Cache MR diffs by diff refs

app/models/merge_request.rb

@@ -368,7 +368,11 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def merge_request_diff_for(diff_refs)
-    merge_request_diffs.viewable.select_without_diff.with_diff_refs(diff_refs).take
+    @merge_request_diffs_by_diff_refs ||= Hash.new do |h, diff_refs|
+      h[diff_refs] = merge_request_diffs.viewable.select_without_diff.with_diff_refs(diff_refs).take
+    end
+
+    @merge_request_diffs_by_diff_refs[diff_refs]
   end
 
   def reload_diff_if_branch_changed

lib/gitlab/diff/diff_refs.rb

@@ -18,6 +18,12 @@ module Gitlab
           head_sha == other.head_sha
       end
 
+      alias_method :eql?, :==
+
+      def hash
+        [base_sha, start_sha, head_sha].hash
+      end
+
       # There is only one case in which we will have `start_sha` and `head_sha`,
       # but not `base_sha`, which is when a diff is generated between an
       # orphaned branch and another branch, which means there _is_ no base, but