Do GraphQL fields support polymorphism based on passed in arguments?

Question

I would like to define the following queries

{
// return all individuals
individuals {
    id
  }
}

// return ONE individual by id
individuals(id:"123") {
   id
  }
}

note that the query name is the same, only the parameters are different.

Today, the only workaround I found is to define different query names.

How can I define polymorphic queries? Is it even possible?

Answer 1

The fields accessible at the root of your GraphQL query are defined in a GraphQL schema as fields on the root query type, commonly simply named Query. This type is exactly the same as any other GraphQL Object type in a schema. So this question can be restated as:

Do GraphQL fields have the ability to return different results based on the arguments or argument types passed in?

The short answer is "no". One important property of GraphQL is that a particular field must always return the same type; this means that a field that returns an array must always return an array, and a field that returns an object type must always return that particular type, regardless of the arguments passed in.

However, there are a few ways you can achieve similar results to avoid needing to create too many different named fields:

1. GraphQL fields can have optional arguments. So for example if you have a field called individuals, you could use optional arguments to supply different kinds of filters, for example: individuals(search: "Dan") or individuals(status: "admin"), or individuals (with no arguments) or individuals(status: "admin", search: "Dan") (both arguments provided at once.
2. GraphQL fields can return a union of several object types. So if your field can return several different types of things, you can use a union. This is represented in the SearchResult example in the docs: http://graphql.org/learn/schema/#union-types Unfortunately that doesn't apply here since you can't have a union of an array with an object type.

In many cases though, you will need to create multiple fields because GraphQL doesn't currently have method overloading or polymorphism.