Support String and Number to be injected as the parent/source

[andimarek]

This simple example fails at the moment:

type Query {
    pet: Pet
}
type Pet {
    name: String
}

   @QueryMapping
    public String pet() {
       return "123";
    }

    @SchemaMapping(typeName = "Pet", field = "name")
    public String petName(String petId) {
     return "Luna";
    }

The reason seems to be that no HandlerMethodArgumentResolver is applicable to String.
Looking at the source resolver (SourceMethodArgumentResolver)

@Override
	public boolean supportsParameter(MethodParameter parameter) {
		Class<?> type = parameter.getParameterType();
		return (!BeanUtils.isSimpleValueType(type) && !type.isArray() && !Collection.class.isAssignableFrom(type));
	}

simple types and arrays and collections are excluded.
I am wondering why.

This example is not so uncommon: I can return just an id like "123", which is then used to lookup more in the child DataFetchers.

Can we relax that to always fall back to SourceMethodArgumentResolver regardless of type?

[rstoyanchev]

I think we always imagined that a non-scalar type would map to a "non-simple" Object in order to provide some field values, and hence the defensive exclusions. Now that you mention it, I can see how the parent could be anything as long as all fields are left to child data fetchers.

SourceMethodArgumentResolver is the last one in the order of resolvers. We should be able to relax this a little, perhaps allowing any CharSequence or Number.