Realm Many to Many Query (Inverse too!) - ios

Question

My app's db has a many to many relationship between a Feed object and a Tweet object. This is to keep track of which feeds every tweet belongs in. If you're familiar with Twitter, imagine the main feed, a list feed, a user profile feed, etc.
How can I make a query using an NSPredicate to get a list of Tweets that exist in a specific Feed (and, inversely, get a list of Feeds that a Tweet exists in)? It seems that queries on inverse relationships does not work in Realm, so what are my options?

Answer 1

If I understand your question correctly this part of the documentation should be helpful:
Inverse Relationships Links are unidirectional. So if a to-many
property Person.dogs links to a Dog instance and a to-one property
Dog.owner links to Person, these links are independent from one
another. Appending a Dog to a Person instance’s dogs property, doesn’t
automatically set the dog’s owner property to this Person. Because
manually synchronizing pairs of relationships is error prone, complex
and duplicates information, Realm exposes an API to retrieve backlinks
described below.
With inverse relationships, you can obtain all objects linking to a
given object through a specific property. For example, calling
Object().linkingObjects(_:forProperty:) on a Dog instance will return
all objects of the specified class linking to the calling instance
with the specified property.
I guess you can do something like:
//assuming your Tweet object has a property like "let feeds = List<Feed>()"
someTweet.linkingObjects(Feed.self, forProperty: "feeds") //should return feeds your Tweet is in
But still I don't think I understand your question clearly. From my point of view your first requirement:
get a list of Tweets that exist in a specific Feed
should have a straightforward solution such as having a property in your Feed object like:
let tweets = List<Tweet>()
I wish you can clarify your situation further.

Answer 2

I wonder if it's possible to simplify the model a bit so many-to-many isn't necessary.
My understanding of Twitter is that tweets aren't 'owned' by any feeds. They simply exist on the platform, and are referenced by any number of feeds, but don't actually belong to any specific feed.
So a model setup like this should be appropriate:
class Tweet : Object {
}
class Feed : Object {
let tweets = List<Tweet>()
}
You can do a reverse lookup on a Tweet to see if there are any feeds in which it is currently visible, and you can simply use the tweets property of Feed objects to see which tweets they're displaying
Since the linkingObjects reverse lookup method of Realm simply returns a standard Swift Array, if you did want to filter that further, you could just use the system APIs (like filter or map) to refine it further.
Otherwise, if you really do want to be able to use Realm's NSPredicate filtering system both ways, then, as messy as it is, you would need to manually have each model linking to a list of the other:
class Tweet : Object {
let feeds = List<Feed>()
}
class Feed : Object {
let tweets = List<Tweet>()
}
While it's not recommended (Since it adds additional work), it's not disallowed.
Good luck!