RFC: OneOf Input Objects

[benjie]

Follow up of the @oneField directive and the Tagged type.

Introducing: OneOf Input Objects.

OneOf Input Objects are a special variant of Input Objects where the type system asserts that exactly one of the fields must be set and non-null, all others being omitted. This is represented in introspection with the __Type.isOneOf: Boolean field, and in SDL via the @oneOf directive on the input object.

This variant of an input object introduces a form of input polymorphism to GraphQL.

Example 1 - addPet

The following PetInput oneof input object lets you choose between a number of potential input types:

input PetInput @oneOf {
  cat: CatInput
  dog: DogInput
  fish: FishInput
}

input CatInput { name: String!, numberOfLives: Int }
input DogInput { name: String!, wagsTail: Boolean }
input FishInput { name: String!, bodyLengthInMm: Int }

type Mutation {
  addPet(pet: PetInput!): Pet
}

Example 2 - user(by:)

Previously you may have had a situation where you had multiple ways to locate a user:

type Query {
  user(id: ID!): User
  userByEmail(email: String!): User
  userByUsername(username: String!): User
  userByRegistrationNumber(registrationNumber: Int!): User
}

with OneOf Input Objects you can now express this via a single field without loss of type safety:

input UserBy @oneOf {
  id: ID
  email: String
  username: String
  registrationNumber: Int
}
type Query {
  user(by: UserBy!): User
}

FAQ

Why is this a directive?

At its core, it's a property of the type that's exposed through introspection - much in the same way that deprecation is. There's nothing in introspection, nor in the types exposed through the reference implementation (new GraphQLInputObjectType({ name: "...", isOneOf: true, ... })) that relates to directives. It just happens to be that after I analysed a number of potential syntaxes (including keywords and alternative syntax) I've found that when representing the schema as SDL, using a directive to do so is the least invasive (all current GraphQL parsers can already parse it!) and none of the alternative syntaxes sufficiently justified the increased complexity they would introduce.

Why is this a good approach?

This approach, as a small change to existing types, is the easiest to adopt of any of the solutions we came up with to the InputUnion problem. It's also more powerful in that it allows additional types to be part of the "input union" - in fact any valid input type is allowed: input objects, scalars, enums, and lists of the same. Further it can be used on top of existing GraphQL tooling, so it can be adopted much sooner. Finally it's very explicit, so doesn't suffer the issues that "duck typed" input unions could face.

Why did you go full circle via the tagged type?

When the @oneField directive was proposed some members of the community felt that augmenting the behaviour of existing types might not be the best approach, so the Tagged type was born. (We also researched a lot of other approaches too.) However, the Tagged type brought with it a lot of complexity and controversy, and the Input Unions Working Group decided that we should revisit the simpler approach again. This time around I'm a lot better versed in writing spec edits 😁

Why are all the fields nullable? Shouldn't they be non-nullable?

To make this change minimally invasive I wanted:

• to make it so that existing GraphQL clients could still validate queries against a oneOf-enabled GraphQL schema (if the fields were non-nullable the clients would think the query was invalid because it didn't supply enough data)
• to allow existing GraphQL implementations to change as little code as possible

To accomplish this, we add the "exactly one value, and that value is non-null" as a validation rule that runs after all the existing validation rules - it's an additive change.

Can this allow a field to accept both a scalar and an object?

Yes!

type Query {
  findUser(by: FindUserBy!): User
}

input FindUserBy @oneOf {
  id: ID
  organizationAndRegistrationNumber: OrganizationAndRegistrationNumberInput
}

input OrganizationAndRegistrationNumberInput {
  organizationId: ID!
  registrationNumber: Int!
}

Can I use existing GraphQL clients to issue requests to OneOf-enabled schemas?

Yes - so long as you stick to the rules of one field / one argument manually - note that GraphQL already differentiates between a field not being supplied and a field being supplied with the value null.

Without explicit client support you may lose a little type safety, but all major GraphQL clients can already speak this language. Given this nonsense schema:

type Query {
  foo(by: FooBy!): String
}
input FooBy @oneOf {
  id: ID
  str1: String
  str2: String
}

the following are valid queries that you could issue from existing GraphQL clients:

• {foo(by:{id: "..."})}
• {foo(by:{str1: "..."})}
• {foo(by:{str2: "..."})}
• query Foo($by: FooBy!) {foo(by: $by)}

If my input object has only one field, should I use @oneOf?

Doing so would preserve your option value - making a OneOf Input Object into a regular Input Object is a non-breaking change (the reverse is a breaking change). In the case of having one field on your type changing it from oneOf (and nullable) to regular and non-null is a non-breaking change (the reverse is also true in this degenerate case). The two Example types below are effectively equivalent - both require that value is supplied with a non-null int:

input Example @oneOf {
  value: Int
}

input Example {
  value: Int!
}

Can we expand @oneOf to output types to allow for unions of objects, interfaces, scalars, enums and lists; potentially replacing the union type?

🤫 👀 😉

[wyfo]

Is @oneOf missing in the example of section Can this allow a field to accept both a scalar and an object?

[wyattjoh]

I’d worry that statements around type safety are a little hard to apply in practice.

It’s not the case typically that a directive would change a types underlying type yet @oneOf seems to imply that “when the server gets this, it expect one of these to be non-null”. Off the bat, a standard GraphQL to Typescript conversion could do something like

type PetInput = { cat?: CatInput; dog?: DogInput; fish?: FishInput; }

When instead I’d expect it to do something like:

type PetInput = { cat: CatInput; } | { dog: DogInput; } | { fish: FishInput; };

I totally understand the motivation around the change to make it as low impact as possible, but I'd worry about the adverse side affects introduced by this subtle change to the ways that the null/non-null properties are determined.

Maybe I’m just applying my understanding incorrectly, but I’d hope that any adoption doesn’t in fact mutate the type system of GraphQL using directives like this.

[benjie]

@wyfo Thanks, fixed!

@wyattjoh It’s not a directive, it’s a new type system constraint that DOES model the type of the input differently and would have different types generated. Have a look at the alternative syntaxes document for other ways this could be exposed via SDL and let us know your preference, perhaps you would prefer the oneof keyword to make it clearer (in SDL only, this would not affect introspection) the change in behaviour.

[cometkim]

It looks like an existing syntax, but the semantics are different? I am worried that if it will end up asking for dirty exception handling for every directive code path.

Have a look at the alternative syntaxes document for other ways this could be exposed via SDL and let us know your preference

Could we consider a new syntax that hasn't been mentioned?

type  Query {
   user(id: ID!): User
   user(email: String!): User
   user(username: String!): User 
   user(registrationNumber: Int!): User
}

pros?:

• it might be easy to apply because it just releases the existing constraints (that field names cannot duplicate on SDL)
• it makes the schema can look intuitive for the possible input type.

cons:

• Conversely, it makes it look like a variant is possible for the output.

[benjie]

@cometkim Can you show how that syntax would be expanded to input objects too, please? And yes we can absolutely consider alternative syntaxes.

[wyfo]

It’s not a directive

Why should it be something else than a directive?

Actually, it's already (almost) possible to implement @oneOf as a directive in a few lines of code.
I've made a Gist to show a possible implementation using Python and graphql-core (quite the reference translation of graphql-js in Python).
In fact, if the field directive is trivial, the input type directive requires in my example a graphql-core specific feature. However, the proposal of input object validation (still opened) could bring the material needed to implement it with graphql-js.

By the way, GraphQL schema is kind of poor in validation stuff (compared to JSON schema for example), so part of the validation is already done by the resolvers/scalar parsing methods. In a schema-first approach, you can also defines directives for repetitive checks, maybe with JSON schema-like annotations, but your code/library will have to translate and inject them into your resolvers/scalar types(/input types when the mentioned proposal will pass).
IMO, @oneOf should not be different as a directive, it could just be a validation marker used to add validation code in the resolvers/input type; no need of the type system validation. Also, in a code-first approach (no directives), it's already possible to support tagged unions, I do it in my own library; there is no need of the SDL.

In fact, I don't really see the interest of making @oneOf something else than a validation directive. And I'm wondering then if a validation directive would be appropriate in the GraphQL specification … Maybe it could be a kind of convention for schema-first libraries. Yet, having it in the specifications could help tooling (linters, code generators) and lighten GraphQL libraries. Anyway, night thoughts.

[sungam3r]

Can we expand @OneOf to output types to allow for unions of objects, interfaces, scalars, enums and lists; potentially replacing the union type?

For input types @oneOf implies one more nesting level. What do you think @oneOf will look like for unions?

[sungam3r]

It’s not a directive, it’s a new type system constraint that DOES model...

@benjie I don't understand. You wrote about @oneOf as a directive in the spec and at the same type talk here that it's not a directive... 😕

[benjie]

For input types @OneOf implies one more nesting level. What do you think @OneOf will look like for unions?

Another nesting level; i.e. instead of querying like:

{
  allEntities {
    ... on User { username }
    ... on Pet { name }
    ... on Car { registrationNumber }
    ... on Building { numberOfFloors }
  }
}

it'd look like:

{
  allEntities {
    user { username }
    pet { name }
    car { registrationNumber }
    building { numberOfFloors }
  }
}

[benjie]

@benjie I don't understand. You wrote about @OneOf as a directive in the spec and at the same type talk here that it's not a directive... confused

The input union working group have not decided what syntax to use for oneOf yet. It might end up as being presented as a directive, or it might be a keyword or any other combination of things. Check out this document for alternatives: https://gist.github.com/benjie/5e7324c64f42dd818b9c3ac2a91b6b12 and note that whichever alternative you pick only affects the IDL, it does not affect the functionality or appearance of GraphQL operations, validation, execution, etc. Please see the FAQ above.

TL;DR: do not judge the functionality of this RFC by its current IDL syntax. We can change the IDL syntax.

[sungam3r]

It might end up as being presented as a directive

OK. In my opinion if something is presented as a directive than ... it is just a directive.

[benjie]

Thanks for the review @sungam3r; good to have additional scrutiny! I don't think any modifications to the RFC are required to address your concerns (other than perhaps writing an alternative IDL syntax, but I don't plan to invest time in that until there's general concensus on what the syntax should be, for now the directive syntax can act as a placeholder). I think all the conversations in your review can be closed except for the oneArg suggestion; that one might require some more bike-shedding 😉

[martinbonnin]

@Nomia it's on the agenda of the June working group. Feel free to add yourself if you want to attend!

[benjie]

@Nomia It's RFC3 (Accepted) so you should feel comfortable using it even before it is merged, further changes will likely be very minor in nature: rephrasing sentences for clarity and the like.

[leebyron]

@benjie - shouldn't this validation happen before execution and be a _request error_?

[benjie]

@leebyron Hopefully this is simpler and yet covers all the cases it needs to?

[benjie]

This is once more ready for your review, @leebyron!