How to query posts by category and its subcategories in one request?

I see you're trying to get all posts related to a specific category and its subcategories in one query. This is a common challenge with hierarchical data in Sanity!

The issue is that GROQ doesn't have built-in recursive queries, so you need to handle the hierarchy manually. Here are a couple of approaches:

Approach 1: Two-step query with subcategory IDs

First, fetch the parent category and collect all its subcategory IDs, then use those to filter posts:

*[_type == "blogPostTag" && key.current == "digital-marketing"][0]{
  _id,
  title,
  "subcategoryIds": [_id] + *[_type == "blogPostTag" && references(^._id)]._id,
  "posts": *[_type == "localizedBlogPostEntry" && count((tags[]->_id)[@ in ^.subcategoryIds]) > 0]
}

This query:

1. Gets your parent category
2. Creates an array of IDs including the parent (_id) and all subcategories that reference it
3. Filters posts where any of their tags match those IDs

Approach 2: Using a subquery in the filter

You can also inline the subcategory lookup directly in the posts filter:

*[_type == "blogPostTag" && key.current == "digital-marketing"][0]{
  _id,
  title,
  "posts": *[_type == "localizedBlogPostEntry" && (
    references(^._id) || 
    count((tags[]->_id)[@ in *[_type == "blogPostTag" && references(^._id)]._id]) > 0
  )]
}

This checks if posts reference either the parent category directly OR any of its subcategories.

Important notes:

• Since your schema has parentCategory as a reference field on subcategories pointing to parents, you're querying "bottom-up" (subcategories reference parents)
• The references() function checks if a document references another, which works perfectly for your use case
• The count() and array filtering syntax [@ in array] lets you check if any tag matches your subcategory list

The key insight from the reference fields documentation is that references are bidirectional when queried, so you can traverse them from either direction even though the reference field only appears in the Studio where it's defined.

If you need to go deeper than one level (subcategories of subcategories), you'd need to either flatten your data structure or handle multiple levels explicitly in your query, as GROQ doesn't support recursive traversal natively.