Fix N+1 in `MergeRequest#merge_request_diff_for`reduce-query-count-for-mergerequestscontroller-show

Previously, this would issue a query for each unique `diff_refs_or_sha`
passed. This was because we didn't want to load other MR diffs into memory, as
they had some very large columns.

Now they are actually very small, and it's more efficient to just load them all
at once and do the finding in Ruby.

1 files changed, 17 insertions, 10 deletions

diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 7e6d89ec9c7..91d8be5559b 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -536,18 +536,25 @@ class MergeRequest < ActiveRecord::Base
     merge_request_diff(true)
   end
 
+  def viewable_diffs
+    @viewable_diffs ||= merge_request_diffs.viewable.to_a
+  end
+
   def merge_request_diff_for(diff_refs_or_sha)
-    @merge_request_diffs_by_diff_refs_or_sha ||= Hash.new do |h, diff_refs_or_sha|
-      diffs = merge_request_diffs.viewable
-      h[diff_refs_or_sha] =
-        if diff_refs_or_sha.is_a?(Gitlab::Diff::DiffRefs)
-          diffs.find_by_diff_refs(diff_refs_or_sha)
-        else
-          diffs.find_by(head_commit_sha: diff_refs_or_sha)
-        end
-    end
+    matcher =
+      if diff_refs_or_sha.is_a?(Gitlab::Diff::DiffRefs)
+        {
+          'start_commit_sha' => diff_refs_or_sha.start_sha,
+          'head_commit_sha' => diff_refs_or_sha.head_sha,
+          'base_commit_sha' => diff_refs_or_sha.base_sha
+        }
+      else
+        { 'head_commit_sha' => diff_refs_or_sha }
+      end
 
-    @merge_request_diffs_by_diff_refs_or_sha[diff_refs_or_sha]
+    viewable_diffs.find do |diff|
+      diff.attributes.slice(*matcher.keys) == matcher
+    end
   end
 
   def version_params_for(diff_refs)