# Data Providers

To retrieve data exposed by the API, API Platform uses classes called **data providers**. A data provider using [Doctrine
ORM](http://www.doctrine-project.org/projects/orm.html) to retrieve data from a database is included with the library and
is enabled by default. This data provider natively supports paged collections and filters. It can be used as is and fits
perfectly with common usages.

However, you sometime want to retrieve data from other sources such as another persistence layer, a webservice, ElasticSearch
or MongoDB.
Custom data providers can be used to do so. A project can include as many data providers as it needs. The first able to
retrieve data for a given resource will be used.

For a given resource, you can implement two kind of interfaces:

* the [`CollectionDataProviderInterface`](https://github.com/api-platform/core/blob/master/src/DataProvider/CollectionDataProviderInterface.php)
  is used when fetching a collection.
* the [`ItemDataProviderInterface`](https://github.com/api-platform/core/blob/master/src/DataProvider/ItemDataProviderInterface.php)
  is used when fetching items.

Both implementations can also implement a third, optional interface called
['RestrictedDataProviderInterface'](https://github.com/api-platform/core/blob/master/src/DataProvider/RestrictedDataProviderInterface.php)
if you want to limit their effects to a single resource or operation.

In the following examples we will create custom data providers for an entity class called `App\Entity\BlogPost`.
Note, that if your entity is not Doctrine-related, you need to flag the identifier property by using `@ApiProperty(identifier=true)` for things to work properly (see also [Entity Identifier Case](serialization.md#entity-identifier-case)).

## Custom Collection Data Provider

First, your `BlogPostCollectionDataProvider` has to implement the [`CollectionDataProviderInterface`](https://github.com/api-platform/core/blob/master/src/DataProvider/CollectionDataProviderInterface.php):

The `getCollection` method must return an `array`, a `Traversable` or a [`ApiPlatform\Core\DataProvider\PaginatorInterface`](https://github.com/api-platform/core/blob/master/src/DataProvider/PaginatorInterface.php) instance.
If no data is available, you should return an empty array.

```php
<?php
// api/src/DataProvider/BlogPostCollectionDataProvider.php

namespace App\DataProvider;

use ApiPlatform\Core\DataProvider\CollectionDataProviderInterface;
use ApiPlatform\Core\DataProvider\RestrictedDataProviderInterface;
use ApiPlatform\Core\Exception\ResourceClassNotSupportedException;
use App\Entity\BlogPost;

final class BlogPostCollectionDataProvider implements CollectionDataProviderInterface, RestrictedDataProviderInterface
{
    public function supports(string $resourceClass, string $operationName = null, array $context = []): bool
    {
        return BlogPost::class === $resourceClass;
    }

    public function getCollection(string $resourceClass, string $operationName = null): \Generator
    {
        // Retrieve the blog post collection from somewhere
        yield new BlogPost(1);
        yield new BlogPost(2);
    }
}
```

If you use the default configuration, the corresponding service will be automatically registered thanks to [autowiring](https://symfony.com/doc/current/service_container/autowiring.html).
To declare the service explicitly, or to set a custom priority, you can use the following snippet:

```yaml
# api/config/services.yaml
services:
    # ...
    'App\DataProvider\BlogPostCollectionDataProvider':
        tags: [ { name: 'api_platform.collection_data_provider', priority: 2 } ]
        # Autoconfiguration must be disabled to set a custom priority
        autoconfigure: false
```

Tagging the service with the tag `api_platform.collection_data_provider` will enable API Platform Core to automatically
register and use this data provider. The optional attribute `priority` allows to define the order in which the
data providers are called. The first data provider not throwing a `ApiPlatform\Core\Exception\ResourceClassNotSupportedException`
will be used.

## Custom Item Data Provider

The process is similar for item data providers. Create a `BlogPostItemDataProvider` implementing the [`ItemDataProviderInterface`](https://github.com/api-platform/core/blob/master/src/DataProvider/ItemDataProviderInterface.php)
interface:

The `getItem` method can return `null` if no result has been found.

```php
<?php
// api/src/DataProvider/BlogPostItemDataProvider.php

namespace App\DataProvider;

use ApiPlatform\Core\DataProvider\ItemDataProviderInterface;
use ApiPlatform\Core\DataProvider\RestrictedDataProviderInterface;
use ApiPlatform\Core\Exception\ResourceClassNotSupportedException;
use App\Entity\BlogPost;

final class BlogPostItemDataProvider implements ItemDataProviderInterface, RestrictedDataProviderInterface
{
    public function supports(string $resourceClass, string $operationName = null, array $context = []): bool
    {
        return BlogPost::class === $resourceClass;
    }

    public function getItem(string $resourceClass, $id, string $operationName = null, array $context = []): ?BlogPost
    {
        // Retrieve the blog post item from somewhere then return it or null if not found
        return new BlogPost($id);
    }
}
```

If service autowiring and autoconfiguration are enabled (it's the case by default), you are done!

Otherwise, if you use a custom dependency injection configuration, you need to register the corresponding service and add the
`api_platform.item_data_provider` tag to it. As for collection data providers, the `priority` attribute can be used to order
providers.

```yaml
# api/config/services.yaml
services:
    # ...
    'App\DataProvider\BlogPostItemDataProvider': ~
        # Uncomment only if autoconfiguration is disabled
        #tags: [ 'api_platform.item_data_provider' ]
```

## Injecting the Serializer in an `ItemDataProvider`

In some cases, you may need to inject the `Serializer` in your `DataProvider`. There are no issues with the
`CollectionDataProvider`, but when injecting it in the `ItemDataProvider` it will throw a `CircularReferenceException`.

For this reason, we implemented the `SerializerAwareDataProviderInterface`:

```php
<?php
// api/src/DataProvider/BlogPostItemDataProvider.php

namespace App\DataProvider;

use ApiPlatform\Core\DataProvider\ItemDataProviderInterface;
use ApiPlatform\Core\DataProvider\SerializerAwareDataProviderInterface;
use ApiPlatform\Core\DataProvider\SerializerAwareDataProviderTrait;
use ApiPlatform\Core\Exception\ResourceClassNotSupportedException;
use App\Entity\BlogPost;

final class BlogPostItemDataProvider implements ItemDataProviderInterface, SerializerAwareDataProviderInterface
{
    use SerializerAwareDataProviderTrait;

    public function supports(string $resourceClass, string $operationName = null, array $context = []): bool
    {
        return BlogPost::class === $resourceClass;
    }

    public function getItem(string $resourceClass, $id, string $operationName = null, array $context = []): ?BlogPost
    {
        // Retrieve data from anywhere you want, in a custom format
        $data = '...';

        // Deserialize data using the Serializer
        return $this->getSerializer()->deserialize($data, BlogPost::class, 'custom');
    }
}
```

## Injecting Extensions (Pagination, Filter, EagerLoading etc.)

ApiPlatform provides a few extensions that you can reuse in your custom DataProvider.
Note that there are a few kind of extensions which are detailed in [their own chapter of the documentation](extensions.md).
Because extensions are tagged services, you can use the [injection of tagged services](https://symfony.com/blog/new-in-symfony-3-4-simpler-injection-of-tagged-services):

```yaml
services:
    'App\DataProvider\BlogPostItemDataProvider':
        arguments:
          $itemExtensions: !tagged api_platform.doctrine.orm.query_extension.item
```

Or in XML:

```xml
<services>
    <service id="App\DataProvider\BlogPostItemDataProvider">
        <argument key="$itemExtensions" type="tagged" tag="api_platform.doctrine.orm.query_extension.item" />
    </service>
</services>
```

Your data provider will now have access to the core extensions, here is an example on how to use them:

```php
<?php
// api/src/DataProvider/BlogPostItemDataProvider.php

namespace App\DataProvider;

use ApiPlatform\Core\Bridge\Doctrine\Orm\Util\QueryNameGenerator;
use ApiPlatform\Core\DataProvider\ItemDataProviderInterface;
use ApiPlatform\Core\DataProvider\RestrictedDataProviderInterface;
use ApiPlatform\Core\Exception\ResourceClassNotSupportedException;
use App\Entity\BlogPost;
use Doctrine\Common\Persistence\ManagerRegistry;

final class BlogPostItemDataProvider implements ItemDataProviderInterface, RestrictedDataProviderInterface
{
    private $itemExtensions;
    private $managerRegistry;

    public function __construct(ManagerRegistry $managerRegistry, iterable $itemExtensions)
    {
      $this->managerRegistry = $managerRegistry;
      $this->itemExtensions = $itemExtensions;
    }

    public function supports(string $resourceClass, string $operationName = null, array $context = []): bool
    {
        return BlogPost::class === $resourceClass;
    }

    public function getItem(string $resourceClass, $id, string $operationName = null, array $context = []): ?BlogPost
    {
        $manager = $this->managerRegistry->getManagerForClass($resourceClass);
        $repository = $manager->getRepository($resourceClass);
        $queryBuilder = $repository->createQueryBuilder('o');
        $queryNameGenerator = new QueryNameGenerator();
        $identifiers = ['id' => $id];

        foreach ($this->itemExtensions as $extension) {
            $extension->applyToItem($queryBuilder, $queryNameGenerator, $resourceClass, $identifiers, $operationName, $context);
            if ($extension instanceof QueryResultItemExtensionInterface && $extension->supportsResult($resourceClass, $operationName, $context))                 {
                return $extension->getResult($queryBuilder, $resourceClass, $operationName, $context);
            }
        }

        return $queryBuilder->getQuery()->getOneOrNullResult();
    }
}

```