How to unit test a Builder design pattern - unit-testing

Question

If we have the following class that uses the Builder design pattern:
class CourseListingBuilder extends Component
{
/**
* #var yii\db\Query
*/
private $query;
private $data = [];
public function init()
{
parent::init();
$this->query = new yii\db\Query();
}
/**
* Return a new instance of the builder
* #return CourseListingBuilder
*/
public static function create()
{
return new CourseListingBuilder();
}
public function selectColumns(array $columns)
{
// #TODO validate and format $columns
$this->query->select($columns);
return $this;
}
public function applyFilters($filters = [])
{
// #TODO validate and parse filters
$this->query->andWhere($filters);
return $this;
}
public function build()
{
// Make the actual DB query
$this->data = $this->query->all();
}
public function getData()
{
return $this->data;
}
}
We use it like this:
$data = CourseListingBuilder::create()
->selectColumns(['id', 'name'])
->applyFilters(['active'=>1])
->build()
->getData();
In time, these classes grow quite large because we have multiple developers working on them. Developers extend the selectColumns() or applyFilters() and occasionally, the code breaks. We've recently started introducing Unit Tests in the company and we would like to make classes that follow the above pattern - unit testable, preferrably through a small refactoring.
How do you properly unit test the above builder design pattern class, given its internal dependency to yii\db\Query (which comes from the Yii2 framework by the way, but this is not relevant to the example). What's relevant to the example is that we DO NOT want to test the internal behaviors of yii\db\Query. It comes from the framework. We know that it's working. In other words - we want to "mock" it as much as possible, while effectively testing the actual methods inside our Builder class and how they will affect the outcome.
The second thing to point out is that we know how to write unit tests in general. This question is not about "how to write unit tests" in general, but "how to write unit tests for a builder class that has an internal dependency to a third party DAO class".
Are we doing something wrong?
Such code is not unit testable but instead goes in the integration tests territory?

Answer 1

The simplest way would be to replace yii\db\Query with some simple class and test its state after method call.
class MockQuery extends \yii\db\Query {
public $select;
public $selectOption;
public function select($columns, $option = null) {
$this->select = $columns;
$this->selectOption = $option;
}
// ...
}
class CourseListingBuilder extends \yii\base\Component {
private $query;
public function selectColumns(array $columns) {
// #TODO validate and format $columns
$this->query->select($columns);
return $this;
}
// ...
}
And test:
public function testQuery() {
$builder = new CourseListingBuilder();
// use reflection to access private property
$reflection = new \ReflectionObject($builder);
$property = $reflection->getProperty('query');
$property->setAccessible(true);
$property->setValue($builder, new MockQuery());
$builder->selectColumns(['id', 'name']);
$query = $property->getValue($builder);
$this->assertSame(['id', 'name'], $query->select);
$this->assertNull($query->selectOption);
}