Schema
GraphQL Unions
Using Union Types
GraphQL unions group unrelated types for flexible queries.
Introduction to GraphQL Unions
GraphQL unions allow you to group different types that are not related to each other into a single field. This feature provides a way to perform flexible and dynamic queries where the result can be one of several types. Unlike interfaces, unions do not share any common fields.
This capability is particularly useful when you need a field that can return multiple types, enabling more powerful and flexible API designs.
Defining Unions in GraphQL Schema
Defining a union in a GraphQL schema is straightforward. You use the union keyword followed by the union name and the possible types it can include. Here's a basic example of how to define a union:
In this example, SearchResult is a union type that can return either a User, a Product, or an Article. Each of these types can be queried under the SearchResult field.
Using Unions in Queries
When querying a union type, you need to use inline fragments to specify which fields you want to retrieve from each possible type. Below is an example of how to query a union type:
In this query, the search field returns a SearchResult. Depending on the actual type of the result, the query retrieves different fields: id and name for User, id and price for Product, and id and title for Article.
Benefits of Using Unions
Unions provide several benefits in GraphQL schemas:
- Flexibility: They allow fields to return different types, making APIs more adaptable to diverse data requirements.
- Expressiveness: Developers can define complex query structures and retrieve varied data sets in a single request.
- Efficiency: By consolidating types into a union, you can optimize the response payload for specific client needs.
Limitations of Unions
While unions are powerful, they have some limitations:
- Unions cannot have common fields, unlike interfaces. This means you can't rely on shared fields across types within a union.
- Resolvers for union types need to include logic to determine which type to return.
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