic) { create(:epic, group: group, title: 'original title', labels: [label_2]) }
 
   let(:attributes) do
     {
@@ -100,6 +103,15 @@ describe 'Updating an Epic' do
         end
       end
 
+      context 'when changing labels of the epic' do
+        let(:attributes) { { add_label_ids: [label_1.id, label_3.id], remove_label_ids: label_2.id } }
+        it 'adds and removes labels correctly' do
+          post_graphql_mutation(mutation, current_user: current_user)
+
+          expect(epic.reload.labels).to match_array([label_1, label_3])
+        end
+      end
+
       context 'when there are ActiveRecord validation errors' do
         let(:attributes) { { title: '' } }
 

spec/services/issues/update_service_spec.rb

@@ -606,6 +606,24 @@ describe Issues::UpdateService, :mailer do
         end
       end
 
+      context 'when same id is passed as add_label_ids and remove_label_ids' do
+        let(:params) { { add_label_ids: [label.id], remove_label_ids: [label.id] } }
+
+        context 'for a label assigned to an issue' do
+          it 'removes the label' do
+            issue.update(labels: [label])
+
+            expect(result.label_ids).to be_empty
+          end
+        end
+
+        context 'for a label not assigned to an issue' do
+          it 'does not add the label' do
+            expect(result.label_ids).to be_empty
+          end
+        end
+      end
+
       context 'when duplicate label titles are given' do
         let(:params) do
           { labels: [label3.title, label3.title] }