How to group Sanity documents by a specific field value?

Great question! To group documents by a specific field dynamically in Sanity, you'll need to use the Structure Builder API with S.documentList() and GROQ filters.

However, there's an important limitation: GROQ can't project objects with dynamic keys. So you can't get exactly the format you showed with { "leadership": [], "radiologist": [] } directly from a query.

Here are your two best options:

Option 1: Use References (Recommended)

If you make category a reference to a separate category document type instead of just a string, you can create a dynamic structure:

*[_type == 'category'] {
  "category": name,
  "results": *[
    _type == "physician" 
    && references(^._id)
  ]
}

This gives you:

[
  {
    "category": "radiologist",
    "results": [/* physicians */]
  },
  {
    "category": "leadership", 
    "results": [/* physicians */]
  }
]

Option 2: Hardcode Categories in Your Query

If you want to keep categories as strings, you'll need to list them explicitly:

{
  "radiologist": *[_type == "physician" && category == "radiologist"],
  "leadership": *[_type == "physician" && category == "leadership"]
}

For Studio Structure Builder

You can create filtered lists in your Studio desk structure:

S.documentTypeList('category')
  .title('Posts by Category')
  .child(categoryId => 
    S.documentList()
      .title('Physicians')
      .filter('_type == "physician" && $categoryId == category')
      .params({ categoryId })
  )

The reference approach is more flexible and maintainable since it automatically adapts when you add new categories. The hardcoded approach works but requires updating your query whenever categories change.