Add divergedFromTargetBranch to MergeRequestType

Update docs and schema
Add tests

app/graphql/types/merge_request_type.rb

@@ -108,6 +108,10 @@ class MergeRequestType < BaseObject
           null: false, calls_gitaly: true,
           method: :target_branch_exists?,
           description: 'Indicates if the target branch of the merge request exists.'
+    field :diverged_from_target_branch, GraphQL::BOOLEAN_TYPE,
+          null: false, calls_gitaly: true,
+          method: :diverged_from_target_branch?,
+          description: 'Indicates if the source branch is behind the target branch.'
     field :mergeable_discussions_state, GraphQL::BOOLEAN_TYPE, null: true,
           description: 'Indicates if all discussions in the merge request have been resolved, allowing the merge request to be merged.'
     field :web_url, GraphQL::STRING_TYPE, null: true,

changelogs/unreleased/275818-be-add-info-error-messages-to-security-widget-summary-diverged.yml

+---
+title: Add divergedFromTargetBranch field to MergeRequestType to indicate the target
+  branch has diverged from the source branch
+merge_request: 53759
+author:
+type: changed

doc/api/graphql/reference/index.md

@@ -2473,6 +2473,7 @@ Autogenerated return type of MarkAsSpamSnippet.
 | `diffStatsSummary` | DiffStatsSummary | Summary of which files were changed in this merge request. |
 | `discussionLocked` | Boolean! | Indicates if comments on the merge request are locked to members only. |
 | `discussions` | DiscussionConnection! | All discussions on this noteable. |
+| `divergedFromTargetBranch` | Boolean! | Indicates if the source branch is behind the target branch. |
 | `downvotes` | Int! | Number of downvotes for the merge request. |
 | `forceRemoveSourceBranch` | Boolean | Indicates if the project settings will lead to source branch deletion after merge. |
 | `hasCi` | Boolean! | Indicates if the merge request has CI. |

spec/graphql/types/merge_request_type_spec.rb

@@ -23,7 +23,7 @@
       merge_error allow_collaboration should_be_rebased rebase_commit_sha
       rebase_in_progress default_merge_commit_message
       merge_ongoing mergeable_discussions_state web_url
-      source_branch_exists target_branch_exists
+      source_branch_exists target_branch_exists diverged_from_target_branch
       upvotes downvotes head_pipeline pipelines task_completion_status
       milestone assignees reviewers participants subscribed labels discussion_locked time_estimate
       total_time_spent reference author merged_at commit_count current_user_todos
@@ -77,4 +77,33 @@
       end
     end
   end
+
+  describe '#diverged_from_target_branch' do
+    subject(:execute_query) { GitlabSchema.execute(query, context: { current_user: current_user }).as_json }
+
+    let!(:merge_request) { create(:merge_request, target_project: project, source_project: project) }
+    let(:project) { create(:project, :public) }
+    let(:current_user) { create :admin }
+    let(:query) do
+      %(
+        {
+          project(fullPath: "#{project.full_path}") {
+            mergeRequests {
+              nodes {
+                divergedFromTargetBranch
+              }
+            }
+          }
+        }
+      )
+    end
+
+    it 'delegates the diverged_from_target_branch? call to the merge request entity' do
+      expect_next_found_instance_of(MergeRequest) do |instance|
+        expect(instance).to receive(:diverged_from_target_branch?)
+      end
+
+      execute_query
+    end
+  end
 end