CODEWITHSUNDEEP
ioscocoacore-dataunique
Dominik
Dominik

Reputation: 599

iOS Core Data unique object fetch

I'm fairly new to iOS/Cocoa and I have a very fundamental question about core data. I was looking over the internet to find a appropriate answer/solution but I wasn't able to.

So the question is how do you handle uniqueness in core data? I know that core data is not a database, its something like an object graph. Lets assume we have an entity called 'User' with the attributes 'id' and 'name' and a few relations to the other entities. Now we want to update the name of a specific user (e.g. a web service gave us the id and the new name).

This was the way I have done that before:

NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:@"User" inManagedObjectContext:context]];
[request setPredicate:[NSPredicate predicateWithFormat:@"id == %@", anId]];

NSError *error = nil;
NSArray *results = [context executeFetchRequest:request error:&error];
User *user = [results lastObject];

But than I've heard this is bad practice. Is it because fetch requests with predicates are very slow? I can't imagine that this is such a big deal. As far as I know there is no other way to get a unique object rather than go over each object and checking for equality..

Would it be more efficient to fetch all objects of the entity, put them in an array and looping through it manually (instead of the fetch request)?

NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:@"User" inManagedObjectContext:context]];

NSError *error = nil;
NSArray *results = [context executeFetchRequest:request error:&error];

User *resultUser;

for(User *user in results){
    if([user.id isEqual:anId]) resultUser = user;
}

Please help me finding the right path and thanks.

Upvotes: 2

Views: 2117

Answers (2)

TechZen
TechZen

Reputation: 64428

You're not fetching a unique object but rather objects containing a notionally unique atrribute value.

All managed objects are unique instances but nothing in Core Data enforces that the attribute values are unique. You could in principle have an arbitrary number of unique managed objects all which had identical attribute values. Only relationships enforce a unique position in the object-graph.

There's no particular reason not to fetch a particular object that contains a particular value if that is what your app requires.

I think what you've read is warnings against trying to cram SQL-like key values into entities and then to try and link managed objects together with those keys using predicates, for example doing something like:

EntityOne{
  sqlKey:string
}

EntityTwo{
  sqlKey:string
}

… and then trying to relate objects of the two entities with predicates. 

   (anEntityOneObject.)sqlKey == anEntityTwoObject.sqlKey

… instead of just setting a relationship in the data model.

EntityOne{
  two<-->EntityTwo.entityOne
}

EntityTwo{
  one<-->EntityOne.two
}

… and just finding the related objects with AnEntityOneObj.two.

Upvotes: 1

omz
omz

Reputation: 53551

If you have an index for the property that you're fetching (there's a checkbox in the model editor), it's definitely a lot more efficient to use an equality predicate for fetching. Actually, it's never more efficient to fetch all objects and iterate over them in memory, but an index makes the difference more significant.

Upvotes: 4

Related Questions