Fix issues missing on epic's page after project import

- This MR fixes issues being missing on Epic page
  after project import. The issues were associated with an epic
  correctly, but were not shown on Epic page, because
  epic_issue relation had relative_position: nil.
- Now epic_issue is exported, that contains epic, which will
  preserve relative_position, which is required in order
  for the issue to be shown on Epic's page.

changelogs/unreleased/georgekoltsov-fix-epic-issues.yml

+---
+title: Fix issues missing on epic's page after project import
+merge_request: 26099
+author:
+type: fixed

ee/spec/lib/ee/gitlab/import_export/project/tree_restorer_spec.rb

@@ -18,6 +18,7 @@ describe Gitlab::ImportExport::Project::TreeRestorer do
     end
 
     context 'with group' do
+      let(:issue) { project.issues.find_by_title('Issue with Epic') }
       let!(:project) do
         create(:project,
                :builds_disabled,
@@ -35,7 +36,8 @@ describe Gitlab::ImportExport::Project::TreeRestorer do
 
           expect { restored_project_json }.not_to change { Epic.count }
           expect(project.group.epics.count).to eq(1)
-          expect(project.issues.find_by_title('Issue with Epic').epic).not_to be_nil
+          expect(issue.epic).to eq(epic)
+          expect(issue.epic_issue.relative_position).not_to be_nil
         end
       end
 
@@ -45,7 +47,9 @@ describe Gitlab::ImportExport::Project::TreeRestorer do
 
           expect { restored_project_json }.to change { Epic.count }.from(0).to(1)
           expect(project.group.epics.count).to eq(1)
-          expect(project.issues.find_by_title('Issue with Epic').epic).not_to be_nil
+
+          expect(issue.epic).not_to be_nil
+          expect(issue.epic_issue.relative_position).not_to be_nil
         end
       end
     end

ee/spec/lib/ee/gitlab/import_export/project/tree_saver_spec.rb

@@ -60,12 +60,21 @@ describe Gitlab::ImportExport::Project::TreeSaver do
     end
 
     context 'epics' do
-      it 'has issue epic' do
-        expect(saved_project_json['issues'].first['epic']).not_to be_empty
+      it 'has epic_issue' do
+        expect(saved_project_json['issues'].first['epic_issue']).not_to be_empty
+        expect(saved_project_json['issues'].first['epic_issue']['id']).to eql(epic_issue.id)
       end
 
-      it 'has issue epic id' do
-        expect(saved_project_json['issues'].first['epic']['id']).to eql(epic.id)
+      it 'has epic' do
+        expect(saved_project_json['issues'].first['epic_issue']['epic']['title']).to eql(epic.title)
+      end
+
+      it 'does not have epic_id' do
+        expect(saved_project_json['issues'].first['epic_issue']['epic_id']).to be_nil
+      end
+
+      it 'does not have issue_id' do
+        expect(saved_project_json['issues'].first['epic_issue']['issue_id']).to be_nil
       end
     end
   end

lib/gitlab/import_export/project/import_export.yml

@@ -318,6 +318,9 @@ excluded_attributes:
     - :state_id
     - :start_date_sourcing_epic_id
     - :due_date_sourcing_epic_id
+  epic_issue:
+    - :epic_id
+    - :issue_id
 methods:
   notes:
     - :type
@@ -370,7 +373,8 @@ ee:
         - design_versions:
           - actions:
             - :design # Duplicate export of issues.designs in order to link the record to both Issue and Action
-        - :epic
+        - epic_issue:
+          - :epic
       - protected_branches:
         - :unprotect_access_levels
       - protected_environments:

spec/fixtures/lib/gitlab/import_export/group/project.json

@@ -205,36 +205,40 @@
         "iid": 1,
         "group_id": 100
       },
-      "epic": {
-        "id": 1,
-        "group_id": 5,
-        "author_id": 1,
-        "assignee_id": null,
-        "iid": 1,
-        "updated_by_id": null,
-        "last_edited_by_id": null,
-        "lock_version": 0,
-        "start_date": null,
-        "end_date": null,
-        "last_edited_at": null,
-        "created_at": "2019-12-08T19:37:07.098Z",
-        "updated_at": "2019-12-08T19:43:11.568Z",
-        "title": "An epic",
-        "description": null,
-        "start_date_sourcing_milestone_id": null,
-        "due_date_sourcing_milestone_id": null,
-        "start_date_fixed": null,
-        "due_date_fixed": null,
-        "start_date_is_fixed": null,
-        "due_date_is_fixed": null,
-        "closed_by_id": null,
-        "closed_at": null,
-        "parent_id": null,
-        "relative_position": null,
-        "state_id": "opened",
-        "start_date_sourcing_epic_id": null,
-        "due_date_sourcing_epic_id": null,
-        "milestone_id": null
+      "epic_issue": {
+        "id": 78,
+        "relative_position": 1073740323,
+        "epic": {
+          "id": 1,
+          "group_id": 5,
+          "author_id": 1,
+          "assignee_id": null,
+          "iid": 1,
+          "updated_by_id": null,
+          "last_edited_by_id": null,
+          "lock_version": 0,
+          "start_date": null,
+          "end_date": null,
+          "last_edited_at": null,
+          "created_at": "2019-12-08T19:37:07.098Z",
+          "updated_at": "2019-12-08T19:43:11.568Z",
+          "title": "An epic",
+          "description": null,
+          "start_date_sourcing_milestone_id": null,
+          "due_date_sourcing_milestone_id": null,
+          "start_date_fixed": null,
+          "due_date_fixed": null,
+          "start_date_is_fixed": null,
+          "due_date_is_fixed": null,
+          "closed_by_id": null,
+          "closed_at": null,
+          "parent_id": null,
+          "relative_position": null,
+          "state_id": "opened",
+          "start_date_sourcing_epic_id": null,
+          "due_date_sourcing_epic_id": null,
+          "milestone_id": null
+        }
       }
     }
   ],

spec/lib/gitlab/import_export/all_models.yml

@@ -606,3 +606,6 @@ epic:
 - events
 - resource_label_events
 - user_mentions
+epic_issue:
+- epic
+- issue

spec/lib/gitlab/import_export/safe_model_attributes.yml

@@ -832,3 +832,6 @@ Epic:
   - start_date_sourcing_epic_id
   - due_date_sourcing_epic_id
   - health_status
+EpicIssue:
+  - id
+  - relative_position