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

 1<?php
 2
 3declare(strict_types=1);
 4
 5namespace DesignPatterns\Structural\DataMapper;
 6
 7class User
 8{
 9    public static function fromState(array $state): User
10    {
11        // validate state before accessing keys!
12
13        return new self(
14            $state['username'],
15            $state['email']
16        );
17    }
18
19    public function __construct(private string $username, private string $email)
20    {
21    }
22
23    public function getUsername(): string
24    {
25        return $this->username;
26    }
27
28    public function getEmail(): string
29    {
30        return $this->email;
31    }
32}

UserMapper.php

 1<?php
 2
 3declare(strict_types=1);
 4
 5namespace DesignPatterns\Structural\DataMapper;
 6
 7use InvalidArgumentException;
 8
 9class UserMapper
10{
11    public function __construct(private StorageAdapter $adapter)
12    {
13    }
14
15    /**
16     * finds a user from storage based on ID and returns a User object located
17     * in memory. Normally this kind of logic will be implemented using the Repository pattern.
18     * However the important part is in mapRowToUser() below, that will create a business object from the
19     * data fetched from storage
20     */
21    public function findById(int $id): User
22    {
23        $result = $this->adapter->find($id);
24
25        if ($result === null) {
26            throw new InvalidArgumentException("User #$id not found");
27        }
28
29        return $this->mapRowToUser($result);
30    }
31
32    private function mapRowToUser(array $row): User
33    {
34        return User::fromState($row);
35    }
36}

StorageAdapter.php

 1<?php
 2
 3declare(strict_types=1);
 4
 5namespace DesignPatterns\Structural\DataMapper;
 6
 7class StorageAdapter
 8{
 9    public function __construct(private array $data)
10    {
11    }
12
13    /**
14     * @return array|null
15     */
16    public function find(int $id)
17    {
18        if (isset($this->data[$id])) {
19            return $this->data[$id];
20        }
21
22        return null;
23    }
24}

2.4.5. Test

Tests/DataMapperTest.php

 1<?php
 2
 3declare(strict_types=1);
 4
 5namespace DesignPatterns\Structural\DataMapper\Tests;
 6
 7use InvalidArgumentException;
 8use DesignPatterns\Structural\DataMapper\StorageAdapter;
 9use DesignPatterns\Structural\DataMapper\User;
10use DesignPatterns\Structural\DataMapper\UserMapper;
11use PHPUnit\Framework\TestCase;
12
13class DataMapperTest extends TestCase
14{
15    public function testCanMapUserFromStorage()
16    {
17        $storage = new StorageAdapter([1 => ['username' => 'domnikl', 'email' => 'liebler.dominik@gmail.com']]);
18        $mapper = new UserMapper($storage);
19
20        $user = $mapper->findById(1);
21
22        $this->assertInstanceOf(User::class, $user);
23    }
24
25    public function testWillNotMapInvalidData()
26    {
27        $this->expectException(InvalidArgumentException::class);
28
29        $storage = new StorageAdapter([]);
30        $mapper = new UserMapper($storage);
31
32        $mapper->findById(1);
33    }
34}