Extend MergeToRefService for creating merge ref from the other ref

Currently, MergeToRefService is specifically designed for
createing merge commits from source branch and target branch of
merge reqeusts. We extend this behavior to source branch and any
target ref paths.

2 files changed, 15 insertions, 5 deletions

diff --git a/app/models/repository.rb b/app/models/repository.rb
index d087a5a7bbd..a408db7ebbe 100644
--- a/app/models/repository.rb
+++ b/app/models/repository.rb
@@ -839,10 +839,10 @@ class Repository
     end
   end
 
-  def merge_to_ref(user, source_sha, merge_request, target_ref, message)
+  def merge_to_ref(user, source_sha, merge_request, target_ref, message, first_parent_ref)
     branch = merge_request.target_branch
 
-    raw.merge_to_ref(user, source_sha, branch, target_ref, message)
+    raw.merge_to_ref(user, source_sha, branch, target_ref, message, first_parent_ref)
   end
 
   def ff_merge(user, source, target_branch, merge_request: nil)
diff --git a/app/services/merge_requests/merge_to_ref_service.rb b/app/services/merge_requests/merge_to_ref_service.rb
index efe4dcd6255..0ea50a5dbf5 100644
--- a/app/services/merge_requests/merge_to_ref_service.rb
+++ b/app/services/merge_requests/merge_to_ref_service.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module MergeRequests
-  # Performs the merge between source SHA and the target branch. Instead
+  # Performs the merge between source SHA and the target branch or the specified first parent ref. Instead
   # of writing the result to the MR target branch, it targets the `target_ref`.
   #
   # Ideally this should leave the `target_ref` state with the same state the
@@ -56,12 +56,22 @@ module MergeRequests
       raise_error(error) if error
     end
 
+    ##
+    # The parameter `target_ref` is where the merge result will be written.
+    # Default is the merge ref i.e. `refs/merge-requests/:iid/merge`.
     def target_ref
-      merge_request.merge_ref_path
+      params[:target_ref] || merge_request.merge_ref_path
+    end
+
+    ##
+    # The parameter `first_parent_ref` is the main line of the merge commit.
+    # Default is the target branch ref of the merge request.
+    def first_parent_ref
+      params[:first_parent_ref] || merge_request.target_branch_ref
     end
 
     def commit
-      repository.merge_to_ref(current_user, source, merge_request, target_ref, commit_message)
+      repository.merge_to_ref(current_user, source, merge_request, target_ref, commit_message, first_parent_ref)
     rescue Gitlab::Git::PreReceiveError => error
       raise MergeError, error.message
     end