How to query posts by dynamically provided category reference in Sanity?

Great question! Filtering posts by category is a common pattern in Sanity. Based on your schema where posts have an array of category references, here's how to query all posts for a specific category using GROQ:

The Solution

The key is using the references() function in GROQ. Here's the query:

*[_type == "post" && references($categoryId)]

Where $categoryId is a parameter you pass in with the _id of the category you want to filter by.

Complete Example

If you want to include the category details in your results, you can dereference it:

*[_type == "post" && references($categoryId)] {
  _id,
  title,
  categories[]->
}

The -> operator dereferences the category references, giving you the full category documents instead of just the reference IDs.

Filtering Out Drafts

In production, you'll want to exclude draft documents:

*[_type == "post" && !(_id in path("drafts.*")) && references($categoryId)]

Using with Client Code

Here's how you'd use this with the Sanity client:

const posts = await client.fetch(
  `*[_type == "post" && references($categoryId)] {
    _id,
    title,
    categories[]->
  }`,
  { categoryId: 'your-category-id-here' }
)

The references() function works perfectly with arrays of references - it checks if any item in your categories array references the provided ID. Check out the GROQ parameters documentation for more details on passing parameters to your queries.