1 files changed, 17 insertions, 7 deletions
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index e83b11f7668..ac08dc0ee1f 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -162,7 +162,7 @@ class MergeRequest < ActiveRecord::Base
     target = unscoped.where(target_project_id: relation).select(:id)
     union  = Gitlab::SQL::Union.new([source, target])
 
-    where("merge_requests.id IN (#{union.to_sql})")
+    where("merge_requests.id IN (#{union.to_sql})") # rubocop:disable GitlabSecurity/SqlInjection
   end
 
   WIP_REGEX = /\A\s*(\[WIP\]\s*|WIP:\s*|WIP\s+)+\s*/i.freeze
@@ -443,7 +443,8 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def reload_diff_if_branch_changed
-    if source_branch_changed? || target_branch_changed?
+    if (source_branch_changed? || target_branch_changed?) &&
+        (source_branch_head && target_branch_head)
       reload_diff
     end
   end
@@ -792,11 +793,7 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def fetch_ref
-    target_project.repository.fetch_ref(
-      source_project.repository.path_to_repo,
-      "refs/heads/#{source_branch}",
-      ref_path
-    )
+    write_ref
     update_column(:ref_fetched, true)
   end
 
@@ -939,4 +936,17 @@ class MergeRequest < ActiveRecord::Base
 
     true
   end
+
+  private
+
+  def write_ref
+    target_project.repository.with_repo_branch_commit(
+      source_project.repository, source_branch) do |commit|
+        if commit
+          target_project.repository.write_ref(ref_path, commit.sha)
+        else
+          raise Rugged::ReferenceError, 'source repository is empty'
+        end
+      end
+  end
 end