Slice in populate throws error

I have two collection, a cart, and product Cart has an array of items with references to products The product has a prices array.

I want to do a query where I get the cart, and populate the products, but with the prices array sliced. I use the following query:

Cart.findById( id )
.populate({
    path:'items.products',
    options: {
        slice: {
            'prices': 1,
        }
    }
})

But it throws this error:

Error: slice() must be used after where() when called with these arguments

[tsabolov]

I have a similar task: We have a collection of messages. Messages can have replies which are messages too. Only last reply (if any) should be returned with each message. I’ve tried to make a population like this:

query.populate({
  path: 'replies',
  options: {
    sort: { 'createdAt': -1 },
    limit: 1
  }
});

It works, but I’ve got another problem. I also need a total number of replies which is the length of replies array. I have a virtual property to get that. The thing is that after population replies.length becomes 1.

[nickydonna]

Great, thanks!