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When and How to Use Data Mappers

When and How to Use Data Mappers

When a form is compound, the initial data needs to be passed to children so each can display their own input value. On submission, children values need to be written back into the form.

Data mappers are responsible for reading and writing data from and into parent forms.

The main built-in data mapper uses the PropertyAccess component and will fit most cases. However, you can create your own implementation that could, for example, pass submitted data to immutable objects via their constructor.

The Difference between Data Transformers and Mappers

It is important to know the difference between data transformers and mappers.

  • Data transformers change the representation of a value (e.g. from "2016-08-12" to a DateTime instance);
  • Data mappers map data (e.g. an object or array) to form fields, and vice versa.

Changing a YYYY-mm-dd string value to a DateTime instance is done by a data transformer. Populating inner fields (e.g year, hour, etc) of a compound date type using a DateTime instance is done by the data mapper.

Creating a Data Mapper

Suppose that you want to save a set of colors to the database. For this, you're using an immutable color object:

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// src/App/Painting/Color.php
namespace App\Painting;

final class Color
{
    private $red;
    private $green;
    private $blue;

    public function __construct(int $red, int $green, int $blue)
    {
        $this->red = $red;
        $this->green = $green;
        $this->blue = $blue;
    }

    public function getRed(): int
    {
        return $this->red;
    }

    public function getGreen(): int
    {
        return $this->green;
    }

    public function getBlue(): int
    {
        return $this->blue;
    }
}

The form type should be allowed to edit a color. But because you've decided to make the Color object immutable, a new color object has to be created each time one of the values is changed.

Tip

If you're using a mutable object with constructor arguments, instead of using a data mapper, you should configure the empty_data option with a closure as described in How to Configure empty Data for a Form Class.

The red, green and blue form fields have to be mapped to the constructor arguments and the Color instance has to be mapped to red, green and blue form fields. Recognize a familiar pattern? It's time for a data mapper!

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// src/App/Form/DataMapper/ColorMapper.php
namespace App\Form\DataMapper;

use App\Painting\Color;
use Symfony\Component\Form\DataMapperInterface;
use Symfony\Component\Form\Exception\UnexpectedTypeException;
use Symfony\Component\Form\FormInterface;

final class ColorMapper implements DataMapperInterface
{
    /**
     * @param Color|null $data
     */
    public function mapDataToForms($data, $forms)
    {
        // there is no data yet, so nothing to prepopulate
        if (null === $data) {
            return;
        }

        // invalid data type
        if (!$data instanceof Color) {
            throw new UnexpectedTypeException($data, Color::class);
        }

        /** @var FormInterface[] $forms */
        $forms = iterator_to_array($forms);

        // initialize form field values
        $forms['red']->setData($data->getRed());
        $forms['green']->setData($data->getGreen());
        $forms['blue']->setData($data->getBlue());
    }

    public function mapFormsToData($forms, &$data)
    {
        /** @var FormInterface[] $forms */
        $forms = iterator_to_array($forms);

        // as data is passed by reference, overriding it will change it in
        // the form object as well
        // beware of type inconsistency, see caution below
        $data = new Color(
            $forms['red']->getData(),
            $forms['green']->getData(),
            $forms['blue']->getData()
        );
    }
}

Caution

The data passed to the mapper is not yet validated. This means that your objects should allow being created in an invalid state in order to produce user-friendly errors in the form.

Using the Mapper

You're ready to use the data mapper for the ColorType form. Use the setDataMapper() method to configure the data mapper:

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// src/App/Form/Type/ColorType.php
namespace App\Form\Type;

use App\Form\DataMapper\ColorMapper;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\IntegerType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

final class ColorType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('red', IntegerType::class, [
                // enforce the strictness of the type to ensure the constructor
                // of the Color class doesn't break
                'empty_data' => '0',
            ])
            ->add('green', IntegerType::class, [
                'empty_data' => '0',
            ])
            ->add('blue', IntegerType::class, [
                'empty_data' => '0',
            ])
            ->setDataMapper(new ColorMapper())
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        // when creating a new color, the initial data should be null
        $resolver->setDefault('empty_data', null);
    }
}

Cool! When using the ColorType form, the custom ColorMapper will create a new Color object now.

Caution

When a form has the inherit_data option set to true, it does not use the data mapper and lets its parent map inner values.

Tip

You can also implement the DataMapperInterface in the ColorType and add the mapDataToForms() and mapFormsToData() in the form type directly to avoid creating a new class. You'll then have to call $builder->setDataMapper($this).

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.