# Handling Relations

API Platform Admin handles `to-one` and `to-many` relations automatically.

Thanks to [the Schema.org support](schema.org.md), you can easily display the name of a related resource instead of its IRI.

## Embedded Relations

If a relation is an array of [embeddeds or an embedded](../core/serialization.md#embedding-relations) resource, the admin will keep them by default.

The embedded data will be displayed as text field and editable as text input: the admin cannot determine the fields present in it.
To display the fields you want, see [this section](handling-relations.md#display-a-field-of-an-embedded-relation).

You can also ask the admin to automatically replace the embedded resources' data by their IRI,
by setting the `useEmbedded` parameter of the Hydra data provider to `false`.
Embedded data is inserted to a local cache: it will not be necessary to make more requests if you reference some fields of the embedded resource later on.

```javascript
// admin/src/App.js

import { HydraAdmin, fetchHydra, hydraDataProvider } from "@api-platform/admin";
import { parseHydraDocumentation } from "@api-platform/api-doc-parser";

const entrypoint = process.env.REACT_APP_API_ENTRYPOINT;

const dataProvider = hydraDataProvider({
    entrypoint,
    httpClient: fetchHydra,
    apiDocumentationParser: parseHydraDocumentation,
    mercure: true,
    useEmbedded: false,
});

export default () => (
  <HydraAdmin
      dataProvider={dataProvider}
      entrypoint={entrypoint}
  />
);
```

## Display a Field of an Embedded Relation

If you have an [embedded relation](../core/serialization.md#embedding-relations) and need to display a nested field, the code you need to write depends of the value of `useEmbedded` of the Hydra data provider.

If `true` (default behavior), you need to use the dot notation to display a field:

```javascript
import {
  HydraAdmin,
  FieldGuesser,
  ListGuesser,
  ResourceGuesser
} from "@api-platform/admin";
import { TextField } from "react-admin";

const BooksList = (props) => (
  <ListGuesser {...props}>
    <FieldGuesser source="title" />
    {/* Use react-admin components directly when you want complex fields. */}
    <TextField label="Author first name" source="author.firstName" />
  </ListGuesser>
);

export default () => (
  <HydraAdmin entrypoint={process.env.REACT_APP_API_ENTRYPOINT}>
    <ResourceGuesser
      name="books"
      list={BooksList}
    />
  </HydraAdmin>
);
```

If `useEmbedded` is explicitly set to `false`, make sure you write the code as if the relation needs to be fetched as a reference.

In this case, you *cannot* use the dot separator to do so.

Note that you cannot edit the embedded data directly with this behavior.

For instance, if your API returns:

```json
{
  "@context": "/contexts/Book",
  "@id": "/books",
  "@type": "hydra:Collection",
  "hydra:member": [
    {
      "@id": "/books/07b90597-542e-480b-a6bf-5db223c761aa",
      "@type": "https://schema.org/Book",
      "title": "War and Peace",
      "author": {
        "@id": "/authors/d7a133c1-689f-4083-8cfc-afa6d867f37d",
        "@type": "https://schema.org/Author",
        "firstName": "Leo",
        "lastName": "Tolstoi"
      }
    }
  ],
  "hydra:totalItems": 1
}
```

If you want to display the author first name in the list, you need to write the following code:

```javascript
import {
  HydraAdmin,
  FieldGuesser,
  ListGuesser,
  ResourceGuesser
} from "@api-platform/admin";
import { ReferenceField, TextField } from "react-admin";

const BooksList = (props) => (
  <ListGuesser {...props}>
    <FieldGuesser source="title" />
    {/* Use react-admin components directly when you want complex fields. */}
    <ReferenceField label="Author first name" source="author" reference="authors">
      <TextField source="firstName" />
    </ReferenceField>
  </ListGuesser>
);

export default () => (
  <HydraAdmin entrypoint={process.env.REACT_APP_API_ENTRYPOINT}>
    <ResourceGuesser
      name="books"
      list={BooksList}
    />
    <ResourceGuesser name="authors" />
  </HydraAdmin>
);
```

## Using an Autocomplete Input for Relations

Let's go one step further thanks to the [customization capabilities](customizing.md) of API Platform Admin by adding autocompletion support to form inputs for relations.

Let's consider an API exposing `Review` and `Book` resources linked by a `many-to-one` relation (through the `book` property).

This API uses the following PHP code:

```php
<?php
// api/src/Entity/Review.php
namespace App\Entity;

use ApiPlatform\Metadata\ApiResource;
use Doctrine\ORM\Mapping as ORM;

#[ORM\Entity]
#[ApiResource]
class Review
{
    #[ORM\Id, ORM\Column, ORM\GeneratedValue]
    public ?int $id = null;

    #[ORM\ManyToOne]
    public Book $book;
}
```

```php
<?php
// api/src/Entity/Book.php
namespace App\Entity;

use ApiPlatform\Metadata\ApiFilter;
use ApiPlatform\Metadata\ApiResource;
use ApiPlatform\Doctrine\Orm\Filter\SearchFilter;
use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\Common\Collections\Collection;
use Doctrine\ORM\Mapping as ORM;

#[ORM\Entity]
#[ApiResource]
class Book
{
    #[ORM\Id, ORM\Column, ORM\GeneratedValue]
    public ?int $id = null;

    #[ORM\Column] 
    #[ApiFilter(SearchFilter::class, strategy: 'ipartial')]
    public string $title;

    #[ORM\OneToMany(targetEntity: Review::class, mappedBy: 'book')] 
    public $reviews;

    public function __construct()
    {
        $this->reviews = new ArrayCollection();
    }
}
```

Notice the "partial search" [filter](../core/filters.md) on the `title` property of the `Book` resource class.

Now, let's configure API Platform Admin to enable autocompletion for the relation selector:

```javascript
import {
  HydraAdmin,
  ResourceGuesser,
  CreateGuesser,
  EditGuesser,
  InputGuesser
} from "@api-platform/admin";
import { ReferenceInput, AutocompleteInput } from "react-admin";

const ReviewsCreate = props => (
  <CreateGuesser {...props}>
    <InputGuesser source="author" />
    <ReferenceInput
      source="book"
      reference="books"
    >
      <AutocompleteInput
        filterToQuery={searchText => ({ title: searchText })}
        optionText="title"
        label="Books"
      />
    </ReferenceInput>

    <InputGuesser source="rating" />
    <InputGuesser source="body" />
    <InputGuesser source="publicationDate" />
  </CreateGuesser>
);

const ReviewsEdit = props => (
  <EditGuesser {...props}>
    <InputGuesser source="author" />

    <ReferenceInput
      source="book"
      reference="books"
    >
      <AutocompleteInput
        filterToQuery={searchText => ({ title: searchText })}
        optionText="title"
        label="Books"
      />
    </ReferenceInput>

    <InputGuesser source="rating" />
    <InputGuesser source="body" />
    <InputGuesser source="publicationDate" />
  </EditGuesser>
);

export default () => (
  <HydraAdmin entrypoint={process.env.REACT_APP_API_ENTRYPOINT}>
    <ResourceGuesser
      name="reviews"
      create={ReviewsCreate}
      edit={ReviewsEdit}
    />
  </HydraAdmin>
);
```

If the book is embedded into a review and if the `useEmbedded` parameter is `true` (default behavior),
you need to change the `ReferenceInput` for the edit component:

```javascript
import {
  HydraAdmin,
  ResourceGuesser,
  CreateGuesser,
  EditGuesser,
  InputGuesser
} from "@api-platform/admin";
import { ReferenceInput, AutocompleteInput } from "react-admin";

const ReviewsCreate = props => (
  <CreateGuesser {...props}>
    <InputGuesser source="author" />
    <ReferenceInput
      source="book"
      reference="books"
    >
      <AutocompleteInput
        filterToQuery={searchText => ({ title: searchText })}
        optionText="title"
        label="Books"
      />
    </ReferenceInput>

    <InputGuesser source="rating" />
    <InputGuesser source="body" />
    <InputGuesser source="publicationDate" />
  </CreateGuesser>
);

const ReviewsEdit = props => (
  <EditGuesser {...props}>
    <InputGuesser source="author" />

    <ReferenceInput
      source="book"
      reference="books"
    >
      <AutocompleteInput
        filterToQuery={searchText => ({ title: searchText })}
        format={v => v['@id'] || v}
        optionText="title"
        label="Books"
      />
    </ReferenceInput>

    <InputGuesser source="rating" />
    <InputGuesser source="body" />
    <InputGuesser source="publicationDate" />
  </EditGuesser>
);

export default () => (
  <HydraAdmin entrypoint={process.env.REACT_APP_API_ENTRYPOINT}>
    <ResourceGuesser
      name="reviews"
      create={ReviewsCreate}
      edit={ReviewsEdit}
    />
  </HydraAdmin>
);
```

The autocomplete field should now work properly!