MongoDB - Query on the last element of an array? - arrays

Question

I know that MongoDB supports the syntax find{array.0.field:"value"}, but I specifically want to do this for the last element in the array, which means I don't know the index. Is there some kind of operator for this, or am I out of luck?
EDIT: To clarify, I want find() to only return documents where a field in the last element of an array matches a specific value.

Answer 1

In 3.2 this is possible. First project so that myField contains only the last element, and then match on myField.
db.collection.aggregate([
{ $project: { id: 1, myField: { $slice: [ "$myField", -1 ] } } },
{ $match: { myField: "myValue" } }
]);

Answer 2

You can use $expr ( 3.6 mongo version operator ) to use aggregation functions in regular query.
Compare query operators vs aggregation comparison operators.
For scalar arrays
db.col.find({$expr: {$gt: [{$arrayElemAt: ["$array", -1]}, value]}})
For embedded arrays - Use $arrayElemAt expression with dot notation to project last element.
db.col.find({$expr: {$gt: [{"$arrayElemAt": ["$array.field", -1]}, value]}})
Spring #Query code
#Query("{$expr:{$gt:[{$arrayElemAt:[\"$array\", -1]}, ?0]}}")
ReturnType MethodName(ArgType arg);

Answer 3

Starting Mongo 4.4, the aggregation operator $last can be used to access the last element of an array:
For instance, within a find query:
// { "myArray": ["A", "B", "C"] }
// { "myArray": ["D"] }
db.collection.find({ $expr: { $eq: [{ $last: "$myArray" }, "C"] } })
// { "myArray": ["A", "B", "C"] }
Or within an aggregation query:
db.collection.aggregate([
{ $addFields: { last: { $last: "$myArray" } } },
{ $match: { last: "C" } }
])

Answer 4

use $slice.
db.collection.find( {}, { array_field: { $slice: -1 } } )
Editing:
You can make use of
{ <field>: { $elemMatch: { <query1>, <query2>, ... } } } to find a match.
But it won't give exactly what you are looking for. I don't think that is possible in mongoDB yet.

Answer 5

I posted on the official Mongo Google group here, and got an answer from their staff. It appears that what I'm looking for isn't possible. I'm going to just use a different schema approach.

Answer 6

Version 3.6 use aggregation to achieve the same.
db.getCollection('deviceTrackerHistory').aggregate([
{
$match:{clientId:"12"}
},
{
$project:
{
deviceId:1,
recent: { $arrayElemAt: [ "$history", -1 ] }
}
}
])

Answer 7

You could use $position: 0 whenever you $push, and then always query array.0 to get the most recently added element. Of course then, you wont be able to get the new "last" element.

Answer 8

Not sure about performance, but this works well for me:
db.getCollection('test').find(
{
$where: "this.someArray[this.someArray.length - 1] === 'pattern'"
}
)

Answer 9

You can solve this using aggregation.
model.aggregate([
{
$addFields: {
lastArrayElement: {
$slice: ["$array", -1],
},
},
},
{
$match: {
"lastArrayElement.field": value,
},
},
]);
Quick explanations. aggregate creates a pipeline of actions, executed sequentially, which is why it takes an array as parameter. First we use the $addFields pipeline stage. This is new in version 3.4, and basically means: Keep all the existing fields of the document, but also add the following. In our case we're adding lastArrayElement and defining it as the last element in the array called array. Next we perform a $match pipeline stage. The input to this is the output from the previous stage, which includes our new lastArrayElement field. Here we're saying that we only include documents where its field field has the value value.
Note that the resulting matching documents will include lastArrayElement. If for some reason you really don't want this, you could add a $project pipeline stage after $match to remove it.

Answer 10

For the answer use $arrayElemAt,if i want orderNumber:"12345" and the last element's value $gt than "value"? how to make the $expr? thanks！
For embedded arrays - Use $arrayElemAt expression with dot notation to project last element.
db.col.find({$expr: {$gt: [{"$arrayElemAt": ["$array.field", -1]}, value]}})

Answer 11

db.collection.aggregate([
{
$match: {
$and: [
{ $expr: { $eq: [{ "$arrayElemAt": ["$fieldArray.name", -1] }, "value"] } },
{ $or: [] }
]
}
}
]);