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.

app/controllers/concerns/issuable_collections.rb

@@ -150,7 +150,7 @@ module IssuableCollections
                                 when 'MergeRequest'
                                   [
                                     :source_project, :target_project, :author, :assignee, :labels, :milestone,
-                                    head_pipeline: :project, target_project: :namespace, merge_request_diff: :merge_request_diff_commits
+                                    head_pipeline: :project, target_project: :namespace, latest_merge_request_diff: :merge_request_diff_commits
                                   ]
                                 end
   end

app/models/ci/build.rb

@@ -243,7 +243,7 @@ module Ci
 
       @merge_request ||=
         begin
-          merge_requests = MergeRequest.includes(:merge_request_diff)
+          merge_requests = MergeRequest.includes(:latest_merge_request_diff)
             .where(source_branch: ref,
                    source_project: pipeline.project)
             .reorder(iid: :desc)

app/models/concerns/manual_inverse_association.rb

@@ -0,0 +1,17 @@
+module ManualInverseAssociation
+  extend ActiveSupport::Concern
+
+  module ClassMethods
+    def manual_inverse_association(association, inverse)
+      define_method(association) do |*args|
+        super(*args).tap do |value|
+          next unless value
+
+          child_association = value.association(inverse)
+          child_association.set_inverse_instance(self)
+          child_association.target = self
+        end
+      end
+    end
+  end
+end

app/models/merge_request.rb

@@ -5,6 +5,8 @@ class MergeRequest < ActiveRecord::Base
   include Referable
   include IgnorableColumn
   include TimeTrackable
+  include ManualInverseAssociation
+  include EachBatch
 
   ignore_column :locked_at,
                 :ref_fetched
@@ -14,9 +16,28 @@ class MergeRequest < ActiveRecord::Base
   belongs_to :merge_user, class_name: "User"
 
   has_many :merge_request_diffs
+
   has_one :merge_request_diff,
     -> { order('merge_request_diffs.id DESC') }, inverse_of: :merge_request
 
+  belongs_to :latest_merge_request_diff, class_name: 'MergeRequestDiff'
+  manual_inverse_association :latest_merge_request_diff, :merge_request
+
+  # This is the same as latest_merge_request_diff unless:
+  # 1. There are arguments - in which case we might be trying to force-reload.
+  # 2. This association is already loaded.
+  # 3. The latest diff does not exist.
+  #
+  # The second one in particular is important - MergeRequestDiff#merge_request
+  # is the inverse of MergeRequest#merge_request_diff, which means it may not be
+  # the latest diff, because we could have loaded any diff from this particular
+  # MR. If we haven't already loaded a diff, then it's fine to load the latest.
+  def merge_request_diff(*args)
+    fallback = latest_merge_request_diff if args.empty? && !association(:merge_request_diff).loaded?
+
+    fallback || super
+  end
+
   belongs_to :head_pipeline, foreign_key: "head_pipeline_id", class_name: "Ci::Pipeline"
 
   has_many :events, as: :target, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
@@ -167,6 +188,22 @@ class MergeRequest < ActiveRecord::Base
     where("merge_requests.id IN (#{union.to_sql})") # rubocop:disable GitlabSecurity/SqlInjection
   end
 
+  # This is used after project import, to reset the IDs to the correct
+  # values. It is not intended to be called without having already scoped the
+  # relation.
+  def self.set_latest_merge_request_diff_ids!
+    update = '
+      latest_merge_request_diff_id = (
+        SELECT MAX(id)
+        FROM merge_request_diffs
+        WHERE merge_requests.id = merge_request_diffs.merge_request_id
+      )'.squish
+
+    self.each_batch do |batch|
+      batch.update_all(update)
+    end
+  end
+
   WIP_REGEX = /\A\s*(\[WIP\]\s*|WIP:\s*|WIP\s+)+\s*/i.freeze
 
   def self.work_in_progress?(title)

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)

app/services/merge_requests/refresh_service.rb

@@ -35,7 +35,7 @@ module MergeRequests
     # target branch manually
     def close_merge_requests
       commit_ids = @commits.map(&:id)
-      merge_requests = @project.merge_requests.preload(:merge_request_diff).opened.where(target_branch: @branch_name).to_a
+      merge_requests = @project.merge_requests.preload(:latest_merge_request_diff).opened.where(target_branch: @branch_name).to_a
       merge_requests = merge_requests.select(&:diff_head_commit)
 
       merge_requests = merge_requests.select do |merge_request|

changelogs/unreleased/use-merge-requests-diff-id-column.yml

@@ -0,0 +1,5 @@
+---
+title: Make finding most recent merge request diffs more efficient
+merge_request:
+author:
+type: performance

db/migrate/20171115164540_populate_merge_requests_latest_merge_request_diff_id_take_two.rb

@@ -0,0 +1,30 @@
+# This is identical to the stolen background migration, which already has specs.
+class PopulateMergeRequestsLatestMergeRequestDiffIdTakeTwo < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  BATCH_SIZE = 1_000
+
+  class MergeRequest < ActiveRecord::Base
+    self.table_name = 'merge_requests'
+
+    include ::EachBatch
+  end
+
+  disable_ddl_transaction!
+
+  def up
+    Gitlab::BackgroundMigration.steal('PopulateMergeRequestsLatestMergeRequestDiffId')
+
+    update = '
+      latest_merge_request_diff_id = (
+        SELECT MAX(id)
+        FROM merge_request_diffs
+        WHERE merge_requests.id = merge_request_diffs.merge_request_id
+      )'.squish
+
+    MergeRequest.where(latest_merge_request_diff_id: nil).each_batch(of: BATCH_SIZE) do |relation|
+      relation.update_all(update)
+    end
+  end
+end

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

lib/gitlab/import_export/project_tree_restorer.rb

@@ -60,6 +60,8 @@ module Gitlab
           end
         end
 
+        @project.merge_requests.set_latest_merge_request_diff_ids!
+
         @saved
       end
 

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

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

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

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(last_diff.reload).to be_latest }
-    it { expect(first_diff.latest?).to be_falsey }
+    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

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 { project.merge_requests.set_latest_merge_request_diff_ids! }
+        .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) }