Support creating nested objects in single mutation

[sorenbs]

Currently to create an object and link it to another object you have to perform two mutations:

mutation {
  createCity(name: "Berlin", knownFor: "cool GraphQL people")
}

mutation {
  createUser(name: "carl", cityId: "cityId")
}

This is a common enough use case that we want to support it in a single mutation. The straight forward implementation would be:

mutation {
  createUser(name: "carl", city: {name: "Berlin", knownFor: "cool GraphQL people"})
}

We still need to support linking existing objects. We could support it by having the input type for city contain optional fields for all types on the City model including the id and do runtime checks to ensure that all required constraints are satisfied:

mutation {
  createUser(name: "carl", city: {id: "cityId"})
}

It would be better if we could encode in the type system that the shape of the city input field should be {id!} || {name!, knownFor}

[sorenbs]

See graphql/graphql-js#207