Big Parent Class

In some application code, especially older legacy code, we can come across some classes that extend a “big parent class” - a parent class that knows and does too much:

class BigParentClass
{
    public function doesEverything()
    {
        // sets up database connections
        // writes to log files
    }
}

class ChildClass extends BigParentClass
{
    public function doesOneThing()
    {
        // but calls on BigParentClass methods
        $result = $this->doesEverything();
        // does something with $result
        return $result;
    }
}

We want to test our ChildClass and its doesOneThing method, but the problem is that it calls on BigParentClass::doesEverything(). One way to handle this would be to mock out all of the dependencies BigParentClass has and needs, and then finally actually test our doesOneThing method. It’s an awful lot of work to do that.

What we can do, is to do something... unconventional. We can create a runtime partial test double of the ChildClass itself and mock only the parent’s doesEverything() method:

$childClass = \Mockery::mock('ChildClass')->makePartial();
$childClass->shouldReceive('doesEverything')
    ->andReturn('some result from parent');

$childClass->doesOneThing(); // string("some result from parent");

With this approach we mock out only the doesEverything() method, and all the unmocked methods are called on the actual ChildClass instance.