RealmDataSource<E,O> is a swift implementation of an interface for Realm conforming to DataSource (GetDataSoruce, PutDataSource and DeleteDataSource). It automatically maps entities to realm objects (it requires object mappers) and can run queries to fetch, store and delete objects.

Its interface exposes two generics:

  • E: The entity type
  • O: The equivalent realm object type

Upon initialization, it requires of two mappers:

  • O to E mapper (realm to entity): Using the default Mapper interface.
  • E to O mapper (entity to realm): Using a custom interface called RealmMapper.


// Swift
let dataSource = RealmDataSource(realmHandler: RealmHandler(RealmFactory()),
toEntityMapper: RealmItemToItemEntityMapper(),
toRealmMapper: ItemEntityToRealmItemMapper())
dataSource.get("aRealmObjectId").then { entity in
print("Entity found: \(entity)")
}.fail { error in
if error is CoreError.NotFound {
print ("Entity for given id not found")

Note that the example above is using the extension methods of DataSoruce that encapsulate queries of type IdQuery<String>.

Query Types

get function:

  • IdQuery<Int>: get an object by its id (typed int).
  • IdQuery<String>: get an object by its id (typed string).

getAll function:

  • AllObjectsQuery: get all objects.
  • RealmQuery: get objects by predicate. Read more below.

put and putAll functions:

  • put: Query is not checked. Object is stored directly.
  • putAll: Query is not checked. Objects are stored directly.

delete function:

  • IdQuery<Int>: delete an object by its id (typed int).
  • IdQuery<String>: delete an object by its id (typed string).
  • ObjectQuery<E>: delete the object contained in the query.

deleteAll function:

  • ObjectsQuery<E>: delete the objects contained in the query.
  • AllObjectsQuery: delete all objects.
  • RealmQuery: delete objects by predicate. Reade more below.

RealmQuery protocol

A RealmQuery is a protocol that exposes a predicate. Any Query can adopt this protocol and generate a predicate to represent its equivalent realm predicate.

public protocol RealmQuery : Query {
var realmPredicate : NSPredicate { get }

Note that NSPredicate implements RealmQuery (therefore, can be used directly as a Query):

extension NSPredicate : RealmQuery {
public var realmPredicate : NSPredicate {
return self

For example, imagine we have a query named SearchTextQuery that contains a text field:

struct SearchTextQuery: Query {
let text : String

And our entities we want to search for have a field called text (also present in our Realm object representations). Then, we can do the follwing:

extension SearchTextQuery: RealmQuery {
public var realmPredicate : NSPredicate {
return NSPredicate(format: "text == %@", text)

And now we can run search queries over our RealmDataSource:

dataSource.getAll(SearchTextQuery(text:"textToSearch")).then { array in
print("Entities found: \(array)")


All realm objects used in RealmDataSource must subclass RealmObject, which adds an id:

open class RealmObject : Object {
@objc dynamic public var id : String = ""
public convenience init(_ id: String?) {
if let id = id { = id
} else { = UUID().uuidString
@objc override open class func primaryKey() -> String? {
return "id"

Then, there are implemented extensions that allows searching an existing object Id by a key-value query:

public extension RealmObject {
public static func findId<T>(key: String, value: T, inRealm realm: Realm) -> String? where T : CVarArg
public static func findId<T>(keyedValues: [String : T], inRealm realm: Realm) -> String? where T : CVarArg

This convenience methods are very useful when mapping entities coming from a server (where the realm id is not stored). If the server object has a primary key or a set of variables representing a primary key, this methods will perform the query search and return the id of the object stored in the local realm database or nil if doesn't exist.


The RealmDataSource maps automatically entities to realm objects, and this is done via a RealmMapper:

open class RealmMapper <In:RealmEntity, Out:Object> {
public init() { }
open func map(_ from: In, inRealm realm: Realm) throws -> Out {

This class is very similar to Mapper, but add an extra parameter in the map function to pass the Realm instance where the map takes place.

Note: very often while mapping entities (that contain server primary key) to realm objects (which have its own primary key) it will be required to first search if there is already a realm object representation of that entity. This is accomplished using the findId functions listed above.