Logical error: Magento 2 checks whether a backend config form field's «backend_model» implements the \Magento\Framework\App\Config\ValueInterface interface only, but actually calls the «backend_model» for much more methods (absent in the interface)

[dmitry_fedyuk (Dmitrii Fediuk)]

https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Config/Model/Config/BackendFactory.php#L32-L39

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https://github.com/magento/magento2/blob/2.0.0/lib/internal/Magento/Framework/App/Config/ValueInterface.php#L17-L37

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https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Config/Block/System/Config/Form.php#L331-L341

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https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Config/Model/Config.php#L244-L254

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https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Config/Model/Config.php#L256-L256

https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Config/Model/Config.php#L455-L458

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https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Config/Model/Config.php#L281-L281

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https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Config/Model/Config.php#L284-L284

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https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Config/Model/Config.php#L290-L290

https://github.com/magento/magento2/blob/2.0.0/lib/internal/Magento/Framework/DB/Transaction.php#L129-L129

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https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Config/Model/Config.php#L292-L292

https://github.com/magento/magento2/blob/2.0.0/lib/internal/Magento/Framework/DB/Transaction.php#L166-L166

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https://github.com/magento/magento2/blob/2.0.0/lib/internal/Magento/Framework/DB/Transaction.php#L95-L95

So actually implementing the \Magento\Framework\App\Config\ValueInterface is not enough for a backend_model and the \Magento\Config\Model\Config\BackendFactory::create method should do more checks: maybe for being an instance of \Magento\Framework\App\Config\Value or \Magento\Framework\Model\AbstractModel class.

https://github.com/magento/magento2/issues/2378

[dmitry_fedyuk (Dmitrii Fediuk)]

See also:

• The methods \Magento\Framework\App\Config\ValueInterface::getFieldsetDataValue() and \Magento\Framework\App\Config\Value::getFieldsetDataValue() are declared but never used
• The method \Magento\Framework\App\Config\ValueInterface::isValueChanged() should be removed from the interface because it is used only internally by a particular interface implementation: \Magento\Framework\App\Config\Value
• The method \Magento\Framework\App\Config\ValueInterface::getOldValue() should be removed from the interface because it is used only internally by a particular interface implementation: \Magento\Framework\App\Config\Value
• A backend model should always be a \Magento\Framework\Model\AbstractModel