GraphQL mutation variables

Question

I'm trying to do a simple mutation using GraphQL with the GraphiQL interface. My mutation looks like this:

mutation M($name: String) {
  addGroup(name:$name) {
    id,
    name
  }
}

with variables:

{
  "name": "ben"
}

But it gives me the error: Variable $name of type "String" used in position expecting type "String!"

If I change my mutation to mutation M($name: String = "default") it works as expected. This looks like it's related to the type system, but I can't seem to figure out what the problem is.

dqc dqc · Accepted Answer · 2016-07-29T13:44:18

You probably defined the input name as a non-null string (something like type: new GraphQLNonNull(GraphQLString) if using js server, or String! in plain GraphQL).

So your input in the mutation must match, which means it must also be a non-null string. If you change to the following, it should work:

mutation M($name: String!) {
  addGroup(name:$name) {
    id,
    name
  }
}

Also if you define a default value as you did, it will be a non-null string.

Finally, you could drop the requirement of being a non-null in the server.

GraphQL mutation variables

2 Answers