Adding reference in block array - how to expand with GROQ?

You can absolutely dereference (expand) those references directly in your GROQ query! The key is using the -> operator, which tells GROQ to follow the reference and pull in the actual document data.

For your specific case with references inside a body array (portable text), you'll want to use array projection to selectively expand only the reference items. Here's how to modify your query:

*[_type == "post" && slug.current == $slug][0] {
  _id,
  title,
  body[] {
    ...,
    _type == "jccClass" => @->
  },
  "slug": slug.current
}

This syntax works by:

1. body[] - iterates over each item in the body array
2. ... - spreads all the existing properties (keeps blocks, images, etc. intact)
3. _type == "jccClass" => @-> - conditionally dereferences: when the item type is "jccClass", the @ refers to the current item and -> follows the reference

If you want to control which fields from the referenced jccClass document get returned (recommended for performance), you can be more specific:

*[_type == "post" && slug.current == $slug][0] {
  _id,
  title,
  body[] {
    ...,
    _type == "jccClass" => @-> {
      _id,
      title,
      description,
      // whatever fields you need from jccClass
    }
  },
  "slug": slug.current
}

This approach is much more efficient than having your component fetch each reference separately, as it resolves everything in a single query. The Content Lake will handle all the dereferencing server-side.

The -> operator works for any reference type in Sanity, and the conditional projection pattern (_type == "something" => ...) is the standard way to handle mixed arrays like portable text where you have blocks, images, and custom reference types all together.