Aggregation at each document level mongodb - mongodb

Question

I have a list of documents like this
[{
"_id": "5dbc95f921d7625303fe2369",
"name": "John",
"itemsPurchased": [{
"offer": "o1",
"items": ["p1"]
},{
"offer": "o1",
"items": ["p1"]
},
{
"offer": "o1",
"items": ["p2"]
},
{
"offer": "o2",
"items": ["p1"]
}, {
"offer": "o7",
"items": ["p1"]
}
]
},
{
"_id": "zbc95f921d7625303fe2363",
"name": "Doe",
"itemsPurchased": [{
"offer": "o1",
"items": ["p11"]
},{
"offer": "o1",
"items": ["p11"]
},
{
"offer": "o2",
"items": ["p13"]
},
{
"offer": "o1",
"items": ["p22"]
},
{
"offer": "o2",
"items": ["p11"]
}, {
"offer": "o3",
"items": ["p11"]
}
]
}
]
And i am trying to compute unique offers on unique products by each customer, expecting the resultant to be like:
[
{
"_id": "5dbc95f921d7625303fe2369",
"name": "John",
"offersAndProducts": {
"o1":2,
"o2":2,
"o3":1
},
{
"_id": "zbc95f921d7625303fe2363",
"name": "Doe",
"offersAndProducts": {
"o1":2,
"o2":1,
"o7":1
}
]
I want to apply aggregations per document, After performing $unwind on itemsPurchased, applied $group on items and then on offer to eliminate the duplication:
{
"$group" : {
"_id" : {
"item" : {
"$arrayElemAt" : [
"$itemsPurchased.item",
0.0
]
},
"count" : {
"$sum" : 1.0
},
"offer" : "$itemsPurchased.offer"
}
}
}
then,
{
"$group" : {
"_id" : "$_id.offer",
"count" : {
"$sum" : 1.0
}
}
}
this gives the array of products and offers for all documents:
[
{o1:4,o2:3,o3:1,o7:1}
]
But i need it at document level.
tried $addFeild, but $unwind and $match operators gives invalid error.
Any other way of achieving this?

Answer 1

Generally speaking, it's an anti-pattern to $unwind an array and then to $group on the original _id since most operations can be done on the array directly, in a single stage. Here is what such a stage would look like:
{$addFields:{
offers:{$arrayToObject:{
$map:{
input:{$setUnion:"$itemsPurchased.offer"},
as:"o",
in:[
"$$o",
{$size:{$setUnion:{$let:{
vars:{items:{$filter:{
input:"$itemsPurchased",
cond:{$eq:["$$this.offer","$$o"]}
}}},
in:{$reduce:{
input:"$$items",
initialValue:[],
in:{$concatArrays:["$$value","$$items.items"]}
}}
}}}
}]
}
}}
}}
What this does is create an array where each element is a two element array (which is a syntax that $arrayToObject can convert to an object where first element is key name and second is value) and the input is a unique set of offers and for each we accumulate an array of products, get rid of duplicates (with $setUnion) and then get the size of the result. What this produces on your input is this:
"offers" : {
"o1" : 2,
"o2" : 2,
"o3" : 1
}

Answer 2

You need to run $unwind and $group twice. To count only unique items you can use $addToSet. To build your keys dynamically you need to use $arrayToObject:
db.collection.aggregate([
{
$unwind: "$itemsPurchased"
},
{
$unwind: "$itemsPurchased.items"
},
{
$group: {
_id: {
_id: "$_id",
offer: "$itemsPurchased.offer"
},
name: { $first: "$name" },
items: { $addToSet: "$itemsPurchased.items" }
}
},
{
$group: {
_id: "$_id._id",
name: { $first: "$name" },
offersAndProducts: { $push: { k: "$_id.offer", v: { $size: "$items" } } }
}
},
{
$project: {
_id: 1,
name: 1,
offersAndProducts: { $arrayToObject: "$offersAndProducts" }
}
}
])
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