Merge branch 'fix-pipeline-for-empty-merge-request-diff' into 'master'

Fix pipeline error when trying to read empty merge request diff

When a user pushed something which resulted an empty merge request diff, `st_commits` would be `nil`. Therefore we also need to check if there exists `st_commits`.

We could tell this from:

``` ruby
def commits
  @commits ||= load_commits(st_commits || [])
end
```

and

``` ruby
def save_commits
  new_attributes = {}

  commits = compare.commits

  if commits.present?
    commits = Commit.decorate(commits, merge_request.source_project).reverse
    new_attributes[:st_commits] = dump_commits(commits)
  end

  update_columns_serialized(new_attributes)
end
```

Closes #22438

See merge request !6470

1 files changed, 5 insertions, 1 deletions

diff --git a/app/models/merge_request_diff.rb b/app/models/merge_request_diff.rb
index 7362886e9f5..36b8b70870b 100644
--- a/app/models/merge_request_diff.rb
+++ b/app/models/merge_request_diff.rb
@@ -30,6 +30,10 @@ class MergeRequestDiff < ActiveRecord::Base
     select(column_names - ['st_diffs'])
   end
 
+  def st_commits
+    super || []
+  end
+
   # Collect information about commits and diff from repository
   # and save it to the database as serialized data
   def save_git_content
@@ -83,7 +87,7 @@ class MergeRequestDiff < ActiveRecord::Base
   end
 
   def commits
-    @commits ||= load_commits(st_commits || [])
+    @commits ||= load_commits(st_commits)
   end
 
   def reload_commits