A union type is a type that may be any of a list of possible objects.
GraphQL Spec
Read about :spec:`unions <Unions>`.
A union is a type defined in terms of different objects:
.. literalinclude:: _examples/union-definition.edn :language: clojure
A union definition must include a :members key, a sequence of object types.
The above example identifies the SearchResult` union type to be either a ``Person (with fields
name and age), or a Photo (with fields imageURL, title, height, and width).
Unions must define at least one type; each member type must be an object type (members may not be reference scalar types, interfaces, or other unions).
When a client makes a union request, they must use the fragment spread syntax to identify what is to be returned based on the runtime type of object:
{ search (term:"ewok") {
... on Person { name }
... on Photo { imageURL title }
}}This breaks down what will be returned in the result map based on the type of the value produced
by the search query. Sometimes there will be a name key in the result, and other times
an image-url and title key.
This may vary result by result even within a single request:
.. literalinclude:: _examples/unions-query-response.edn :language: clojure
Tip
When a field or operation type is a union, the field resolver may return any of a number of different concrete object types, and Lacinia has no way to determine which; this information must be :doc:`explicitly provided <resolve/type-tags>`.