CODEWITHSUNDEEP
unit-testingphpunit
Andrew
Andrew

Reputation: 239097

Unit Testing: How to test a subclass in isolation from its parent?

I'm getting started unit testing my PHP application with PHPUnit. I understand that it's important for unit tests to run in isolation so you know where to look when a test fails. One thing I am struggling to understand is how to test subclasses in isolation from their parent. For example, most of my models extend a "base model" which has most of the features that a model should have.

<?php

class BaseModel
{
    public function save($data) {
        // write $data to the database
        $dbAdapter->save($data);
    }
}

class RegularModel extends BaseModel
{
    public function save($data) {
        // clean up $data before passing it to parent
        if (isset($data['foo'])) {
            unset($data['foo']);
            $data['bar'] = 'foo';
        }
        parent::save($data);
    }
}

# Unit Test
class RegularModelTest extends PHPUnit_Framework_TestCase
{
    public function testSaveMethodCallsParent() {
        $data = array('foo' => 'yes');
        $model = new RegularModel();
        $model->save($data);
        // assert parent received data correctly
    }
}

I'm not sure how to test my RegularModel without calling a bunch of unnecessary code. I'm also doing some autoloading so when it calls save on the parent, it will actually try to save to the test database. I'd rather mock this out since I don't care about whether or not it actually writes to the database when I'm testing my RegularModel only when I am testing my BaseModel. Or am I thinking about this all wrong? What do you recommend when it comes to testing situations like this?

Upvotes: 4

Views: 888

Answers (2)

GordonM
GordonM

Reputation: 31780

Subclasses are, by definition, tightly coupled to their superclasses so practically speaking there is no way to test a subclass in isolation.

However, if the superclass has an extensive test suite that passes then you normally can be confident that you can test the subclass by just covering the methods implemented in the subclass. The superclass test suite covers the superclass functionality.

Upvotes: 2

David Harkness
David Harkness

Reputation: 36562

Your best bet is to mock the $dbAdapter when testing RegularModel. While you are still executing the parent class's code, that code really is part of the RegularModel unit due to the is-a relationship.

The only way around it would be to provide a different implementation of BaseModel. You can either run these tests in a separate process or use Runkit to swap in the other implementation at runtime. Both of these have drawbacks--complexity, performance degredation, and instability--that come at too high a price in my view.

Upvotes: 3

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