Generating Resources

Generating Resources from PHP Objects

In the previous chapter, we discussed links and resources. The primitive objects allow us to create representations easily, but do not answer one critical question: how can we create resources based on existing PHP object types?

To answer that question, we provide two related features: metadata, and a resource generator.

Metadata

Metadata allows you to detail the requirements for generating a HAL representation of a PHP object. Metadata might include:

The PHP class name to represent.
A URI to use for the generated resource's self relational link.
Alternately, routing information to use with the LinkGenerator.
A laminas-hydrator extractor to use to serialize the object to a representation.
Whether or not the resource is a collection, and, if so, whether pagination is handled as a path parameter or a query string argument, the name of the parameter, etc.

All metadata types inherit from Mezzio\Hal\Metadata\AbstractMetadata, which defines a single method, getClass(), for retrieving the name of the PHP class to represent; all metadata are expected to inherit from this class.

The component also provides four concrete metadata types, requiring the following information:

Mezzio\Hal\Metadata\RouteBasedCollectionMetadata:
- string $class
- string $collectionRelation
- string $route
- string $paginationParam = 'page' (name of the parameter indicating the current page of results)
- string $paginationParamType = self::TYPE_QUERY (one of "query" or "placeholder")
- array $routeParams = [] (associative array of substitutions to use with the designated route)
- array $queryStringArguments = [] (associative array of query string arguments to include in the generated URI)
Mezzio\Hal\Metadata\RouteBasedResourceMetadata:
- string $class
- string $route
- string $extractor (string service name of the laminas-hydrator hydrator to use for extracting data from the instance)
- string $resourceIdentifier = 'id' (name of the property uniquely identifying the resource)
- array $routeParams = [] (associative array of additional routing parameters to substitute when generating the URI)
- array $identifiersToPlacholdersMapping = [] (associative array mapping resource properties to route parameters, for use when generating the URI)
- int $maxDepth = 10 max allowed nesting levels.
Mezzio\Hal\Metadata\UrlBasedCollectionMetadata:
- string $class
- string $collectionRelation
- string $url
- string $paginationParam = 'page' (name of the parameter indicating the current page of results)
- string $paginationParamType = self::TYPE_QUERY (one of "query" or "placeholder")
Mezzio\Hal\Metadata\UrlBasedResourceMetadata
- string $class
- string $url
- string $extractor (string service name of the laminas-hydrator hydrator to use for extracting data from the instance)

We aggregate metadata in a Mezzio\Hal\Metadata\MetadataMap instance:

$bookMetadata = new RouteBasedResourceMetadata(
    Book::class,
    'book',
    ObjectPropertyHydrator::class
);
$booksMetadata = new RouteBasedCollectionMetadata(
    BookCollection::class,
    'book',
    'books',
);

$metadataMap = new MetadataMap();
$metadataMap->add($bookMetadata);
$metadataMap->add($booksMetadata);

Configuration-based metadata

To automate generation of the MetadataMap, we provide Mezzio\Hal\Metadata\MetadataMapFactory. This factory may be used with any PSR-11 container. It utilizes the config service, and pulls its configuration from a key named after the Mezzio\Hal\Metadata\MetadataMap class.

Each item in the map will be an associative array. The member __class__ will describe which metadata class to create, and the remaining properties will then be used to generate an instance. As an example, the above could be configured as follows:

use Api\Books\Book;
use Api\Books\BookCollection;
use Mezzio\Hal\Metadata\MetadataMap;
use Mezzio\Hal\Metadata\RouteBasedCollectionMetadata;
use Mezzio\Hal\Metadata\RouteBasedResourceMetadata;
use Laminas\Hydrator\ObjectProperty;

return [
    'Mezzio\Hal\Metadata\MetadataMap' => [
        [
            '__class__' => RouteBasedResourceMetadata::class,
            'resource_class' => Book::class,
            'route' => 'book',
            'extractor' => ObjectProperty::class,
        ],
        [
            '__class__' => RouteBasedCollectionMetadata::class,
            'collection_class' => BookCollection::class,
            'collection_relation' => 'book',
            'route' => 'books',
        ],
    ],
];

The rest of the parameters follow underscore delimiter naming convention:

RouteBasedResourceMetadata::class
- resource_identifier (name of the property uniquely identifying the resource)
- route_params (associative array of substitutions to use with the designated route)
- identifiers_to_placeholders_mapping (associative array mapping resource properties to routing parameters, for use when generating the URI)
- max_depth (maximum depth to allow when rendering nested resources; defaults to 10)
RouteBasedCollectionMetadata::class
- pagination_param (name of the parameter indicating the current page of results)
- pagination_param_type (one of "query" or "placeholder")
- route_params (associative array of substitutions to use with the designated route)
- query_string_arguments (associative array of additional routing parameters to substitute when generating the URI)
UrlBasedResourceMetadata::class
- resource_class
- url
- extractor
UrlBasedCollectionMetadata::class
- collection_class
- collection_relation
- url
- pagination_param (name of the parameter indicating the current page of results)
- pagination_param_type (one of "query" or "placeholder")

ResourceGenerator

Once you have defined the metadata for the various objects you will represent in your API, you can start generating resources.

Mezzio\Hal\ResourceGenerator has the following constructor:

public function __construct(
    Mezzio\Hal\Metadata\MetadataMap $metadataMap,
    Psr\Container\ContainerInterface $hydrators,
    Mezzio\Hal\LinkGenerator $linkGenerator
) {

We described the MetadataMap in the previous section, and the LinkGenerator in the previous chapter.

Hydrators are defind in the laminas-hydrator component, and are objects which can hdyrate associative arrays to object instances and extract associative arrays from object instances. Generally speaking, the $hydrators instance may be any PSR-11 container, but you will generally want to use the Laminas\Hydrator\HydratorPluginManager.

Once you have your instance created, you can start generating resources:

$resource = $resourceGenerator->fromObject($book, $request);

(Where $request is a Psr\Http\Message\ServerRequestInterface instance; the instance is passed along to the LinkGenerator in order to generate route-based URIs for Link instances.)

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