Examples

Mutable

In these cases the DTO is just an explicit transport tool, and it is OK/expected that the DTO changes over its lifetime, and code will set, add, remove from it. Values are also often expected to be not set.

Most importantly: Copies of such an entity will cause side-effects to the original(s). This fact of mutability must be known to you as programmer to avoid bugs in your software.

$ownerDto = new OwnerDto();
$ownerDto->setName('The Owner');

$carDto = new CarDto();
$carDto->setOwner($ownerDto);

$otherCarDto = $carDto;

// A trivial example
$otherCarDto->getOwner()->setName('The new owner');

// You might not expect the original $carDto to also change it's value... Hopefully you do :)
$this->assertSame('The new owner', $otherCarDto->getOwner()->getName());
$this->assertSame('The new owner', $carDto->getOwner()->getName());

Immutable

Using immutable.dto.xml.

In these cases we want to make sure, the DTO, once created, is not changing anymore that easily. Required values are for sure set. Modifications create a new object, keeping the previous one unchanged for further use with that dataset.

$array = [
    'id' => 2,
    'author' => [
        'id' => 1,
        'name' => 'me'
    ],
    'title' => 'My title',
    'created' => new FrozenTime(time() - DAY),
];

$articleDto = new ArticleDto($array);
$this->assertInstanceOf(AbstractImmutableDto::class, $articleDto);

// A trivial example
$modifiedArticleDto = $articleDto->withTitle('My new title');
$this->assertSame('My new title', $modifiedArticleDto->getTitle());
$this->assertSame('My title', $articleDto->getTitle());

// A reason why we want to use immutable datetime objects (FrozenTime):
$created = $articleDto->getCreated();
$isToday = $created->addDay()->isToday();
// A mutable datetime inside $articleDto->getCreated() would now accidentally be modified
$this->assertTrue($isToday);

// But luckily we don't have this side effect with our immutable one.
$this->assertSame($created, $articleDto->getCreated());

ORM Entity to DTO

Using orm.dto.xml and immutable DTOs matching the entities (including primary keys, but without foreign keys).

This is not so much that it should be done, but that it can be done :) This would be much more valuable (and necessary) for active record pattern ORMs (maybe someone can provide a Laravel example?^^).

Let's first get a record from the DB, including its relations:

$articleEntity = $this->Articles->find()->contain(['Author', 'Tags')
    ->all()->toArray();

The data will most likely look like (this is from the OrmTest.php test case run):

object(TestApp\Model\Entity\Article) {
    'id' => (int) 2,
    'author' => object(TestApp\Model\Entity\Author) {
        'id' => (int) 1,
        'name' => 'me',
        ...
    
    },
    'title' => 'My title',
    'created' => object(Cake\I18n\FrozenTime) {
        ...
    },
    'tags' => [
        object(TestApp\Model\Entity\Tag) {

            'id' => (int) 3,
            'name' => 'Awesome',
            ...
        },
        object(TestApp\Model\Entity\Tag) {
            'id' => (int) 4,
            'name' => 'Shiny',
            ...
        }
    ],
    ...
}

Now let's put it into our DTO (including the relations):

$articleDto = new ArticleDto($articleEntity->toArray(), true);

We use the 2nd param (ignoreMissing) as true to allow fields to be present in the array that are not part of the DTO. Those will then just get lost (foreign keys like author_id for example, since we already got it in Author.id).

Let's then just toArray() that one to verify:

$result = $articleDto->touchedToArray();

And this is the result (you can verify it in the test case):

[
    'id' => 2,
    'author' => [
        'id' => 1,
        'name' => 'me'
    ],
    'title' => 'My title',
    'tags' => [
        [
            'id' => 3,
            'name' => 'Awesome'
        ],
        [
            'id' => 4,
            'name' => 'Shiny'
        ]
    ]
];

Since the view layer (or other parts of the system that only need the read-only data from your entities) does not need to care about the state of the entities (persisted yes/no, errors yes/no, ...), you can easily break it down to these simple containers to be read in the template:

echo h($articleDto->getTitle());

And you also then know: Based on the fact that required fields in this (immutable) entity must be there, this method cannot return an empty value.

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