Fix parsing epic_id id type param

ee/app/graphql/ee/mutations/issues/update.rb

@@ -16,6 +16,12 @@ module EE
                    required: false,
                    description: 'The ID of the parent epic. NULL when removing the association'
         end
+
+        def resolve(project_path:, iid:, **args)
+          args[:epic_id] = ::GitlabSchema.parse_gid(args[:epic_id], expected_type: ::Epic).model_id if args[:epic_id]
+
+          super
+        end
       end
     end
   end

ee/spec/graphql/mutations/issues/update_spec.rb

@@ -15,7 +15,7 @@ RSpec.describe Mutations::Issues::Update do
     let_it_be(:issue) { create(:issue, project: project) }
     let_it_be(:epic) { create(:epic, group: group) }
 
-    let(:epic_id) { epic.id }
+    let(:epic_id) { epic.to_global_id.to_s }
     let(:params) { { project_path: project.full_path, iid: issue.iid, epic_id: epic_id } }
     let(:mutated_issue) { subject[:issue] }
     let(:mutation) { described_class.new(object: nil, context: { current_user: user }, field: nil) }
@@ -48,14 +48,26 @@ RSpec.describe Mutations::Issues::Update do
 
         context 'when a valid epic is given' do
           it 'updates the epic' do
+            expect { subject }.to change { issue.reload.epic }.from(nil).to(epic)
+          end
+
+          it 'returns the updated issue' do
             expect(mutated_issue.epic).to eq(epic)
           end
         end
 
         context 'when nil epic is given' do
+          before do
+            issue.update!(epic: epic)
+          end
+
           let(:epic_id) { nil }
 
           it 'set the epic to nil' do
+            expect { subject }.to change { issue.reload.epic }.from(epic).to(nil)
+          end
+
+          it 'returns the updated issue' do
             expect(mutated_issue.epic).to be_nil
           end
         end