Objectmapper: Realm models that have a primary key seem to crash when mapped to JSON

[bennichols]

I'm trying to integrate my ObjectMapper codebase (which I love!) with Realm, and I'm having a really hard time getting it to work with even the most simple setup. I have a class that just has a string "id" that I'm telling Realm is the primary key. When I call Mapper().toJSONString() on it, I get a crash: "Terminating app due to uncaught exception 'RLMException', reason: 'Primary key can't be changed after an object is inserted.'" If I remove the primary key, the crash goes away and behaves as intended, but I don't see the point of using a database like Realm if I can't use primary keys.

class IronObject: Object, Mappable {
    dynamic var id: String? = ""

    override static func primaryKey() -> String? {
        return "id"
    }

    convenience init(id: String) {
        self.init()
        self.id = id
    }

    convenience required init?(_ map: Map) {
        self.init()
        mapping(map)
    }

    func mapping(map: Map) {
        id              <- map["id"]
    }
}

let i = IronObject(id: "12345")
let realm = try! Realm()
try! realm.write {
    realm.add(i)
    print(Mapper().toJSONString(i))
}

[tristanhimmelman]

I think you may need to put an if clause around the primary key so that it is only set once:

if id == nil {
    id <- map["id"]
}

The ObjectMapper operator <- has an inout flag on the left hand side variable. This is only necessary when mapping data from JSON. However since the <- operator is also used for writing JSON, Realm assumes that the values are being modified when toJSONString is called.

[bennichols]

Ok, that appears to work. Thanks Tristan!

[j2damax]

when checking below condition toJsonString omit the id in json string, any solution for that?

if id == nil {
id <- map["id"]
}

[tristanhimmelman]

I think this might work:

if map.mappingType == .ToJSON || id == nil {
    id <- map["id"]
}

[j2damax]

Hi thanks for the reply, but when do this still same issue when trying to convert object into json, it says 'RLMException', reason: 'Primary key can't be changed after an object is inserted.'

What i need is when trying to convert ToJson id should be included in json string.

[tristanhimmelman]

I think you may need to store the primaryKey in another variable that is not used directly in the mapping.

id <- map["id"]
if primaryKey == nil {
    primaryKey = id
}

[ivanruizscm]

theres ir a better solution other than cheking if its not set?

[ivanruizscm]

Its weird, plus i need the primary key in the json

[ivanruizscm]

@tristanhimmelman

[tristanhimmelman]

@ivanruizscm, I have never really used Realm with ObjectMapper so I'm not sure if there is a better solution. Perhaps you can just map the primary key to a different variable and set the real primary key variable once during the first mapping?

[thacilima]

@ivanruizscm I also need the primary key in the json and I did like this:

if map.mappingType == .ToJSON {
    var id = self.id
    id <- map["id"]
}
else {
    id <- map["id"]
}

'id' is the realm object primary key. It worked!

[pvsilvestrin]

It works like a charm, thanks @thacilima

[vitovalov]

Thanks for the workarounds,
However on Swift 4 it gives me this compile error:
Binary operator '==' cannot be applied to operands of type 'MappingType' and '_'
And I'm using pod branch swift-4
cc @thacilima

[rafaelkgobara]

@vitovalov

For Swift 4, use:

if map.mappingType == .toJSON

[kkizlaitis]

For those also wondering, I have come up with a _slightly_ more elegant solution:

switch map.mappingType {
    case .fromJSON:
        id <- map["id"]
    case .toJSON:
        id >>> map["id"]
}

Here id is the _primaryKey_ in _Realm_.