GROQ: Filter product attributes by category's attribute references

Great question! You're looking to filter an array of references based on whether those references match certain criteria. The key is using the @ operator to reference the current element within an array filter.

Here's the solution for your use case:

*[_type == "product"] {
  "attrs": attributes[@._ref in ^.category->attributeRefs[]._ref]
}

However, if you want to filter based on the actual attribute values (not just the references), you'll want to use this pattern:

*[_type == "product"] {
  "attrs": attributes[(@->._id in ^.category->attributeRefs[]._ref)]->
}

The most robust solution based on this answer from the Sanity community would be:

*[_type == "product"] {
  "attrs": attributes[(@._ref in ^.category->attributeRefs[]._ref)]->,
  // ... other fields
}

Key concepts here:

• @ refers to the current element being filtered in the array
• ^ allows you to reference the parent scope (the product document)
• -> dereferences the reference to get the full document
• The in operator checks if a value exists in an array

If you want to go a step further and ensure you only get products that have at least one matching attribute, you can add a filter at the document level:

*[_type == "product"] {
  "attrs": attributes[(@._ref in ^.category->attributeRefs[]._ref)]->,
  _id
}[length(attrs) > 0]

Note: This pattern requires API version v2021-03-25 or later to work properly with the @ operator in array filters.