Updating object properties in an array on a document with the Sanity client

Unfortunately, dec() (and inc()) operations don't support JSONMatch path syntax with array selectors like variants[_key == "..."].quantity. These operations only work with simple dot-notation paths like stock or nested.field. That's why you're getting a 400 error.

The patches documentation shows that inc and dec work with simple paths, while the JSONMatch syntax (like arrayField[_key == "value"].property) is supported by set, unset, and setIfMissing operations.

Here's the solution using set with the JSONMatch path syntax:

const promises = data.overview.cartContents.map(async (item) => {
  // First, fetch the current quantity
  const doc = await client.fetch(
    `*[_id == $id][0]{"currentQty": variants[_key == $variantKey][0].quantity}`,
    { id: item._id, variantKey: item.variant }
  );
  
  // Then set the new decremented value
  return await client
    .patch(item._id)
    .set({ 
      [`variants[_key == "${item.variant}"].quantity`]: doc.currentQty - item.quantity 
    })
    .commit();
});

The key changes:

1. Use set() instead of dec(): The JSONMatch path syntax works with set, not with inc/dec
2. Computed property names: The square brackets [] around the template literal are required to create a dynamic property name
3. Fetch first, then calculate: Since you can't use dec() directly, fetch the current value, calculate the new value, and set it

Here's a cleaner syntax example from the array patching documentation:

await client
  .patch('document-id')
  .set({ 
    'variants[_key == "abc123"].quantity': 42 
  })
  .commit();

Alternative: Use a transaction for atomic updates

If you're concerned about race conditions between the fetch and set operations, you could use Sanity Functions or handle the calculation server-side where you have better control over concurrency.

The limitation exists because inc and dec are designed as atomic operations that work on simple paths, while JSONMatch expressions with array filtering require more complex evaluation that isn't supported with these numeric operations.