Use latest_merge_request_diff association

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.

1 files changed, 4 insertions, 1 deletions

diff --git a/app/models/merge_request_diff.rb b/app/models/merge_request_diff.rb
index 5382f5cc627..9ee561b5161 100644
--- a/app/models/merge_request_diff.rb
+++ b/app/models/merge_request_diff.rb
@@ -2,6 +2,7 @@ class MergeRequestDiff < ActiveRecord::Base
   include Sortable
   include Importable
   include Gitlab::EncodingHelper
+  include ManualInverseAssociation
 
   # Prevent store of diff if commits amount more then 500
   COMMITS_SAFE_SIZE = 100
@@ -10,6 +11,8 @@ class MergeRequestDiff < ActiveRecord::Base
   VALID_CLASSES = [Hash, Rugged::Patch, Rugged::Diff::Delta].freeze
 
   belongs_to :merge_request
+  manual_inverse_association :merge_request, :merge_request_diff
+
   has_many :merge_request_diff_files, -> { order(:merge_request_diff_id, :relative_order) }
   has_many :merge_request_diff_commits, -> { order(:merge_request_diff_id, :relative_order) }
 
@@ -194,7 +197,7 @@ class MergeRequestDiff < ActiveRecord::Base
   end
 
   def latest?
-    self == merge_request.merge_request_diff
+    self.id == merge_request.latest_merge_request_diff_id
   end
 
   def compare_with(sha)