Unit Test for overwrite collection class in magento2

Question

Suppose I overwrite 'Magento\Sales\Model\Resource\Order\Collection' to 'SR\RewriteSales\Model\Resource\Order\Collection', code looks like

public function addFieldToFilter($field, $condition = null)
    {
        // custom condition applied here

        return parent::addFieldToFilter($field, $condition);
    }

Now I need some test case for 'AddFieldToFilter' method.

I create a file inside SR\RewriteSales\Test\Unit\Model\Resource\Order\CollectionTest.php code looks like

class CollectionTest extends \PHPUnit_Framework_TestCase
{
    /**
     * @var Collection|\PHPUnit_Framework_MockObject_MockObject
     */
    protected $collection;

    /**
     * @var \Magento\Framework\DB\Select|\PHPUnit_Framework_MockObject_MockObject
     */
    protected $select;

    protected function setUp()
    {
        $this->select = $this->getMockBuilder('Magento\Framework\DB\Select')
            ->disableOriginalConstructor()
            ->getMock();

        $connection = $this->getMockBuilder('Magento\Framework\DB\Adapter\Pdo\Mysql')
            ->disableOriginalConstructor()
            ->getMock();
        $connection->expects($this->any())
            ->method('select')
            ->will($this->returnValue($this->select));

        $resource = $this->getMockBuilder('Magento\Framework\Model\Resource\Db\AbstractDb')
            ->disableOriginalConstructor()
            ->setMethods(['__wakeup', 'getReadConnection'])
            ->getMockForAbstractClass();
        $resource->expects($this->any())
            ->method('getReadConnection')
            ->will($this->returnValue($connection));

        $objectManagerHelper = new \Magento\Framework\TestFramework\Unit\Helper\ObjectManager($this);
        $arguments = $objectManagerHelper->getConstructArguments(
            'SR\RewriteSales\Model\Resource\Order\Collection',
            ['resource' => $resource, 'connection' => $connection]
        );

        $this->collection = $this->getMockBuilder('SR\RewriteSales\Model\Resource\Order\Collection')
            ->setConstructorArgs($arguments)
            ->setMethods(['addFilter', '_translateCondition', 'getMainTable', '__wakeup'])
            ->getMock();
    }

    public function testAddFieldToFilter()
    {

    }
}

It's come following error-> "RuntimeException: ObjectManager isn't initialized".

Is that possible to create some test case for overwrite class?

Answer 1

First of all better to do not mock class under test.

Second one, better to use plugins instead of class overwrite.

And last, you must pass all parameters of parent constructor to your class.

Simplest way to instantiate class is using objectMangerHelper

    $objectManager = new \Magento\Framework\TestFramework\Unit\Helper\ObjectManager($this);
    $this->collection = $objectManager->getObject(
        'SR\RewriteSales\Model\Resource\Order\Collection',
        [
            'resource' => $resource, 
            'connection' => $connection,

        ]
    );

Answer 2

Are you sure that you use phpunit.xml from dev/tests/unit/ dir? Also you shouldn't mock class which you want test (@KAndy comment said about that)