Adding validation to MutationOptions

Initial discussion in #160.

Validating data prior to triggering a mutation is a common (and critical) part of any action. I think it could be beneficial to add a validation key on the MutationOptions object that represents a method with the following signature ({ variables }) => boolean.

View layer integrations could consume this and even add in more data to the method (like props for React) ({ variables, ownProps }) => boolean.

Prior to creating the mutation, this method would run (defaulting to a true stub?) and could return an error if invalid.

Thoughts?

[ryannealmes]

Really wish there was guidance in the docs for this. The only thing I could figure out is throwing an exception from my resolver.

Another example would be returning errors from queries - e.g. When searching for a user using graphql and getting a 404 Not found from the server.

Is there anyone who can give guidance on this?

[ciwolsey]

Can’t validation errors on the server be returned from the resolver? or is that the wrong place for them?

I like the way redux-form does it’s validation which is to have a function like:

(values) => {
  const errors = {};
  if(values.category !== 'news') errors.category = "Category must be news";
  return errors;
}

Where errors is a free form object allowing you to use errors from an ORM or client side validation library etc.