Add labels to issues update mutation

-  Ability to add and remove labels from an
issue using  GraphQL

app/graphql/mutations/issues/update.rb

@@ -31,6 +31,16 @@ module Mutations
                required: false,
                description: copy_field_description(Types::IssueType, :discussion_locked)
 
+      argument :add_label_ids,
+               [GraphQL::ID_TYPE],
+               required: false,
+               description: 'The IDs of labels to be added to the issue.'
+
+      argument :remove_label_ids,
+               [GraphQL::ID_TYPE],
+               required: false,
+               description: 'The IDs of labels to be removed from the issue.'
+
       def resolve(project_path:, iid:, **args)
         issue = authorized_find!(project_path: project_path, iid: iid)
         project = issue.project

changelogs/unreleased/208782-graphql-api-mutation-for-changing-labels-of-an-issue.yml

+---
+title: Allow user to update issue labels via GraphQL
+merge_request: 37728
+author:
+type: added

doc/api/graphql/reference/gitlab_schema.graphql

@@ -14128,6 +14128,11 @@ type UpdateImageDiffNotePayload {
 Autogenerated input type of UpdateIssue
 """
 input UpdateIssueInput {
+  """
+  The IDs of labels to be added to the issue.
+  """
+  addLabelIds: [ID!]
+
   """
   A unique identifier for the client performing the mutation.
   """
@@ -14168,6 +14173,11 @@ input UpdateIssueInput {
   """
   projectPath: ID!
 
+  """
+  The IDs of labels to be removed from the issue.
+  """
+  removeLabelIds: [ID!]
+
   """
   Title of the issue
   """

doc/api/graphql/reference/gitlab_schema.json

@@ -41855,6 +41855,42 @@
               },
               "defaultValue": null
             },
+            {
+              "name": "addLabelIds",
+              "description": "The IDs of labels to be added to the issue.",
+              "type": {
+                "kind": "LIST",
+                "name": null,
+                "ofType": {
+                  "kind": "NON_NULL",
+                  "name": null,
+                  "ofType": {
+                    "kind": "SCALAR",
+                    "name": "ID",
+                    "ofType": null
+                  }
+                }
+              },
+              "defaultValue": null
+            },
+            {
+              "name": "removeLabelIds",
+              "description": "The IDs of labels to be removed from the issue.",
+              "type": {
+                "kind": "LIST",
+                "name": null,
+                "ofType": {
+                  "kind": "NON_NULL",
+                  "name": null,
+                  "ofType": {
+                    "kind": "SCALAR",
+                    "name": "ID",
+                    "ofType": null
+                  }
+                }
+              },
+              "defaultValue": null
+            },
             {
               "name": "healthStatus",
               "description": "The desired health status",

spec/graphql/mutations/issues/update_spec.rb

@@ -3,8 +3,11 @@
 require 'spec_helper'
 
 RSpec.describe Mutations::Issues::Update do
-  let_it_be(:issue) { create(:issue) }
+  let_it_be(:project) { create(:project) }
   let_it_be(:user) { create(:user) }
+  let_it_be(:project_label) { create(:label, project: project) }
+  let_it_be(:issue) { create(:issue, project: project, labels: [project_label]) }
+
   let(:expected_attributes) do
     {
       title: 'new title',
@@ -22,7 +25,7 @@ RSpec.describe Mutations::Issues::Update do
   describe '#resolve' do
     let(:mutation_params) do
       {
-        project_path: issue.project.full_path,
+        project_path: project.full_path,
         iid: issue.iid
       }.merge(expected_attributes)
     end
@@ -37,7 +40,7 @@ RSpec.describe Mutations::Issues::Update do
 
     context 'when the user can update the issue' do
       before do
-        issue.project.add_developer(user)
+        project.add_developer(user)
       end
 
       it 'updates issue with correct values' do
@@ -53,6 +56,52 @@ RSpec.describe Mutations::Issues::Update do
           expect { subject }.to raise_error(Gitlab::Graphql::Errors::ResourceNotAvailable)
         end
       end
+
+      context 'when changing labels' do
+        let_it_be(:label_1) { create(:label, project: project) }
+        let_it_be(:label_2) { create(:label, project: project) }
+        let_it_be(:external_label) { create(:label, project: create(:project)) }
+
+        it 'adds and removes labels correctly' do
+          mutation_params[:add_label_ids] = [label_1.id, label_2.id]
+          mutation_params[:remove_label_ids] = [project_label.id]
+
+          subject
+
+          expect(issue.reload.labels).to match_array([label_1, label_2])
+        end
+
+        it 'does not add label if label id is nil' do
+          mutation_params[:add_label_ids] = [nil, label_2.id]
+
+          subject
+
+          expect(issue.reload.labels).to match_array([project_label, label_2])
+        end
+
+        it 'does not add label if label is not found' do
+          mutation_params[:add_label_ids] = [external_label.id, label_2.id]
+
+          subject
+
+          expect(issue.reload.labels).to match_array([project_label, label_2])
+        end
+
+        it 'does not modify labels if label is already present' do
+          mutation_params[:add_label_ids] = [project_label.id]
+
+          expect(issue.reload.labels).to match_array([project_label])
+        end
+
+        it 'does not modify labels if label is addded and removed in the same request' do
+          mutation_params[:add_label_ids] = [label_1.id, label_2.id]
+          mutation_params[:remove_label_ids] = [label_1.id]
+
+          subject
+
+          expect(issue.reload.labels).to match_array([project_label, label_2])
+        end
+      end
     end
   end
 end