Combining queries in Realm? - ios

Question

I have these two objects in my model:
Message:
class Message: Object {
//Precise UNIX time the message was sent
dynamic var sentTime: NSTimeInterval = NSDate().timeIntervalSince1970
let images = List<Image>()
}
Image:
class Image: Object {
dynamic var mediaURL: String = ""
var messageContainingImage: Message {
return linkingObjects(Message.self, forProperty: "images")[0]
}
}
I want to form a query which returns messages and images, messages sorted by sentTime and images sorted by their messageContainingImage's sent time. They'd be sorted together.
The recommended code for a query is this:
let messages = Realm().objects(Message).sorted("sentTime", ascending: true)
This returns a Result<Message> object. A Result doesn't have a way to be joined to another Result. There are other issues in my way too, such as, if I could combine them, how would I then perform a sort.
Additional thoughts:
I could also add a property to Image called sentTime, then once they're combined I'd be able to call that property on both of them.
I could make them both subclass from a type which has sentTime. The problem is, doing Realm().objects(Message) would only returns things which are messages, and not subclasses of Message.
How would I be able to do this?
My end goal is to display these message and image results in a tableview, messages separately from their attached image.

Answer 1

I think, inheritance is not the right solution here, this introduces more drawbacks by complicating your object schema, than it's worth for your use case.
Let's go back to what you wrote is your end goal: I guess you want to display messages and images together in one table view as separated rows, where the images follow their message. Do I understand that correctly?
You don't need to sort both, sorting the messages and accessing them and their images in a suitable way will ensure that everything is sorted correctly. The main challenge is more how to enumerate / random-access this two-dimensional data structure as an one-dimensional sequence.
Depending on the amount of data, you query, you have to decide, whether you can go a simple approach by keeping them all in memory at once, or introducing a view object on top of Results, which takes care of accessing all objects in order.
The first solution could just look like this:
let messages = Realm().objects(Message).sorted("sentTime", ascending: true)
array = reduce(messages, [Object]()) { (var result, message) in
result.append(message)
result += map(message.images) { $0 }
return result
}
While the latter solution is more complex, but could look like this:
// Let you iterate a list of nodes with their related objects as:
// [a<list: [a1, a2]>, b<list: [b1, b2, b3]>]
// in pre-order like:
// [a, a1, a2, b, b1, b2, b3]
// where listAccessor returns the related objects of a node, e.g.
// listAccessor(a) = [a1, a2]
//
// Usage:
// class Message: Object {
// dynamic var sentTime = NSDate()
// let images = List<Image>()
// }
//
// class Image: Object {
// …
// }
//
// FlattenedResultsView(Realm().objects(Message).sorted("sentTime"), listAccessor: { $0.images })
//
class FlattenedResultsView<T: Object, E: Object> : CollectionType {
typealias Index = Int
typealias Element = Object
let array: Results<T>
let listAccessor: (T) -> (List<E>)
var indexTransformVectors: [(Int, Int?)]
var notificationToken: NotificationToken? = nil
init(_ array: Results<T>, listAccessor: T -> List<E>) {
self.array = array
self.listAccessor = listAccessor
self.indexTransformVectors = FlattenedResultsView.computeTransformVectors(array, listAccessor)
self.notificationToken = Realm().addNotificationBlock { note, realm in
self.recomputeTransformVectors()
}
}
func recomputeTransformVectors() {
self.indexTransformVectors = FlattenedResultsView.computeTransformVectors(array, listAccessor)
}
static func computeTransformVectors(array: Results<T>, _ listAccessor: T -> List<E>) -> [(Int, Int?)] {
let initial = (endIndex: 0, array: [(Int, Int?)]())
return reduce(array, initial) { (result, element) in
var array = result.array
let list = listAccessor(element)
let vector: (Int, Int?) = (result.endIndex, nil)
array.append(vector)
for i in 0..<list.count {
let vector = (result.endIndex, Optional(i))
array.append(vector)
}
return (endIndex: result.endIndex + 1, array: array)
}.array
}
var startIndex: Index {
return indexTransformVectors.startIndex
}
var endIndex: Index {
return indexTransformVectors.endIndex
}
var count: Int {
return indexTransformVectors.count
}
subscript (position: Index) -> Object {
let vector = indexTransformVectors[position]
switch vector {
case (let i, .None):
return array[i]
case (let i, .Some(let j)):
return listAccessor(array[i])[j]
}
}
func generate() -> GeneratorOf<Object> {
var arrayGenerator = self.array.generate()
var lastObject: T? = arrayGenerator.next()
var listGenerator: GeneratorOf<E>? = nil
return GeneratorOf<Object> {
if listGenerator != nil {
let current = listGenerator!.next()
if current != nil {
return current
} else {
// Clear the listGenerator to jump back on next() to the first branch
listGenerator = nil
}
}
if let currentObject = lastObject {
// Get the list of the currentObject and advance the lastObject already, next
// time we're here the listGenerator went out of next elements and we check
// first whether there is anything on first level and start over again.
listGenerator = self.listAccessor(currentObject).generate()
lastObject = arrayGenerator.next()
return currentObject
} else {
return nil
}
}
}
}