Fragments

A GraphQL fragment is a piece of logic that can be shared between multiple queries and mutations.

Here's the declaration of a NameParts fragment that can be used with any Person object:

fragment NameParts on Person {
  firstName
  lastName
}

Every fragment includes a subset of the fields that belong to its associated type. In the above example, the Person type must declare firstName and lastName fields for the NameParts fragment to be valid.

We can now include the NameParts fragment in any number of queries and mutations that refer to Person objects, like so:

query GetPerson {
  people(id: "7") {
    ...NameParts
    avatar(size: LARGE)
  }
}

You precede an included fragment with three periods (...), much like JavaScript spread syntax.

Based on our NameParts definition, the above query is equivalent to:

query GetPerson {
  people(id: "7") {
    firstName
    lastName
    avatar(size: LARGE)
  }
}

If we later change which fields are included in the NameParts fragment, we automatically change which fields are included in operations that use the fragment. This reduces the effort required to keep fields consistent across a set of operations.

Example usage

Let's say we have a blog application that executes several GraphQL operations related to comments (submitting a comment, fetching a post's comments, etc.). These operations probably all include certain fields of a Comment type.

To specify this core set of fields, we can define a fragment on the Comment type, like so:

fragments.js

import { gql } from '@apollo/client';

export const CORE_COMMENT_FIELDS = gql`
  fragment CoreCommentFields on Comment {
    id
    postedBy {
      username
      displayName
    }
    createdAt
    content
  }
`;

You can declare fragments in any file of your application. The example above exports the fragment from a fragments.js file.

We can then include the CoreCommentFields fragment in a GraphQL operation like so:

PostDetails.jsx

import { gql } from '@apollo/client';
import { CORE_COMMENT_FIELDS } from './fragments';
export const GET_POST_DETAILS = gql`
  ${CORE_COMMENT_FIELDS}  query CommentsForPost($postId: ID!) {
    post(postId: $postId) {
      title
      body
      author
      comments {
        ...CoreCommentFields      }
    }
  }
`;

// ...PostDetails component definition...

We first import CORE_COMMENT_FIELDS because it's declared in another file.
We add our fragment definition to the GET_POST_DETAILS gql template literal via a placeholder (${CORE_COMMENT_FIELDS})
We include the CoreCommentFields fragment in our query with standard ... notation.

Colocating fragments

The tree-like structure of a GraphQL response resembles the hierarchy of a frontend's rendered components. Because of this similarity, you can use fragments to split query logic up between components, so that each component requests exactly the fields that it uses. This helps you make your component logic more succinct.

Consider the following view hierarchy for an app:

FeedPage
└── Feed
    └── FeedEntry
        ├── EntryInfo
        └── VoteButtons

In this app, the FeedPage component executes a query to fetch a list of FeedEntry objects. The EntryInfo and VoteButtons subcomponents need specific fields from the enclosing FeedEntry object.

Creating colocated fragments

A colocated fragment is just like any other fragment, except it's attached to a particular component that uses the fragment's fields. For example, the VoteButtons child component of FeedPage might use the fields score and vote { choice } from the FeedEntry object:

VoteButtons.jsx

VoteButtons.fragments = {
  entry: gql`
    fragment VoteButtonsFragment on FeedEntry {
      score
      vote {
        choice
      }
    }
  `,
};

After you define a fragment in a child component, the parent component can refer to it in its own colocated fragments, like so:

FeedEntry.jsx

FeedEntry.fragments = {
  entry: gql`
    fragment FeedEntryFragment on FeedEntry {
      commentCount
      repository {
        full_name
        html_url
        owner {
          avatar_url
        }
      }
      ...VoteButtonsFragment
      ...EntryInfoFragment
    }
    ${VoteButtons.fragments.entry}
    ${EntryInfo.fragments.entry}
  `,
};

There's nothing special about the naming of VoteButtons.fragments.entry or EntryInfo.fragments.entry. Any naming convention works as long as you can retrieve a component's fragments given the component.

Importing fragments when using Webpack

When loading .graphql files with graphql-tag/loader, we can include fragments using import statements. For example:

#import "./someFragment.graphql"

This makes the contents of someFragment.graphql available to the current file. See the Webpack Fragments section for additional details.

Using fragments with unions and interfaces

You can define fragments on unions and interfaces.

Here's an example of a query that includes three in-line fragments:

query AllCharacters {
  all_characters {

    ... on Character {
      name
    }

    ... on Jedi {
      side
    }

    ... on Droid {
      model
    }
  }
}

The all_characters query above returns a list of Character objects. The Character type is an interface that both the Jedi and Droid types implement. Each item in the list includes a side field if it's an object of type Jedi, and it includes a model field if it's of type Droid.

However, for this query to work, your client needs to understand the polymorphic relationship between the Character interface and the types that implement it. To inform the client about these relationships, you can pass a possibleTypes option when you initialize your InMemoryCache.

Defining `possibleTypes` manually

The possibleTypes option is available in Apollo Client 3.0 and later.

You can pass a possibleTypes option to the InMemoryCache constructor to specify supertype-subtype relationships in your schema. This object maps the name of an interface or union type (the supertype) to the types that implement or belong to it (the subtypes).

Here's an example possibleTypes declaration:

const cache = new InMemoryCache({
  possibleTypes: {
    Character: ["Jedi", "Droid"],
    Test: ["PassingTest", "FailingTest", "SkippedTest"],
    Snake: ["Viper", "Python"],
  },
});

This example lists three interfaces (Character, Test, and Snake) and the object types that implement them.

If your schema includes only a few unions and interfaces, you can probably specify your possibleTypes manually without issue. However, as your schema grows in size and complexity, you should consider generating possibleTypes automatically from your schema.

Generating `possibleTypes` automatically

The following example script translates a GraphQL introspection query into a possibleTypes configuration object:

const fetch = require('cross-fetch');
const fs = require('fs');

fetch(`${YOUR_API_HOST}/graphql`, {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({
    variables: {},
    query: `
      {
        __schema {
          types {
            kind
            name
            possibleTypes {
              name
            }
          }
        }
      }
    `,
  }),
}).then(result => result.json())
  .then(result => {
    const possibleTypes = {};

    result.data.__schema.types.forEach(supertype => {
      if (supertype.possibleTypes) {
        possibleTypes[supertype.name] =
          supertype.possibleTypes.map(subtype => subtype.name);
      }
    });

    fs.writeFile('./possibleTypes.json', JSON.stringify(possibleTypes), err => {
      if (err) {
        console.error('Error writing possibleTypes.json', err);
      } else {
        console.log('Fragment types successfully extracted!');
      }
    });
  });

You can then import the generated possibleTypes JSON module into the file where you create your InMemoryCache:

import possibleTypes from './path/to/possibleTypes.json';

const cache = new InMemoryCache({
  possibleTypes,
});