Relay: [Modern] Conditionally fetch fields with required input

[robrichard]

Consider a query like this:

query rootQuery ($userId: String, $loggedIn: Boolean!) {
  viewer {
    user(userId: $userId) @include(if: $loggedIn) {
      id
    }
  }
}

I would like to fetch this query passing in either {userId: null, loggedIn: false} (representing a logged out user) or {userId: <id>, loggedIn true} (representing a logged in user). This query is invalid GraphQL because userId is a required string on the user field. relay-compiler will fail to compile it (Variable "$userId" of type "String" used in position expecting type "String!").

We were able to do this in Relay Classic because Relay would remove fields that were excluded at runtime. I don't see a way to do it in Relay Modern without making $userId required for the query and passing a dummy value, or modifying the schema. Both of those seem hacky. How do you recommend approaching this?

[josephsavona]

Great question. For context, Relay Modern is enforcing standard GraphQL validation rules - this query wouldn't (shouldn't?) be valid in graphiql either, for example. This is because the query's validity depends on the variable values at runtime.

A good workaround is probably to use an empty string, or to specify a default value for the variable, e.g. $userId: String! = "".

[robrichard]

Thanks! I didn't realize you can provide default values like that. I'll go with that solution.