Fix epic assignment in the createIssue mutation

This was broken in a few ways, but most noteably by the change
in the method name in the FOSS class which is overridden in EE.

This change introduces fixes for this, and a wider range of tests
(missing regression tests were the reason this was not detected),
and marks the method as overriding the FOSS implementation, guaranteeing
that we will notice if a similar upstream change happens again.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/298840

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

@@ -5,6 +5,7 @@ module EE
     module Issues
       module Create
         extend ActiveSupport::Concern
+        extend ::Gitlab::Utils::Override
 
         prepended do
           include ::Mutations::Issues::CommonEEMutationArguments
@@ -14,10 +15,21 @@ module EE
                    description: 'The ID of an epic to associate the issue with.'
         end
 
+        override :resolve
+        def resolve(**args)
+          super
+        rescue ActiveRecord::RecordNotFound => e
+          { errors: [e.message], issue: nil }
+        end
+
         private
 
-        def create_issue_params(params)
-          params[:epic_id] &&= params[:epic_id]&.model_id
+        override :build_create_issue_params
+        def build_create_issue_params(params)
+          # TODO: remove this line when the compatibility layer is removed
+          # See: https://gitlab.com/gitlab-org/gitlab/-/issues/257883
+          params[:epic_id] = ::Types::GlobalIDType[::Epic].coerce_isolated_input(params[:epic_id]) if params[:epic_id]
+          params[:epic_id] = params[:epic_id]&.model_id if params.key?(:epic_id)
 
           super(params)
         end

ee/changelogs/unreleased/298840-ajk-fix-epic-id-processing.yml

+---
+title: Fix epic assignment in the createIssue mutation
+merge_request: 51817
+author:
+type: fixed

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

@@ -3,7 +3,10 @@
 require 'spec_helper'
 
 RSpec.describe Mutations::Issues::Create do
-  let_it_be(:project) { create(:project) }
+  include GraphqlHelpers
+
+  let_it_be(:group) { create(:group) }
+  let_it_be(:project) { create(:project, group: group) }
   let_it_be(:user) { create(:user) }
   let_it_be(:assignee1) { create(:user) }
   let_it_be(:assignee2) { create(:user) }
@@ -19,12 +22,16 @@ RSpec.describe Mutations::Issues::Create do
     }
   end
 
-  let(:mutation_params) do
+  let(:inputs) do
     {
       project_path: project.full_path,
       assignee_ids: [assignee1.to_global_id, assignee2.to_global_id],
       health_status: Issue.health_statuses[:at_risk]
-    }.merge(expected_attributes)
+    }
+  end
+
+  let(:mutation_params) do
+    inputs.merge(expected_attributes)
   end
 
   let(:mutation) { described_class.new(object: nil, context: { current_user: user }, field: nil) }
@@ -43,7 +50,7 @@ RSpec.describe Mutations::Issues::Create do
 
     context 'when user can create issues' do
       before do
-        project.add_developer(user)
+        group.add_developer(user)
       end
 
       it 'creates issue with correct EE values' do
@@ -51,6 +58,49 @@ RSpec.describe Mutations::Issues::Create do
         expect(mutated_issue.assignees.pluck(:id)).to eq([assignee1.id, assignee2.id])
         expect(mutated_issue.health_status).to eq('at_risk')
       end
+
+      context 'when the epic_id parameter is passed' do
+        let(:epic) { create(:epic, group: group) }
+
+        let(:mutation_params) do
+          inputs.merge(expected_attributes).merge(epic_id: epic.to_global_id)
+        end
+
+        context 'epics are available' do
+          before do
+            stub_licensed_features(epics: true)
+          end
+
+          it 'is successful' do
+            expect(subject[:errors]).to be_empty
+          end
+
+          it 'assigns the issue to the epic' do
+            expect(mutated_issue).to have_attributes(epic: epic)
+          end
+
+          context 'the project is not in a group' do
+            let(:project) { create(:project) }
+
+            it 'is successful, but it does not add the epic' do
+              project.add_developer(user)
+
+              expect(subject[:errors]).to be_empty
+              expect(mutated_issue).not_to have_attributes(epic: epic)
+            end
+          end
+        end
+
+        context 'epics are unavailable' do
+          it 'is unsuccessful' do
+            expect(subject[:errors]).to contain_exactly("Couldn't find Epic")
+          end
+
+          it 'does not create an issue' do
+            expect(mutated_issue).to be_nil
+          end
+        end
+      end
     end
   end
 end