graphql using nested query arguments on parent or parent arguments on nested query

I agree with @DenisCappelini's answer. If possible, you can create a new type which represents only Products that have an Item.

However, if you don't want to do that, or if you're just interested in general about how a top-level selector can know about arguments on child selectors, here is a way to do that:

There are 2 ways to do it.

---

To do this:

products {
  id
  style_id
  items(size: ["S","M"]) {
    id
    size
  }
}

In graphql, resolvers have this signature:

(obj, args, context, info) => {}

The 4th argument, info, contains information about the entire request. Namely, it knows about arguments on the child selectors.

Use this package, or a similar one because there are others, to parse info for you: https://www.npmjs.com/package/graphql-parse-resolve-info

---

The above is quite a lot of work, so if you want to do this instead:

products(size: ["S","M"]) {
  id
  style_id
  items {
    id
    size
  }
}

Then in your resolver for products, you need to also return size. Suppose this is your resolver for products:

(parent, args) => {
  ...
  return {
    id: '',
    style_id: ''
  }
}

Modify your resolver to also return size like this:

(parent, args) => {
  ...
  return {
    id: '',
    style_id: '',
    size: ["S", "M"]
  }
}

Now in your resolve for products.items, you will have access to the size, like this:

(product, args) => {
  const size = product.size
}