Add support for kotlin immutable collections with a backing mutable collection [DATAMONGO-2229]

[spring-projects-issues]

Knut Schleßelmann opened DATAMONGO-2229 and commented

Currently I'm struggling with the design of some aggregates in kotlin.

I'd like to have a collection exposed as a property but I need the mutable collection to handle things internally. AFAIK the current way to go in kotlin would be

@Document
class Product(
        @Id val id: String,
        attributes: Set<String> = emptySet()
) {

    private val _attributes: MutableSet<String> = attributes.toMutableSet()
    val attributes: Set<String>
        get() = _attributes
}

Sadly this fails because Spring Data Mongo cannot bind the document to my class any more. Even if I annotate _attributes with @Field("attributes") the constructor parameter is only used to determine the property field name (which then doesn't match again).

I'd be great to have better support for such use cases which do not involve hacks using the @Value("#root...") on properties or naming constructor arguments like some internal private field

---

Affects: 2.1.5 (Lovelace SR5)

[spring-projects-issues]

Knut Schleßelmann commented

The best solution I could come up with in the meantime is

@Document
data class Product(
        @Id val id: String
) {

    private var _attributes: MutableSet<String> = mutableSetOf()
    val attributes: Set<String>
        get() = _attributes

    constructor(id: String, attributes: Set<String>) : this(id) {
        _attributes = attributes.toMutableSet()
    }
}

which is propably what I want (equals on id), nice constructor … only "ugly" names in the database. But that's not that tragic.

The var on the mutable collection isn't that tragic either since it's private.

Maybe there's no need of an improvement after all. What do you think?