How to get last document of each day in MongoDB collection? - mongodb

Question

I have a model Entry to which includes details of a hospital at a particular time. The data looks like this:
{
"_id": "5ef9c7337874820008c1a026",
"date": 1593427763640,
//... some data
"hospital": {
"_id": "5ef8d06630c364000840bb6d",
"name": "City Hospital",
//... some data
},
}
I want to get the last query of each day grouped by the hospital ID. In MySQL, it can be achieved using INNER JOIN. How can I do it using MongoDB?

Answer 1

Given a day, calculate start and end of a day.
This is to be used for filtering records, $match
start_of_day_ephocs=
end_of_day_ephocs=
Aggregate Query
sort by date, Group by hospital id,and select first document
db.Entry.aggregate(
[
{ "$match": { "date": {"$gte":start_of_day_ephocs,"$lte":end_of_day_ephocs }} },
{ "$sort": { "date": -1 } },
{
$group:
{
"_id": "$hospital._id",
"last_document": { "$first": "$$ROOT" }
}
}
]
)

Answer 2

Consider a sales collection with the following documents:
{ "_id" : 1, "item" : "abc", "date" : ISODate("2014-01-01T08:00:00Z"), "price" : 10, "quantity" : 2 }
{ "_id" : 2, "item" : "jkl", "date" : ISODate("2014-02-03T09:00:00Z"), "price" : 20, "quantity" : 1 }
{ "_id" : 3, "item" : "xyz", "date" : ISODate("2014-02-03T09:05:00Z"), "price" : 5, "quantity" : 5 }
{ "_id" : 4, "item" : "abc", "date" : ISODate("2014-02-15T08:00:00Z"), "price" : 10, "quantity" : 10 }
{ "_id" : 5, "item" : "xyz", "date" : ISODate("2014-02-15T09:05:00Z"), "price" : 5, "quantity" : 10 }
{ "_id" : 6, "item" : "xyz", "date" : ISODate("2014-02-15T12:05:10Z"), "price" : 5, "quantity" : 5 }
{ "_id" : 7, "item" : "xyz", "date" : ISODate("2014-02-15T14:12:12Z"), "price" : 5, "quantity" : 10 }
The following operation first sorts the documents by item and date, and then in the following $group stage, groups the now sorted documents by the item field and uses the $last accumulator to compute the last sales date for each item:
db.sales.aggregate(
[
{ $sort: { item: 1, date: 1 } },
{
$group:
{
_id: "$item",
lastSalesDate: { $last: "$date" }
}
}
]
)
The operation returns the following results:
{ "_id" : "xyz", "lastSalesDate" : ISODate("2014-02-15T14:12:12Z") }
{ "_id" : "jkl", "lastSalesDate" : ISODate("2014-02-03T09:00:00Z") }
{ "_id" : "abc", "lastSalesDate" : ISODate("2014-02-15T08:00:00Z") }
Resource