How to patch nested array object field with Sanity JS client?

Good news - you can definitely do this in a cleaner way without fetching the entire document! Sanity's patch operations support targeting specific items within nested arrays using path conditions.

For your use case of inserting data into a nested subentries array, you can use the insert operation with a path that targets the specific parent entry by its _key. Here's how:

client
  .patch('aFuiYGWUw0SSAvMnYQafry')
  .insert('after', 'entries[_key=="E0vulMwINU3H9ETBqXnRT7"].subentries[-1]', [
    {
      _key: 'new-unique-key', // generate a unique key
      data: 'your new data'
    }
  ])
  .commit()

This approach:

• Targets the specific entry in entries array using [_key=="E0vulMwINU3H9ETBqXnRT7"]
• References the subentries array within that entry
• Uses [-1] to reference the last item (or use a specific index like [0])
• Inserts your new item after that position

If you want to update an existing item in the nested array instead of inserting, you can use set:

client
  .patch('aFuiYGWUw0SSAvMnYQafry')
  .set({
    'entries[_key=="E0vulMwINU3H9ETBqXnRT7"].subentries[_key=="6b8c867d-36b9-4eff-97e0-9fd5be6c6ee4"].data': 'updated data'
  })
  .commit()

The key is using JSONMatch-style path syntax with conditions like [_key=="value"] to target specific objects in arrays. This way you're only sending the minimal change needed, avoiding the fetch-edit-patch cycle and preventing data loss issues.

These operations are also "real-time safe," meaning they work correctly even when multiple users are editing simultaneously, which wouldn't be guaranteed with your fetch-then-replace approach. You can read more about patch operations in the docs.