Magento – Magento 2 – override product attribute in product_form.xml

When adding the product attribute, set used_in_product_listing, used_for_sort_by and filterable to true.

You can do that with the following update script:

$installer = Mage::getResourceModel('catalog/setup', 'catalog_setup');
$installer->startSetup();
$installer->updateAttribute('catalog_product', 'product_type', 'used_in_product_listing', 1);
$installer->updateAttribute('catalog_product', 'product_type', 'used_for_sort_by', 1);
$installer->updateAttribute('catalog_product', 'product_type', 'is_filterable', 1);
$installer->endSetup();

Thats all, if if you are NOT using the flat catalog.

If you enable the flat catalog product feature, then you need to ensure that your custom source model implements the methods getFlatColums(), getFlatIndexes() and getFlatUpdateSelect().
If they are missing, your attribute won't be included in the generated flat tables, even if used_in_product_listing is enabled.
Have a look at the class Mage_Eav_Model_Entity_Attribute_Source_Boolean for an example. You can probably copy over the methods with minimal adjustments into your custom source model from there. Otherwise, please ask a new question.

One of the solution is to add a backend model to your attribute which is used to format / validate your attribute value before save and/or after load.

Add a backend class :

[
    'type' => 'int',
    'backend' => '\Foo\Bar\Model\Attribute\Backend\YourAttribute',
    'frontend' => '',
    'label' => 'XXXX',
    'input' => 'text',
    'frontend_class' => 'validate-greater-than-zero',
    'source' => '',
    'global' => \Magento\Eav\Model\Entity\Attribute\ScopedAttributeInterface::SCOPE_GLOBAL,
    'visible' => true,
    'required' => true,
    'user_defined' => false,
    'default' => 0,
    'searchable' => false,
    'filterable' => true,
    'comparable' => false,
    'visible_on_front' => false,
    'used_in_product_listing' => true,
    'unique' => false
]

Here is an example of your custom class \Foo\Bar\Model\Attribute\Backend\YourAttribute

<?php

namespace Foo\Bar\Model\Attribute\Backend;

/**
 * Class YourAttribute
 */
class YourAttribute extends \Magento\Eav\Model\Entity\Attribute\Backend\AbstractBackend
{

    /**
     * @var int $minimumValueLength
     */
    protected $minimumValueLength = 0;

    /**
     * @param \Magento\Framework\DataObject $object
     *
     * @return $this
     */
    public function afterLoad($object)
    {
        // your after load logic

        return parent::afterLoad($object);
    }

    /**
     * @param \Magento\Framework\DataObject $object
     *
     * @return $this
     */
    public function beforeSave($object)
    {
        $this->validateLength($object);

        return parent::beforeSave($object);
    }

    /**
     * Validate length
     *
     * @param \Magento\Framework\DataObject $object
     *
     * @return bool
     * @throws \Magento\Framework\Exception\LocalizedException
     */
    public function validateLength($object)
    {
        /** @var string $attributeCode */
        $attributeCode = $this->getAttribute()->getAttributeCode();
        /** @var int $value */
        $value = (int)$object->getData($attributeCode);
        /** @var int $minimumValueLength */
        $minimumValueLength = $this->getMinimumValueLength();

        if ($this->getAttribute()->getIsRequired() && $value <= $minimumValueLength) {
            throw new \Magento\Framework\Exception\LocalizedException(
                __('The value of attribute "%1" must be greater than %2', $attributeCode, $minimumValueLength)
            );
        }

        return true;
    }

    /**
     * Get minimum attribute value length
     * 
     * @return int
     */
    public function getMinimumValueLength()
    {
        return $this->minimumValueLength;
    }
}

If you want a simple example of that kind of class you can check

• \Magento\Customer\Model\Customer\Attribute\Backend\Website
• all the classes which extend \Magento\Eav\Model\Entity\Attribute\Backend\AbstractBackend
• the classes into backend_model column in eav_attribute table

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EDIT
If you want a class that do nearly the same thing as you want you can take a look at the SKU attribute validation \Magento\Catalog\Model\Product\Attribute\Backend\Sku
I also added the method in the example class

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EDIT
Another solution (maybe not the best one) is to create a plugin on the function \Magento\Eav\Helper\Data::getFrontendClasses and add your frontend class here that can be validated in front.