Auto-assign merge request author when creating from issue

When creating a merge request from an issue via "Create merge
request" button, whoever clicked it will be the author of
the merge request.

Additional to that, we are now auto-assigning the merge request
to the author. This is to make it more convenient for the
author to assign themselves.

app/services/merge_requests/create_from_issue_service.rb

@@ -85,7 +85,8 @@ module MergeRequests
         source_project_id: target_project.id,
         source_branch: branch_name,
         target_project_id: target_project.id,
-        target_branch: target_branch
+        target_branch: target_branch,
+        assignee_ids: [current_user.id]
       }
     end
 

changelogs/unreleased/287636-auto-assign-author.yml

+---
+title: Auto-assign merge request author when creating from issue
+merge_request: 50263
+author:
+type: added

spec/services/merge_requests/create_from_issue_service_spec.rb

@@ -66,10 +66,11 @@ RSpec.describe MergeRequests::CreateFromIssueService do
         expect { service.execute }.to change(target_project.merge_requests, :count).by(1)
       end
 
-      it 'sets the merge request author to current user', :sidekiq_might_not_need_inline do
+      it 'sets the merge request author to current user and assigns them', :sidekiq_might_not_need_inline do
         result = service.execute
 
         expect(result[:merge_request].author).to eq(user)
+        expect(result[:merge_request].assignees).to eq([user])
       end
 
       it 'sets the merge request source branch to the new issue branch', :sidekiq_might_not_need_inline do