One "Weird Trick" To Instantly Break usesSource

Published: August 5, 2016

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As you might know, Magento features several frontend input types for catalog attributes. For example the “New From Date” attribute, is stored as DATETIME in the catalog_product_entity_datetime table, whereas the product’s “Name” is stored as VARCHAR(255) in the catalog_product_entity_varchar table.

For both of these attribute types, the values can be pulled directly from the db and are ready to display to the end user with no further manipulation. However, there are a couple types which are initially stored numerically (e.g. in catalog_product_entity_int) and need to be translated via eav_attribute_option and eav_attribute_option_value before they are ready to be displayed to the end user.

There is a method in Mage_Eav_Model_Entity_Attribute_Abstract (an ancestor of Mage_Catalog_Model_Resource_Eav_Attribute) called usesSource. It is uncommented, but at first glance it looks like it can reliably used to determine whether or not an attribute’s values need to be translated in that way…

public function usesSource()
{
    return $this->getFrontendInput() === 'select' || $this->getFrontendInput() === 'multiselect'
        || $this->getData('source_model') != '';
}

However, as I learned the hard way, there’s this “one weird trick” that will cause usesSource to return true no matter what.

The “Trick”

The “trick” is simple. Before calling usesSource() on the attribute, call getSource() which is defined in Mage_Eav_Model_Entity_Attribute_Abstract. Watch and be amazed…

The Code

<?php

require_once('app/Mage.php');
Mage::app();

$product = Mage::getModel('catalog/product')->load(384);

$attribute = $product->getResource()->getAttribute('name');
echo 'Before:' . PHP_EOL;
var_dump($attribute->usesSource()) . PHP_EOL;
echo 'After:' . PHP_EOL;
$attribute->getSource();
var_dump($attribute->usesSource()) . PHP_EOL;

The Result

Before:
bool(false)
After:
bool(true)

Why Does This Happen?

To understand why this happens, you need to look at the internals of getSource(). Here they are…

/**
 * Retrieve source instance
 *
 * @return Mage_Eav_Model_Entity_Attribute_Source_Abstract
 */
public function getSource()
{
    if (empty($this->_source)) {
        if (!$this->getSourceModel()) {
            $this->setSourceModel($this->_getDefaultSourceModel());
        }
        $source = Mage::getModel($this->getSourceModel());
        if (!$source) {
            throw Mage::exception('Mage_Eav',
                Mage::helper('eav')->__('Source model "%s" not found for attribute "%s"',$this->getSourceModel(), $this->getAttributeCode())
            );
        }
        $this->_source = $source->setAttribute($this);
    }
    return $this->_source;
}

Specifically, this line is of interest…

$this->setSourceModel($this->_getDefaultSourceModel());

Remember we’re working with a Mage_Catalog_Model_Resource_Eav_Attribute. Here’s how it defines _getDefaultSourceModel()

/**
 * Get default attribute source model
 *
 * @return string
 */
public function _getDefaultSourceModel()
{
    return 'eav/entity_attribute_source_table';
}

Now let’s take one more look at usesSource()…

public function usesSource()
{
    return $this->getFrontendInput() === 'select' || $this->getFrontendInput() === 'multiselect'
        || $this->getData('source_model') != '';
}

Specifically this line…

$this->getData('source_model') != '';

As demonstrated above, calling getSource() on the attributes leads to the source_model being set. As a result usesSource() will now return true, even for attributes that aren’t selects or multiselects.

Why Should I Care?

One would think that a method called usesSource() is a safe means for determining whether a not a given attribute uses a source model. However, because calling getSource() on an attribute makes usesSource() return true always, it turns out it’s not. usesSource() is not documented, so we can’t say for sure whether or not this is expected behavior, but in my mind this is a Magento bug.

In my case, I ran into a lot of issues getting Vinai Kopp’s awesome Nicer Image Names plugin to work because usesSource() is used to determine whether the an attribute value needs translation. This works fine on an out of box installation. However, in practice, community and local code is likely to call getSource() on attribute that don’t use source models, hence “corrupting” usesSource(). Here’s an example from BlueAcorn_UniversalAnalytics which attempts to find the value of an attribute for a product

/**
 * Initial entry point for finding product attribute values
 *
 * @name findAttributeValue
 * @param Mage_Catalog_Model_Product $product
 * @param string $attribute
 * @return mixed
 */
protected function findAttributeValue($product, $attribute) {
    $newValue = null;

    foreach (Array('getListAttributeValue', 'getNormalAttributeValue') as $method) {
        $newValue = $this->$method($product, $attribute);
        if ($newValue !== null) break;
    }
    
    $newValue = ($attribute == 'coupon_code') ? strtoupper($newValue) : $newValue;
    $newValue = str_replace(array("\n", "\t", "\r"), ' ', $newValue);

    return $newValue;
}

getListAttributeValue() will wind up calling getSource() on the attribute first. If that fails, then it will fall back to getNormalAttributeValue which will try to fetch the attribute value using a magic getter.

While I was working through this issue I found that there were actually two plugins within the same project that were doing this!

What To Do?

At this point, Magento should just leave this one as is. Changing the API for getSource() may have other downstream impacts. However, as a responsible developer, you should…

Not rely on usesSource() for determining whether an attribute has a source model
Not call getSource() on attribute unless you know it has a source model.