2.4. Data Mapper

2.4.1. Purpose

A Data Mapper, is a Data Access Layer that performs bidirectional transfer of data between a persistent data store (often a relational database) and an in memory data representation (the domain layer). The goal of the pattern is to keep the in memory representation and the persistent data store independent of each other and the data mapper itself. The layer is composed of one or more mappers (or Data Access Objects), performing the data transfer. Mapper implementations vary in scope. Generic mappers will handle many different domain entity types, dedicated mappers will handle one or a few.

The key point of this pattern is, unlike Active Record pattern, the data model follows Single Responsibility Principle.

2.4.2. Examples

  • DB Object Relational Mapper (ORM) : Doctrine2 uses DAO named as “EntityRepository”

2.4.3. UML Diagram

Alt DataMapper UML Diagram

2.4.4. Code

You can also find this code on GitHub

User.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\DataMapper;

class User
{
    private string $username;
    private string $email;

    public static function fromState(array $state): User
    {
        // validate state before accessing keys!

        return new self(
            $state['username'],
            $state['email']
        );
    }

    public function __construct(string $username, string $email)
    {
        // validate parameters before setting them!

        $this->username = $username;
        $this->email = $email;
    }

    public function getUsername(): string
    {
        return $this->username;
    }

    public function getEmail(): string
    {
        return $this->email;
    }
}

UserMapper.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\DataMapper;

use InvalidArgumentException;

class UserMapper
{
    private StorageAdapter $adapter;

    public function __construct(StorageAdapter $storage)
    {
        $this->adapter = $storage;
    }

    /**
     * finds a user from storage based on ID and returns a User object located
     * in memory. Normally this kind of logic will be implemented using the Repository pattern.
     * However the important part is in mapRowToUser() below, that will create a business object from the
     * data fetched from storage
     */
    public function findById(int $id): User
    {
        $result = $this->adapter->find($id);

        if ($result === null) {
            throw new InvalidArgumentException("User #$id not found");
        }

        return $this->mapRowToUser($result);
    }

    private function mapRowToUser(array $row): User
    {
        return User::fromState($row);
    }
}

StorageAdapter.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\DataMapper;

class StorageAdapter
{
    private array $data = [];

    public function __construct(array $data)
    {
        $this->data = $data;
    }

    /**
     * @param int $id
     *
     * @return array|null
     */
    public function find(int $id)
    {
        if (isset($this->data[$id])) {
            return $this->data[$id];
        }

        return null;
    }
}

2.4.5. Test

Tests/DataMapperTest.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\DataMapper\Tests;

use InvalidArgumentException;
use DesignPatterns\Structural\DataMapper\StorageAdapter;
use DesignPatterns\Structural\DataMapper\User;
use DesignPatterns\Structural\DataMapper\UserMapper;
use PHPUnit\Framework\TestCase;

class DataMapperTest extends TestCase
{
    public function testCanMapUserFromStorage()
    {
        $storage = new StorageAdapter([1 => ['username' => 'domnikl', 'email' => '[email protected]']]);
        $mapper = new UserMapper($storage);

        $user = $mapper->findById(1);

        $this->assertInstanceOf(User::class, $user);
    }

    public function testWillNotMapInvalidData()
    {
        $this->expectException(InvalidArgumentException::class);

        $storage = new StorageAdapter([]);
        $mapper = new UserMapper($storage);

        $mapper->findById(1);
    }
}