public function test()
 {
     $one = new Collection([new Entity(['id' => 1]), new Entity(['id' => 2]), new Entity(['id' => 1]), new Entity(['id' => null]), new Entity(['noid' => 1])]);
     $two = new Collection([new Entity(['id2' => 1]), new Entity(['id2' => 5]), new Entity(['id2' => 1]), new Entity(['id2' => null])]);
     $one->bindCollection($two, ['id2' => 'id'], 'two');
     $this->assertInstanceOf(Collection::class, $one[0]->two);
     $this->assertInstanceOf(Collection::class, $one[1]->two);
     $this->assertInstanceOf(Collection::class, $one[2]->two);
     $this->assertEquals(2, $one[0]->two->count());
     $this->assertEquals(0, $one[1]->two->count());
     $this->assertEquals(2, $one[2]->two->count());
 }