MongoDB aggregation: $unwind after grouping by date - mongodb

Question

I have this model for purchases:
{
purchase_date: 2018-03-11 00:00:00.000,
total_cost: 400,
items: [
{
title: 'Pringles',
price: 200,
quantity: 2,
category: 'Snacks'
}
]
}
What I'm trying to do is to, first of all, to group the purchases by date, by doing so:
{$group: {
_id: {
date: $purchase_date,
items: '$items'
}
}}
However, now what I want to do is group the purchases of each day by items[].category and calculate how much was spent for each category in that day. I was able to do that with one day, but when I grouped each purchase by date I no longer able to $unwind the items.
I tried passing the path $items and it doesn't find it at all. If I try to use $_id.$items or _id.$items in both cases I get an error stating that it is not a valid path for $unwind.

Answer 1

You can use purchase_data and items.category as a grouping _id but you need to use $unwind on items before and then you can add another $group to get all groups per day
db.col.aggregate([
{ $unwind: "$items" },
{
$group: {
_id: {
purchase_date: "$purchase_date",
category: "$items.category",
},
total: { $sum: { $multiply: [ "$items.price", "$items.quantity" ] } }
}
},
{
$group: {
_id: "$_id.purchase_date",
categories: { $push: { name: "$_id.category", total: "$total" } }
}
}
])