Config field backend model (system.xml) not used for default value (config.xml)

[kweij]

Preconditions

Magento 2.1.2

Steps to reproduce

Create a config field with a custom backend model, such as:

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <tab id="oops" sortOrder="110">
            <label>Oops</label>
        </tab>
        <section id="oops_theme" type="text" sortOrder="10" showInDefault="1" showInWebsite="1" showInStore="1">
            <label>Theme</label>
            <tab>oops</tab>
            <resource>Oops_Theme::config</resource>
            <group id="homepage" type="text" sortOrder="30" showInDefault="1" showInWebsite="1" showInStore="1">
                <label>Homepage</label>
                <field id="menu" type="text" sortOrder="20" showInDefault="1" showInWebsite="1" showInStore="1">
                    <label>Primary Actions</label>
                    <frontend_model>Oops\Theme\Block\Adminhtml\Form\Field\Menu</frontend_model>
                    <backend_model>Oops\Theme\Model\System\Config\Backend\Json</backend_model>
                </field>
            </group>
        </section>
    </system>
</config>

Here I use backend model Oops\Theme\Model\System\Config\Backend\Json. This model is looks like this:

namespace Oops\Theme\Model\System\Config\Backend;

use Magento\Framework\App\Config\Value;

class Json extends Value
{
    /**
     * Process data after load
     *
     * @return void
     */
    protected function _afterLoad()
    {
        $value = $this->getValue();
        $value = json_decode($value, true);
        $this->setValue($value);
    }

    /**
     * Prepare data before save
     *
     * @return void
     */
    public function beforeSave()
    {
        $value = $this->getValue();
        unset($value['__empty']);
        $value = json_encode($value, JSON_PRETTY_PRINT);
        $this->setValue($value);
    }
}

Expected result

Performs json_decode() after loading data from the configuration and json_encode() before saving the data to the database. Expected is that the _afterLoad() method gets evoked, unregardless of wether the configuration is retrieved from database or config.xml, however ...

Actual result

The _afterLoad() method gets evoked only after loading the configuration value from the database, and not after loading the configuration from config.xml. This causes default configuration from config.xml to be passed to the form-element as string in stead of Array, resulting in empty configuration fields in the admin interface.

[kweij]

This can be fixed easily, as in pull request #7742

[kweij]

Ok, well since my pull-request won't build due to unexpected results (it's not crashing, but it alters the way default values in config.xml are treated, so it's not backwards compatible) this bug/change may require a different approach. Nevertheless, I stand by my expected result in the original post.

[okorshenko]

Here is a ticket MAGETWO-70797 to track the issue progress