Maps

Kotlin has the Map type, which doesn't exist in BSON. Most serialization libraries treat Map<String, V> specially, and serialize it as an object (the keys become fields and the value their value).

KtMongo assumes your serialization library works like this and provides a few helpers. If these types are serialized differently, these operators may not work.

Using an index

The easiest way to access a map element is using its index. For example, if we know that we want to update Bob's physics score, we can use:

class User(
    val name: String,
    val scores: Map<String, Int>,
)

users.updateOne(
    filter = { User::name eq "Bob" }
) {
    User::scores["physics"] set 20
}