KtMongo: MongoDB request DSL/opensavvy.ktmongo.dsl.aggregation.stages/LookupStageOperators/into

into

abstract fun into(field: Field<LocalDocument, List<ForeignDocument>>)(source)

open fun into(field: KProperty1<LocalDocument, List<ForeignDocument>>)(source)

Specifies in which field the result of the lookup will be stored into.

Specifying this parameter is mandatory.

To learn more about the $lookup stage, see HasLookup.lookup.

Example

class Department(
    val _id: ObjectId,
    val name: String,
)

class User(
    val _id: ObjectId,
    val name: String,
    val department: ObjectId,
    val departments: List<Department>,
)

users.aggregate()
    .lookup {
        into(User::departments)
        from(departments.aggregate())
        on(User::department, Department::_id)
    }

Arrays and subdocuments

If you specify a field in a subdocument, the subdocument will be created even if no results are found. The field itself will not.

For example:

class Department(
    val _id: ObjectId,
    val name: String,
)

class DepartmentInfo(
    val _id: ObjectId,
    val department: Department? = null,
)

class User(
    val _id: ObjectId,
    val name: String,
    val departmentInfo: DepartmentInfo? = null,
)

users.aggregate()
    .lookup {
        into(User::departmentInfo / DepartmentInfo::department)
        from(departments.aggregate())
        on(User::departmentInfo / DepartmentInfo::_id, Department::_id)
    }

If no departments are found, here is a potential returned user:

{
    "_id": { "$oid": "699dfad90ca573f85c0eec1c" },
    "name": "Bob",
    "departmentInfo": {}
}

Even though no results were found, "departmentInfo" is still set to an empty document. Since DepartmentInfo._id does not have a default value, this request will fail during deserialization (missing mandatory field).

External resources

Official documentation: this method corresponds to the parameter as.