MongoDB - Query on the last element of an array?

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\'

Answer 1

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 2

Not sure about performance, but this works well for me:

db.getCollection('test').find(
  {
    $where: "this.someArray[this.someArray.length - 1] === 'pattern'"
  }
)

Answer 3

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 4

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 5

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 6

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" } }
]);