MONGOID-5887 Include server hint for aggregate operations

lib/mongoid/contextual/aggregable/mongo.rb

@@ -27,7 +27,12 @@ module Mongo
         #   If no documents are found, then returned Hash will have
         #   count, sum of 0 and max, min, avg of nil.
         def aggregates(field)
-          result = collection.aggregate(pipeline(field), session: _session).to_a
+          result = collection.aggregate(
+                    pipeline(field),
+                    session: _session,
+                    hint: view.hint
+                  ).to_a
+
           if result.empty?
             Aggregable::EMPTY_RESULT.dup
           else

spec/mongoid/contextual/aggregable/mongo_spec.rb

@@ -244,6 +244,39 @@
         end
       end
     end
+
+    context 'regarding hints' do
+      let(:client) { Person.collection.client }
+      let(:subscriber) { Mrss::EventSubscriber.new }
+
+      before do
+        client.subscribe(Mongo::Monitoring::COMMAND, subscriber)
+        maybe_hint.aggregates(:age)
+      end
+
+      after do
+        client.unsubscribe(Mongo::Monitoring::COMMAND, subscriber)
+      end
+
+      let(:event) { subscriber.single_command_started_event('aggregate') }
+      let(:command) { event.command }
+
+      context 'when no hint is provided' do
+        let(:maybe_hint) { Person }
+
+        it 'does not include the hint in the command' do
+          expect(command['hint']).to be_nil
+        end
+      end
+
+      context 'when a hint is provided' do
+        let(:maybe_hint) { Person.hint(age: 1) }
+
+        it 'includes the hint with the command' do
+          expect(command['hint']).to eq({ 'age' => 1 })
+        end
+      end
+    end
   end
 
   describe "#avg" do