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.

5 files changed, 98 insertions, 3 deletions

diff --git a/spec/lib/gitlab/import_export/all_models.yml b/spec/lib/gitlab/import_export/all_models.yml
index bf1e97654e5..0ecb50f7110 100644
--- a/spec/lib/gitlab/import_export/all_models.yml
+++ b/spec/lib/gitlab/import_export/all_models.yml
@@ -88,6 +88,7 @@ merge_requests:
 - metrics
 - timelogs
 - head_pipeline
+- latest_merge_request_diff
 merge_request_diff:
 - merge_request
 - merge_request_diff_commits
diff --git a/spec/lib/gitlab/import_export/project_tree_restorer_spec.rb b/spec/lib/gitlab/import_export/project_tree_restorer_spec.rb
index e4b4cf5ba85..5f6db0b3dfd 100644
--- a/spec/lib/gitlab/import_export/project_tree_restorer_spec.rb
+++ b/spec/lib/gitlab/import_export/project_tree_restorer_spec.rb
@@ -117,6 +117,12 @@ describe Gitlab::ImportExport::ProjectTreeRestorer do
         expect(st_commits.first[:committed_date]).to be_kind_of(Time)
       end
 
+      it 'has the correct data for merge request latest_merge_request_diff' do
+        MergeRequest.find_each do |merge_request|
+          expect(merge_request.latest_merge_request_diff_id).to eq(merge_request.merge_request_diffs.maximum(:id))
+        end
+      end
+
       it 'has labels associated to label links, associated to issues' do
         expect(Label.first.label_links.first.target).not_to be_nil
       end
diff --git a/spec/models/concerns/manual_inverse_association_spec.rb b/spec/models/concerns/manual_inverse_association_spec.rb
new file mode 100644
index 00000000000..aad40883854
--- /dev/null
+++ b/spec/models/concerns/manual_inverse_association_spec.rb
@@ -0,0 +1,51 @@
+require 'spec_helper'
+
+describe ManualInverseAssociation do
+  let(:model) do
+    Class.new(MergeRequest) do
+      belongs_to :manual_association, class_name: 'MergeRequestDiff', foreign_key: :latest_merge_request_diff_id
+      manual_inverse_association :manual_association, :merge_request
+    end
+  end
+
+  before do
+    stub_const("#{described_class}::Model", model)
+  end
+
+  let(:instance) { create(:merge_request).becomes(model) }
+
+  describe '.manual_inverse_association' do
+    context 'when the relation exists' do
+      before do
+        instance.create_merge_request_diff
+        instance.reload
+      end
+
+      it 'loads the relation' do
+        expect(instance.manual_association).to be_an_instance_of(MergeRequestDiff)
+      end
+
+      it 'does not perform extra queries after loading' do
+        instance.manual_association
+
+        expect { instance.manual_association.merge_request }
+          .not_to exceed_query_limit(0)
+      end
+
+      it 'passes arguments to the default association method, to allow reloading' do
+        query_count = ActiveRecord::QueryRecorder.new do
+          instance.manual_association
+          instance.manual_association(true)
+        end.count
+
+        expect(query_count).to eq(2)
+      end
+    end
+
+    context 'when the relation does not return a value' do
+      it 'does not try to set an inverse' do
+        expect(instance.manual_association).to be_nil
+      end
+    end
+  end
+end
diff --git a/spec/models/merge_request_diff_spec.rb b/spec/models/merge_request_diff_spec.rb
index 0cfaa17676e..e2a9233a496 100644
--- a/spec/models/merge_request_diff_spec.rb
+++ b/spec/models/merge_request_diff_spec.rb
@@ -18,8 +18,8 @@ describe MergeRequestDiff do
     let!(:first_diff) { mr.merge_request_diff }
     let!(:last_diff) { mr.create_merge_request_diff }
 
-    it { expect(last_diff.latest?).to be_truthy }
-    it { expect(first_diff.latest?).to be_falsey }
+    it { expect(last_diff.reload).to be_latest }
+    it { expect(first_diff.reload).not_to be_latest }
   end
 
   describe '#diffs' do
@@ -29,7 +29,7 @@ describe MergeRequestDiff do
     context 'when the :ignore_whitespace_change option is set' do
       it 'creates a new compare object instead of loading from the DB' do
         expect(mr_diff).not_to receive(:load_diffs)
-        expect(Gitlab::Git::Compare).to receive(:new).and_call_original
+        expect(mr_diff.compare).to receive(:diffs).and_call_original
 
         mr_diff.raw_diffs(ignore_whitespace_change: true)
       end
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index d250ad50713..e510cc3ba47 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -79,6 +79,43 @@ describe MergeRequest do
     end
   end
 
+  describe '.set_latest_merge_request_diff_ids' do
+    def create_merge_request_with_diffs(source_branch, diffs: 2)
+      params = {
+        target_project: project,
+        target_branch: 'master',
+        source_project: project,
+        source_branch: source_branch
+      }
+
+      create(:merge_request, params).tap do |mr|
+        diffs.times { mr.merge_request_diffs.create }
+      end
+    end
+
+    let(:project) { create(:project) }
+
+    it 'sets IDs for merge requests, whether they are already set or not' do
+      merge_requests = [
+        create_merge_request_with_diffs('feature'),
+        create_merge_request_with_diffs('feature-conflict'),
+        create_merge_request_with_diffs('wip', diffs: 0),
+        create_merge_request_with_diffs('csv')
+      ]
+
+      merge_requests.take(2).each do |merge_request|
+        merge_request.update_column(:latest_merge_request_diff_id, nil)
+      end
+
+      expected = merge_requests.map do |merge_request|
+        merge_request.merge_request_diffs.maximum(:id)
+      end
+
+      expect { MergeRequest.set_latest_merge_request_diff_ids(project.merge_requests) }
+        .to change { merge_requests.map { |mr| mr.reload.latest_merge_request_diff_id } }.to(expected)
+    end
+  end
+
   describe '#target_branch_sha' do
     let(:project) { create(:project, :repository) }