Using Fragments

A GraphQL fragment is a shared piece of query logic.

fragment NameParts on Person {
  firstName
  lastName
}

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

There are two principal uses for fragments in Apollo:

• Sharing fields between multiple queries, mutations or subscriptions.
• Breaking your queries up to allow you to co-locate field access with the places they are used.

Reusing Fragments

The most straightforward use of fragments is to reuse parts of queries (or mutations or subscriptions) in various parts of your application. For instance, in GitHunt on the comments page, we want to fetch the same fields after posting a comment as we originally query. This way we can be sure that we render consistent comment objects as the data changes.

To do so, we can simply share a fragment describing the fields we need for a comment:

import { gql } from 'apollo-angular';

CommentsPage.fragments = {
  comment: gql`
    fragment CommentsPageComment on Comment {
      id
      postedBy {
        login
        html_url
      }
      createdAt
      content
    }
  `,
};

We put the fragment on CommentsPage.fragments.comment by convention, and use the familiar gql helper to create it.

When it’s time to embed the fragment in a query, we simply use the ...Name syntax in our GraphQL, and embed the fragment inside our query GraphQL document:

const SUBMIT_COMMENT_MUTATION = gql`
  mutation submitComment($repoFullName: String!, $commentContent: String!) {
    submitComment(repoFullName: $repoFullName, commentContent: $commentContent) {
      ...CommentsPageComment
    }
  }
  ${CommentsPage.fragments.comment}
`;

export const COMMENT_QUERY = gql`
  query Comment($repoName: String!) {
    # ...
    entry(repoFullName: $repoName) {
      # ...
      comments {
        ...CommentsPageComment
      }
      # ...
    }
  }
  ${CommentsPage.fragments.comment}
`;

Collocating Fragments

A key advantage of GraphQL is the tree-like nature of the response data, which in many cases mirrors your rendered component hierarchy. This, combined with GraphQL’s support for fragments, allows you to split your queries up in such a way that the various fields fetched by the queries are located right alongside the code that uses the field.

Although this technique doesn’t always make sense (for instance it’s not always the case that the GraphQL schema is driven by the UI requirements), when it does, it’s possible to use some patterns in Apollo client to take full advantage of it.

Imagine this view hierarchy:

FeedPage
└── Feed
    └── FeedEntry
        ├── RepoInfo
        └── VoteButtons

The FeedPage conducts a query to fetch a list of Entrys, and each of the subcomponents requires different subfields of each Entry.

Creating Fragments

To create the fragments, we again use the gql helper and attach to subfields of ComponentClass.fragments, for example:

VoteButtons.fragments = {
  entry: gql`
    fragment VoteButtons on Entry {
      score
      vote {
        vote_value
      }
    }
  `,
};

If our fragments include sub-fragments then we can pass them into the gql helper:

FeedEntry.fragments = {
  entry: gql`
    fragment FeedEntry on Entry {
      commentCount
      repository {
        full_name
        html_url
        owner {
          avatar_url
        }
      }
      ...VoteButtons
      ...RepoInfo
    }
    ${VoteButtons.fragments.entry}
    ${RepoInfo.fragments.entry}
  `,
};

Importing Fragments When Using webpack

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

#import "./someFragment.graphql"

Will make 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 creating the InMemoryCache.

Defining possibleTypes Manually

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.

Auto-Generating possibleTypes

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.

Here’s an example of using possibleTypes declaration generated by GraphQL Codegen:

// generated by Fragment Matcher plugin
import { possibleTypes } from '../possible-types';

const cache = new InMemoryCache({ possibleTypes });