MongoDB get size of unwinded array - mongodb

Question

I'm trying to return size of 'orders' and sum of 'item' values for each 'order' for each order from documents like the example document:
orders: [
{
order_id: 1,
items: [
{
item_id: 1,
value:100
},
{
item_id: 2,
value:200
}
]
},
{
order_id: 2,
items: [
{
item_id: 3,
value:300
},
{
item_id: 4,
value:400
}
]
}
]
I'm using following aggregation to return them, everything works fine except I can't get size of 'orders' array because after unwind, 'orders' array is turned into an object and I can't call $size on it since it is an object now.
db.users.aggregate([
{
$unwind: "$orders"
},
{
$project: {
_id: 0,
total_values: {
$reduce: {
input: "$orders.items",
initialValue: 0,
in: { $add: ["$$value", "$$this.value"] }
}
},
order_count: {$size: '$orders'}, //I get 'The argument to $size must be an array, but was of type: object' error
}
},
])
the result I expected is:
{order_count:2, total_values:1000} //For example document
{order_count:3, total_values:1500}
{order_count:5, total_values:2500}

Answer 1

I found a way to get the results that I wanted. Here is the code
db.users.aggregate([
{
$project: {
_id: 1, orders: 1, order_count: { $size: '$orders' }
}
},
{ $unwind: '$orders' },
{
$project: {
_id: '$_id', items: '$orders.items', order_count: '$order_count'
}
},
{ $unwind: '$items' },
{
$project: {
_id: '$_id', sum: { $sum: '$items.value' }, order_count: '$order_count'
}
},
{
$group: {
_id: { _id: '$_id', order_count: '$order_count' }, total_values: { $sum: '$sum' }
}
},
])
output:
{ _id: { _id: ObjectId("5dffc33002ef525620ef09f1"), order_count: 2 }, total_values: 1000 }
{ _id: { _id: ObjectId("5dffc33002ef525620ef09f2"), order_count: 3 }, total_values: 1500 }