Use latest_merge_request_diff associationuse-merge-requests-diff-id-column

Compared to the merge_request_diff association:

1. It's simpler to query. The query uses a foreign key to the
   merge_request_diffs table, so no ordering is necessary.
2. It's faster for preloading. The merge_request_diff association has to load
   every diff for the MRs in the set, then discard all but the most recent for
   each. This association means that Rails can just query for N diffs from N
   MRs.
3. It's more complicated to update. This is a bidirectional foreign key, so we
   need to update two tables when adding a diff record. This also means we need
   to handle this as a special case when importing a GitLab project.

There is some juggling with this association in the merge request model:

* `MergeRequest#latest_merge_request_diff` is _always_ the latest diff.
* `MergeRequest#merge_request_diff` reuses
  `MergeRequest#latest_merge_request_diff` unless:
    * Arguments are passed. These are typically to force-reload the association.
    * It doesn't exist. That means we might be trying to implicitly create a
      diff. This only seems to happen in specs.
    * The association is already loaded. This is important for the reasons
      explained in the comment, which I'll reiterate here: if we a) load a
      non-latest diff, then b) get its `merge_request`, then c) get that MR's
      `merge_request_diff`, we should get the diff we loaded in c), even though
      that's not the latest diff.

Basically, `MergeRequest#merge_request_diff` is the latest diff in most cases,
but not quite all.

2 files changed, 7 insertions, 1 deletions

diff --git a/lib/api/merge_requests.rb b/lib/api/merge_requests.rb
index 726f09e3669..5b4642a2f57 100644
--- a/lib/api/merge_requests.rb
+++ b/lib/api/merge_requests.rb
@@ -21,7 +21,7 @@ module API
         return merge_requests if args[:view] == 'simple'
 
         merge_requests
-          .preload(:notes, :author, :assignee, :milestone, :merge_request_diff, :labels, :timelogs)
+          .preload(:notes, :author, :assignee, :milestone, :latest_merge_request_diff, :labels, :timelogs)
       end
 
       params :merge_requests_params do
diff --git a/lib/gitlab/import_export/project_tree_restorer.rb b/lib/gitlab/import_export/project_tree_restorer.rb
index 639f4f0c3f0..94bd71f2563 100644
--- a/lib/gitlab/import_export/project_tree_restorer.rb
+++ b/lib/gitlab/import_export/project_tree_restorer.rb
@@ -60,9 +60,15 @@ module Gitlab
           end
         end
 
+        restore_latest_merge_request_diff_ids
+
         @saved
       end
 
+      def restore_latest_merge_request_diff_ids
+        MergeRequest.set_latest_merge_request_diff_ids(@project.merge_requests)
+      end
+
       def save_relation_hash(relation_hash_batch, relation_key)
         relation_hash = create_relation(relation_key, relation_hash_batch)