How to use $slice a $filter result in MongoDB? - mongodb

Question

I have a item collection with the following format:
{
"_id": 123,
"items": [{
"name": "item1",
"status" : "inactive",
"created" : ISODate("2018-02-14T10:39:28.321Z")
},
{
"name": "item2",
"status" : "active",
"created" : ISODate("2018-02-14T10:39:28.321Z")
},
{
"name": "item3",
"status" : "active",
"created" : ISODate("2018-02-14T10:39:28.321Z")
},
{
"name": "item4",
"status" : "inactive",
"created" : ISODate("2018-02-14T10:39:28.321Z")
},
{
"name": "item5",
"status" : "active",
"created" : ISODate("2018-02-14T10:39:28.321Z")
}
]
}
I want to query on status field of items such that the object with status as 'active' is only returned in the array and also skip last 1 and limit 2 are returned in the query.
At present I am using $filter(aggregation) for this operation, by using following query it return only last record:
db.item.aggregate([
{ "$match": { "items.status": "active" } },
{ "$project": {
"items": {
"$slice": [
{ "$filter": {
"input": "$items",
"as": "item",
"cond": { "$eq": [ "$$item.status", "active" ] }
}},
-1,2
]
}
}}
])
Output should be:
{
"_id": 123,
"items": [
{
"name": "item2",
"status" : "active",
"created" : ISODate("2018-02-14T10:39:28.321Z")
},
{
"name": "item3",
"status" : "active",
"created" : ISODate("2018-02-14T10:39:28.321Z")
}]
}
Please help me to achieve this result.

Answer 1

One way of doing it would like this (there's no need for the $match stage depending on the structure of your data and your indexing setup you might still want to keep it, though, for performance reasons):
db.item.aggregate([
{ "$project": {
"items": {
"$slice": [
{ "$filter": {
"input": "$items",
"as": "item",
"cond": { "$eq": [ "$$item.status", "active" ] }
}},
-3,2
]
}
}}
])
I would think it's probably better, though, to use the following query:
db.items.db.aggregate([
{
$project: {
"items": {
"$filter": {
"input": "$items",
"as": "item",
"cond": { "$eq": [ "$$item.status", "active" ] }
}
}
}
}, {
$project: {
"items": {
$slice: [
{
$slice: [
"$items",
{
$subtract: [ { $size: "$items" }, 1 ] // length of items array minus one
}
]
}, 2 // max two elements
]
}
}
}])
as this one will first get rid of the last element and then limit the output to two items which is probably more what you want in a situation with less than 3 "active" elements.