Can't filter products by brand in Sanity desk structure

Looking at your code, the structure itself is actually valid! Your filter syntax $brandId in brands[]._ref is correct GROQ, and the way you're chaining .child() should work. However, there are a few common issues that could prevent it from displaying products:

Most likely culprit: Schema field mismatch

Double-check your product schema. Your filter assumes brands is an array of references, like this:

{
  name: 'brands',
  type: 'array',
  of: [{type: 'reference', to: [{type: 'brand'}]}]
}

If your schema is actually different, you'll need to adjust:

• If it's a single reference (not an array): Change your filter to:

  .filter('_type == "product" && brands._ref == $brandId')

• If the field has a different name (like brand singular or brandRef): Update your filter to match that field name

Alternative filter approach

You could also try using the references() function, which checks all reference fields automatically:

.filter('_type == "product" && references($brandId)')

This is more forgiving and doesn't require you to specify the exact field path.

Other things to verify:

1. Do you have data? Make sure you have products with brand references actually set
2. Check for typos in field names between your schema and filter
3. Try a simpler filter first to isolate the issue:

   .filter('_type == "product"')

   If this shows products, then you know the issue is specifically with the brand filtering logic

About .schemaType()

You might see suggestions to add .schemaType('product') - while this can be helpful for tooling and autocomplete, it's not strictly required for the filter to work. It won't fix a filtering issue, but it doesn't hurt to include it:

S.documentList()
  .schemaType('product')  // optional
  .title('Products')
  .filter('_type == "product" && $brandId in brands[]._ref')
  .params({ brandId })

The most common issue here is that the filter doesn't match your actual schema structure, so start by verifying your brands field configuration!