How could I not only preserve the max value but also the record having the max value - mongodb

Question

I want to use $max operator to select the max value.
And also keep the max record with the key "original_document"
How could I do it in mongoDB
expect result
{ "_id" : "abc", "maxTotalAmount" : 100,
"maxQuantity" : 10,
"original_document": {{ "_id" : 4, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-02-15T08:00:00Z") }}}
current result
{ "_id" : "abc", "maxTotalAmount" : 100, "maxQuantity" : 10 }
documents
{ "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-01-01T08:00:00Z") }
{ "_id" : 4, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-02-15T08:00:00Z") }
aggregation
db.sales.aggregate(
[
{
$group:
{
_id: "$item",
maxTotalAmount: { $max: { $multiply: [ "$price", "$quantity" ] } },
maxQuantity: { $max: "$quantity" }
}
}
]
)

Answer 1

When you want detail from the same grouping item then you use $sort and $first for the field(s) from the document you wish to preserve:
db.sales.aggregate([
{ "$project": {
"item": 1,
"TotalAmount": { "$multiply": [ "$price", "$quantity" ] },
"quantity": 1
}},
{ "$sort": { "TotalAmount": -1 } },
{ "$group": {
"_id": "$item",
"maxTotalAmount": { "$max": "$TotalAmount" },
"maxQuantity": { "$max": "$quantity" },
"doc_id": { "$first": "$_id" },
"doc_quantity": { "$first": "$quantity" }
}}
])
The aggregation "accumulators" cannot use embedded fields, and pushing all to an array makes little sense. But you can name like above and even rename with another $project or in your code if you want to.
Just to demonstrate how impractical this is to do otherwise, there is this example:
db.sales.aggregate([
{ "$group": {
"_id": "$item",
"maxTotalAmount": { "$max": { "$multiply": [ "$price", "$quantity" ] } },
"maxQuantity": { "$max": "$quantity" },
"docs": { "$push": {
"_id": "$_id",
"quantity": "$quantity",
"TotalAmount": { "$multiply": [ "$price", "$quantity" ] }
}}
}},
{ "$project": {
"maxTotalAmount": 1,
"maxQuantity": 1,
"maxTotalDocs": {
"$setDifference": [
{ "$map": {
"input": "$docs",
"as": "doc",
"in": {
"$cond": [
{ "$eq": [ "$maxTotalAmount", "$$doc.TotalAmount" ] },
"$$doc",
false
]
}
}},
[false]
]
}
}}
])
Which is not a great idea since you are pushing every document within the grouping condition into an array, only to filter out the ones you want later. On any reasaonable data size this is not practical and likely to break.

Answer 2

Please check the below :
db.qt.aggregate([
{ "$project": { "maxTotalAmount" : { "$multiply" :
[ "$price", "$quantity" ]
} ,
"currentDocumnet" : { "_id" : "$_id" ,
"item" : "$item", "price" : "$price",
"quantity" : "$quantity",
"date" : "$date" } }
},
{"$sort" : { "currentDocumnet.item" : 1 , maxTotalAmount : -1}},
{"$group" :{ _id : "$currentDocumnet.item" ,
currentDocumnet : { "$first" : "$currentDocumnet"} ,
maxTotalAmount : { "$first" : "$maxTotalAmount"} ,
maxQuantity: { "$max" : "$currentDocumnet.quantity" }}
}
]);