ReactiveData.sketch.swift

// `Entity` protocol.
protocol Entity: Codeable {
  var identifier: UInt { get }
}

protocol EntityIdentifer: Codeable {
  var identifier: UInt { get }
}

struct Identifier<E: Entity>: EntityIdentifer {
  var identifier: UInt { get }
}

struct Tweet: Entity {
  var user: Identifier<User>
  var content: String
  var date: Date
}

// Query
let tweets: SignalProducer<[Tweet], NoError> = context.fetch(Tweet.self)
  .filter(by: .date, condition: .greaterThan(Date().addingTimeInterval(-1000.0)))
  .filter(by: .user, condition: .equal(Identifier<User>(1)))

// Query and Live Updating Instances
let tweets: SignalProducer<[Property<Tweet>], NoError> = context.properties(Tweet.self)
  .filter(by: .date, condition: .greaterThan(Date().addingTimeInterval(-1000.0)))
  .filter(by: .user, condition: .equal(Identifier<User>(1)))

// Live Updating Query
let tweets: SignalProducer<ArrayDelta<Tweet>, NoError> = context.collection(Tweet.self)
  .filter(by: .date, condition: .greaterThan(Date().addingTimeInterval(-1000.0)))
  .filter(by: .user, condition: .equal(Identifier<User>(1)))

// Live updating instance
let tweet: Property<Tweet> = context.property(for: Identifier<Tweet>(0))

// Live updating relationship
let tweet: Property<User> = context.property(for: Identifier<Tweet>(0))
  .flatMap(.latest) { context.property(for: $0.user) }