Mongo query on last sorted array item - mongodb

Question

Would like to query the following to obtain all item documents such that the last sale (ordered by soldDate) has a status of 2.
db.items.insertMany([
{ item: 1,
sales: [
{ soldDate: ISODate("2021-10-04"), status: 1 },
{ soldDate: ISODate("2021-10-05"), status: 2 }
]
},
{ item: 2,
sales: [
{ soldDate: ISODate("2021-09-29"), status: 3 },
{ soldDate: ISODate("2021-09-24"), status: 1 }
]
},
{ item: 3,
sales: [
{ soldDate: ISODate("2021-06-01"), status: 3 },
{ soldDate: ISODate("2021-06-12"), status: 2 },
{ soldDate: ISODate("2021-06-07"), status: 1 }
]
}
]);
So in this example, the query would return the following two documents:
{ item: 1,
sales: [
{ soldDate: ISODate("2021-10-04"), status: 1 },
{ soldDate: ISODate("2021-10-05"), status: 2 } // triggered by this
]
},
{ item: 3,
sales: [
{ soldDate: ISODate("2021-06-01"), status: 3 },
{ soldDate: ISODate("2021-06-12"), status: 2 }, // triggered by this
{ soldDate: ISODate("2021-06-07"), status: 1 }
]
}
Thanks for any help.

Answer 1

You stated: ordered by soldDate which can actually mean two things. Perhaps you want the documents sorted by the array, or perhaps you mean the array is sorted. I assumed the later.
Solution (Array sorted)
db.items.aggregate([
{ $match: { "sales.status": 2} },
{ $unwind: "$sales" },
{ $sort: { "item": 1, "sales.soldDate": 1} },
{ $group: { "_id": "$_id", "item": { $first: "$item" }, "sales": { $push: "$sales" } } }
])
Results
Enterprise replSet [primary] barrydb> db.items.aggregate([
... { $match: { "sales.status": 2} },
... { $unwind: "$sales" },
... { $sort: { "item": 1, "sales.soldDate": 1} },
... { $group: { "_id": "$_id", "item": { $first: "$item" }, "sales": { $push: "$sales" } } }
... ])
[
{
_id: ObjectId("617064519be05d9f1cbab346"),
item: 1,
sales: [
{ soldDate: ISODate("2021-10-04T00:00:00.000Z"), status: 1 },
{ soldDate: ISODate("2021-10-05T00:00:00.000Z"), status: 2 }
]
},
{
_id: ObjectId("617064519be05d9f1cbab348"),
item: 3,
sales: [
{ soldDate: ISODate("2021-06-01T00:00:00.000Z"), status: 3 },
{ soldDate: ISODate("2021-06-07T00:00:00.000Z"), status: 1 },
{ soldDate: ISODate("2021-06-12T00:00:00.000Z"), status: 2 }
]
}
]
But, to be complete here is a solution if you want the documents sorted (and the array not necessarily sorted).
Solution (Documents sorted)
db.items.aggregate([
{ $match: { "sales.status": 2} },
{ $sort: { "sales.soldDate": 1} }
])
Results
Enterprise replSet [primary] barrydb> db.items.aggregate([
... { $match: { "sales.status": 2} },
... { $sort: { "sales.soldDate": 1} }
... ])
[
{
_id: ObjectId("617064519be05d9f1cbab348"),
item: 3,
sales: [
{ soldDate: ISODate("2021-06-01T00:00:00.000Z"), status: 3 },
{ soldDate: ISODate("2021-06-12T00:00:00.000Z"), status: 2 },
{ soldDate: ISODate("2021-06-07T00:00:00.000Z"), status: 1 }
]
},
{
_id: ObjectId("617064519be05d9f1cbab346"),
item: 1,
sales: [
{ soldDate: ISODate("2021-10-04T00:00:00.000Z"), status: 1 },
{ soldDate: ISODate("2021-10-05T00:00:00.000Z"), status: 2 }
]
}
]
EDIT - After re-reading I believe you want only where the record having a status of 2 is also has the greatest date in the array
Solution (Only last having status of value 2 - docs and array unsorted)
db.items.aggregate([
{ $unwind: "$sales" },
{ $sort: { "item": 1, "sales.soldDate": -1} },
{ $group: { "_id": "$_id", "item": { $first: "$item" }, "sales": { $push: "$sales" } } },
{ $match : { "sales.0.status" : 2 } }
])
Results
Enterprise replSet [primary] barrydb> db.items.aggregate([
... { $unwind: "$sales" },
... { $sort: { "item": 1, "sales.soldDate": -1} },
... { $group: { "_id": "$_id", "item": { $first: "$item" }, "sales": { $push: "$sales" } } },
... { $match : { "sales.0.status" : 2 } }
... ])
[
{
_id: ObjectId("617064519be05d9f1cbab346"),
item: 1,
sales: [
{ soldDate: ISODate("2021-10-05T00:00:00.000Z"), status: 2 },
{ soldDate: ISODate("2021-10-04T00:00:00.000Z"), status: 1 }
]
},
{
_id: ObjectId("617064519be05d9f1cbab348"),
item: 3,
sales: [
{ soldDate: ISODate("2021-06-12T00:00:00.000Z"), status: 2 },
{ soldDate: ISODate("2021-06-07T00:00:00.000Z"), status: 1 },
{ soldDate: ISODate("2021-06-01T00:00:00.000Z"), status: 3 }
]
}
]
EDIT - Add Self Referencing Lookup
db.items.aggregate([
{ $unwind: "$sales" },
{ $sort: { "item": 1, "sales.soldDate": -1} },
{ $group: { "_id": "$_id", "item": { $first: "$item" }, "sales": { $push: "$sales" } } },
{ $match : { "sales.0.status" : 2 } },
{ $lookup : {
from: "items",
localField: "_id",
foreignField: "_id",
as: "results"
}
},
{ $unwind: "$results" },
{ $replaceRoot: { "newRoot": "$results" } }
])
With the self-referencing lookup we are treating MongoDB as a relational database. We find the documents that meet our requirements, but in doing so we have destroyed the original shape and content. By performing a lookup on the same records we can restore the shape but at a performance penalty.
Retain Copy
Rather than performing a lookup, which has a performance concern, a different approach is to leverage memory on the server. Keep a copy of the original while moving through the pipeline and manipulating the original to identify desired records...
db.items.aggregate([
{ $addFields: { "_original": "$$ROOT" } },
{ $unwind: "$sales" },
{ $sort: { "item": 1, "sales.soldDate": -1} },
{ $group: { "_id": "$_id", "_original": { $first: "$_original" }, "sales_status": { $push: "$sales.status" } } },
{ $match : { "sales_status.0" : 2 } },
{ $replaceRoot: { "newRoot": "$_original" } }
])
In this example we keep a copy of the original in the field _original then once we have identified the records we want we pivot the root back to _original. This may put pressure on the WiredTiger cache as we are keeping a duplicate of all selected records in memory during the execution of the pipeline. A $lookup approach also has this memory concern. Two queries would eliminate the cache pressure issues, but behaves like a $lookup and would not perform as well.