Increase batchloader efficiency

lib/gitlab/graphql/loaders/batch_model_loader.rb

@@ -12,14 +12,11 @@ module Gitlab
 
         # rubocop: disable CodeReuse/ActiveRecord
         def find
-          BatchLoader::GraphQL.for({ model: model_class, id: model_id.to_i }).batch do |loader_info, loader|
-            per_model = loader_info.group_by { |info| info[:model] }
-            per_model.each do |model, info|
-              ids = info.map { |i| i[:id] }
-              results = model.where(id: ids)
+          BatchLoader::GraphQL.for(model_id.to_i).batch(key: model_class) do |ids, loader, args|
+            model = args[:key]
+            results = model.where(id: ids)
 
-              results.each { |record| loader.call({ model: model, id: record.id }, record) }
-            end
+            results.each { |record| loader.call(record.id, record) }
           end
         end
         # rubocop: enable CodeReuse/ActiveRecord

spec/lib/gitlab/graphql/loaders/batch_model_loader_spec.rb

@@ -4,8 +4,9 @@ require 'spec_helper'
 
 RSpec.describe Gitlab::Graphql::Loaders::BatchModelLoader do
   describe '#find' do
-    let(:issue) { create(:issue) }
-    let(:user) { create(:user) }
+    let_it_be(:issue) { create(:issue) }
+    let_it_be(:other_user) { create(:user) }
+    let_it_be(:user) { create(:user) }
 
     it 'finds a model by id' do
       issue_result = described_class.new(Issue, issue.id).find
@@ -16,15 +17,25 @@ RSpec.describe Gitlab::Graphql::Loaders::BatchModelLoader do
     end
 
     it 'only queries once per model' do
-      other_user = create(:user)
-      user
-      issue
-
       expect do
         [described_class.new(User, other_user.id).find,
          described_class.new(User, user.id).find,
          described_class.new(Issue, issue.id).find].map(&:sync)
       end.not_to exceed_query_limit(2)
     end
+
+    it 'does not force values unnecessarily' do
+      expect do
+        a = described_class.new(User, user.id).find
+        b = described_class.new(Issue, issue.id).find
+
+        b.sync
+
+        c = described_class.new(User, other_user.id).find
+
+        a.sync
+        c.sync
+      end.not_to exceed_query_limit(2)
+    end
   end
 end