I'm using MongoDB and i have the following records:
{
name: "a",
status: [
{ age: 15, closed: true},
{ age: 38, closed: true},
{ age: 42, closed: false},
]
},
{
name: "b",
status: [
{ age: 29, closed: true},
{ age: 5, closed: false},
]
}
I want to check if the before last object in status has for example age = 29.
is there anyway to do that without using aggregation ?
You can use $arrayElemAt and pass -2 as an argument to get before last element (-1 represents last one),
If negative, $arrayElemAt returns the element at the idx position, counting from the end of the array.
Try:
db.col.aggregate([
{
$addFields: {
beforeLast: { $arrayElemAt: [ "$status", -2 ] }
}
},
{
$match: {
"beforeLast.age": 29
}
},
{
$project: {
beforeLast: 0
}
}
])
EDIT: alternatively you can use $expr with $let keyword to define a temporary variable:
db.col.find({
$expr: {
$eq: [
29,
{
$let: {
vars: { beforeLast: { $arrayElemAt: [ "$status", -2 ] } },
in: "$$beforeLast.age"
}
}
]
}
})
$arrayElemAt operator. Any possiblites of upgrading your mongo ?$let with $expr and it works without aggregation