Enums and Custom Types

ObjectBox Swift supports enums and custom types: here's how.

Custom types allow entities to have properties of any type. This is based on mapping custom classes to built-in types. ObjectBox recognizes the following built-in (Swift) types:

Int8, UInt8
Int16, UInt16
Int32, UInt32
Int64, Int, UInt64, UInt
Float, Double
Data, [UInt8]


ObjectBox has built-in support for RawRepresentable enums. All that is needed is an annotation to tell ObjectBox to convert an enum:

enum TestEnum: Int {
    case unknown = 0
    case first = 100
    case second = 200
    case third = 300

class EnumEntity: Entity {
    var id: Id = 0
    // objectbox: convert = { "default": ".unknown" }
    var custom: TestEnum

Specify a default value to use when the value in the database is missing or invalid as the default argument to the // objectbox: convert annotation.

A database can have a missing value even for non-optional fields if you add a property to your entity and then open a database that was written by an older version of your app that did not have that field yet. A database may contain an invalid value if an older version of your app opens a file created by a newer version of your app that has added a new case to this enum.

When persisting enums, there are a couple of best practices:

  • Always assign explicit rawValues to each enum case: Implicitly-assigned rawValues are unstable, and can easily change the next time you edit your enum definitions.

  • Prepare for the unknown: Define an unknown enum value and specify it as the default. It can serve to handle missing or unknown values. This will allow you to gracefully handle cases like an old enum value getting removed without having to constantly unwrap optionals.

You can leave away the "default" annotation if your property is an optional. In that case nil will be the automatic default.

Enums in queries

QueryBuilder is unaware of enums. You have to use the enum's rawValue for queries.

So for the EnumEntity example above you would get users with the custom property of second with the query condition box.query { User.role == TestEnum.second.rawValue }.

Custom Property Converters

To add support for a custom type, you can map properties to one of the built-in types using an // objectbox: convert annotation. You also need to provide a class to serve as a property converter.

For example, you could define a color in your entity using a custom Color class and map it to a String.

Here is an example mapping an enum to an Int manually:

class RoleConverter {
    static func convert(_ enumerated: Role) -> Int {
        return enumerated.rawValue
    static func convert(_ num: Int?) -> Role {
        guard let num = num else { return Role.default }
        return TestEnum(rawValue: num) ?? Role.default

enum Role: Int {
    case default = 0
    case author = 1
    case admin = 2

class User: Entity, CustomDebugStringConvertible {
    var id: Id = 0
    // objectbox: convert = { "dbType": "Int", "converter": "RoleConverter" }
    var role = Role.default

Things to look out for

Be sure to correctly handle nil and invalid values: If you add a field to your entity later on, old records in a database will not have a value for this field. Your converter will be handed a nil value for those instead. Or if a user opens a database created with a newer version of your app that supports additional values for an enum with an older version that doesn't know about these, you will have to supply a fallback value from your converter (In the above example, those are the two Role.default returns in convert(_: Int?)).

You must not interact with the database (such as using Box or Store) inside the converter. The converter methods are called within a transaction, so for example getting or putting entities into a box will fail.

If you implement your database-to-class converter method to not take an optional, ObjectBox will supply an appropriate 0 value for missing values, Same as it would for the underlying type without a converter.

Note: Make sure the converter is thread safe, because it might be called concurrently on multiple entities.

List/Array types

At the moment it is not possible to use arrays with converters, apart from [UInt8], which is treated like Data. However, you could convert a List of Strings to a JSON array resulting in a single string for the database.

Alternately, you can replace arrays with relations and create a new entity for the array elements. So, for example instead of

class Document {
    var id: Id = 0
    var userNames: [String] // ObjectBox doesn't know how to store this

You would have two entities:

class Document {
    var id: Id = 0
    var users: ToMany<User>

class User {
    var id: Id = 0
    var document: ToOne<Document> = nil
    var name: String

Custom types in queries

QueryBuilder is unaware of custom types. You have to use the raw database value for queries.

So for the User example above you would get users with the role of admin with the query condition box.query { User.role == Role.admin.rawValue } or box.query { User.role == 2 }.

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