MongoDB: Find and then modify the resulting object - mongodb

Question

is it possible in MongoDB to find some objects that match a query and then to modify the result without modifying the persistent data?
For example, let
students = [
{ name: "Alice", age: 25 },
{ name: "Bob", age: 22 },
{ name: "Carol", age: 19 },
{ name: "Dave", age: 18}
]
Now, I want to query all students that are younger than 20 and in the search result, I just want to replace "age: X" with "under20: 1" resulting in the following:
result = [
{ name: "Carol", under20: 1 },
{ name: "Dave", under20: 1}
]
without changing anything in the database.
Sure, it is possible to get the result and then call a forEach on it, but that sounds so inefficient because I have to rerun every object again, so I'm searching for an alternative. Or is there no one?

Answer 1

A possible solution would be to use an aggregation pipline with a $match followed by a $project:
db.students.aggregate(
[
{
$match: { age: { $lt: 20 } }
},
{
$project:
{
_id: false,
name: true,
under20: { $literal: 1 }
}
}
]);
The $literal: 1 is required as just using under20: 1 is the same as under20: true, requesting that field under20 be included in the result: which would fail as under20 does not exist in the document produced by the match.
Or to return all documents in students and conditionally generate the value for under20 a possible solution would be to use $cond:
db.students.aggregate(
[
{
$project:
{
_id: false,
name: true,
under20:
{
$cond: { if: { $lt: [ "$age", 20 ] }, then: 1, else: 0 }
}
}
}
]);