Fix pipelines for merge requests in project exports

Previously an import for a pipeline for merge request would fail because
the `project.json` did not export the `merge_request` hash in the tree.
The pipeline would fail to validate with, "Validation failed: Merge
request can't be blank".

Unlike https://gitlab.com/gitlab-org/gitlab/merge_requests/17799, this
change won't make existing exports importable, but it will properly
establish a relationship between a pipeline and a merge request.

Relates to https://gitlab.com/gitlab-org/gitlab/issues/31414

changelogs/unreleased/sh-fix-project-export-for-pipelines-for-mrs-mark2.yml

+---
+title: Fix pipelines for merge requests in project exports
+merge_request: 17844
+author:
+type: fixed

lib/gitlab/import_export/import_export.yml

@@ -66,6 +66,7 @@ tree:
       - stages:
         - :statuses
       - :external_pull_request
+      - :merge_request
     - :external_pull_requests
     - :auto_devops
     - :triggers
@@ -168,6 +169,12 @@ excluded_attributes:
     - :external_diff_size
   issues:
     - :milestone_id
+  merge_request:
+    - :milestone_id
+    - :ref_fetched
+    - :merge_jid
+    - :rebase_jid
+    - :latest_merge_request_diff_id
   merge_requests:
     - :milestone_id
     - :ref_fetched

lib/gitlab/import_export/relation_factory.rb

@@ -38,7 +38,7 @@ module Gitlab
 
       IMPORTED_OBJECT_MAX_RETRIES = 5.freeze
 
-      EXISTING_OBJECT_CHECK = %i[milestone milestones label labels project_label project_labels group_label group_labels project_feature].freeze
+      EXISTING_OBJECT_CHECK = %i[milestone milestones label labels project_label project_labels group_label group_labels project_feature merge_request].freeze
 
       TOKEN_RESET_MODELS = %i[Project Namespace Ci::Trigger Ci::Build Ci::Runner ProjectHook].freeze
 
@@ -293,6 +293,7 @@ module Gitlab
 
       def find_or_create_object!
         return relation_class.find_or_create_by(project_id: @project.id) if @relation_name == :project_feature
+        return find_or_create_merge_request! if @relation_name == :merge_request
 
         # Can't use IDs as validation exists calling `group` or `project` attributes
         finder_hash = parsed_relation_hash.tap do |hash|
@@ -303,6 +304,11 @@ module Gitlab
 
         GroupProjectObjectBuilder.build(relation_class, finder_hash)
       end
+
+      def find_or_create_merge_request!
+        @project.merge_requests.find_by(iid: parsed_relation_hash['iid']) ||
+          relation_class.new(parsed_relation_hash)
+      end
     end
   end
 end

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

@@ -6161,7 +6161,7 @@
     {
       "id": 36,
       "project_id": 5,
-      "ref": "master",
+      "ref": null,
       "sha": "sha-notes",
       "before_sha": null,
       "push_data": null,
@@ -6289,19 +6289,21 @@
     {
       "id": 37,
       "project_id": 5,
-      "ref": null,
+      "ref": "master",
       "sha": "048721d90c449b244b7b4c53a9186b04330174ec",
       "before_sha": null,
       "push_data": null,
       "created_at": "2016-03-22T15:20:35.757Z",
       "updated_at": "2016-03-22T15:20:35.757Z",
-      "tag": null,
+      "tag": false,
       "yaml_errors": null,
       "committed_at": null,
       "status": "failed",
       "started_at": null,
       "finished_at": null,
       "duration": null,
+      "source": "merge_request_event",
+      "merge_request_id": 27,
       "stages": [
         {
           "id": 21,
@@ -6372,7 +6374,32 @@
             }
           ]
         }
-      ]
+      ],
+      "merge_request": {
+        "id": 27,
+        "target_branch": "feature",
+        "source_branch": "feature_conflict",
+        "source_project_id": 999,
+        "author_id": 1,
+        "assignee_id": null,
+        "title": "MR1",
+        "created_at": "2016-06-14T15:02:36.568Z",
+        "updated_at": "2016-06-14T15:02:56.815Z",
+        "state": "opened",
+        "merge_status": "unchecked",
+        "target_project_id": 5,
+        "iid": 9,
+        "description": null,
+        "position": 0,
+        "updated_by_id": null,
+        "merge_error": null,
+        "diff_head_sha": "HEAD",
+        "source_branch_sha": "ABCD",
+        "target_branch_sha": "DCBA",
+        "merge_params": {
+          "force_remove_source_branch": null
+        }
+      }
     },
     {
       "id": 38,

spec/lib/gitlab/import_export/model_configuration_spec.rb

@@ -12,9 +12,9 @@ describe 'Import/Export model configuration' do
 
     # Remove duplicated or add missing models
     # - project is not part of the tree, so it has to be added manually.
-    # - milestone, labels have both singular and plural versions in the tree, so remove the duplicates.
+    # - milestone, labels, merge_request have both singular and plural versions in the tree, so remove the duplicates.
     # - User, Author... Models we do not care about for checking models
-    names.flatten.uniq - %w(milestones labels user author) + ['project']
+    names.flatten.uniq - %w(milestones labels user author merge_request) + ['project']
   end
   let(:all_models_yml) { 'spec/lib/gitlab/import_export/all_models.yml' }
   let(:all_models_hash) { YAML.load_file(all_models_yml) }