Unions and interfaces

Unions and interfaces are abstract GraphQL types that enable a schema field to return one of multiple object types.

Union type

When you define a union type, you declare which object types are included in the union:

union Media = Book | Movie

A field can have a union as its return type. In this case, it can return any object type that's included in the union:

 type Query {
  allMedia: [Media] # This list can include both Books and Movies
}

All of a union's included types must be object types (not scalars, input types, etc.). Included types do not need to share any fields.

Example

The following schema defines a Result union type that can return either a Book or an Author:

union Result = Book | Author

type Book {
  title: String
}

type Author {
  name: String
}

type Query {
  search(contains: String): [Result]
}

The Result union enables Query.search to return a list that includes both Books and Authors.

Querying a union

GraphQL clients don't know which object type a field will return if the field's return type is a union. To account for this, a query can include the subfields of multiple possible types.

Here's a valid query for the schema above:

query GetSearchResults {
  search(contains: "Shakespeare") {
    ... on Book {
      title
    }
    ... on Author {
      name
    }
  }
}

This query uses inline fragments to fetch a Result's title (if it's a Book) or its name (if it's an Author).

For more information, see Using fragments with unions and interfaces.

Resolving a union

To fully resolve a union, Apollo Server needs to specify which of the union's types is being returned. To achieve this, you define a __resolveType function for the union in your resolver map.

The __resolveType function uses a returned object's fields to determine its type. It then returns the name of that type as a string.

Here's an example __resolveType function for the Result union defined above:

const resolvers = {
  Result: {
    __resolveType(obj, context, info){      if(obj.name){        return 'Author';      }      if(obj.title){        return 'Book';      }      return null; // GraphQLError is thrown    },  },
  Query: {
    search: () => { ... }
  },
};

const server = new ApolloServer({
  typeDefs,
  resolvers,
});

server.listen().then(({ url }) => {
  console.log(`🚀 Server ready at ${url}`)
});

If a __resolveType function returns any value that isn't the name of a valid type, the associated operation produces a GraphQL error.

Interface type

An interface specifies a set of fields that multiple object types can include:

interface Book {
  title: String
  author: Author
}

If an object type implements an interface, it must include all of that interface's fields:

type Textbook implements Book {
  title: String # Must be present
  author: Author # Must be present
  courses: [Course]
}

A field can have an interface as its return type. In this case, it can return any object type that implements that interface:

 type Query {
  schoolBooks: [Book] # Can include Textbooks
}

Example

The following schema defines a Book interface, along with two object types that implement it:

interface Book {
  title: String
  author: Author
}

type Textbook implements Book {
  title: String
  author: Author
  courses: [Course]
}

type ColoringBook implements Book {
  title: String
  author: Author
  colors: [Color]
}

type Query {
  schoolBooks: [Book]
}

In this schema, Query.schoolBooks returns a list that can include both Textbooks and ColoringBooks.

Querying an interface

If a field's return type is an interface, clients can query that field for any subfields included in the interface:

query GetBooks {
  schoolBooks {
    title
    author
  }
}

Clients can also query for subfields that aren't included in the interface:

query GetBooks {
  schoolBooks {
    title # Always present (part of Book interface)
    ... on Textbook {
      courses { # Only present in Textbook
        name
      }
    }
    ... on ColoringBook {
      colors { # Only present in ColoringBook
        name
      }
    }
  }
}

This query uses inline fragments to fetch a Book's courses (if it's a Textbook) or its colors (if it's a ColoringBook).

For more information, see Using fragments with unions and interfaces.

Resolving an interface

As with union types, Apollo Server requires interfaces to define a __resolveType function to determine which implementing object type is being returned.

Here's an example __resolveType function for the Book interface defined above:

const resolvers = {
  Book: {
    __resolveType(book, context, info){      if(book.courses){        return 'Textbook';      }      if(book.colors){        return 'ColoringBook';      }      return null; // GraphQLError is thrown    },  },
  Query: {
    schoolBooks: () => { ... }
  },
};