Question

I am setting a managedObject up from data I am getting off the web, before I add this new object to the managedObjectContext I want to check if its all ready in the database. Is there a way to compare two managed objects in one hit, or do I have to compare each attribute individually to work out if they are identical or one contains a difference?

Simple Example:

Entity:Pet (Created but not inserted into database)
Attribute, Name: Brian
Attribute, Type: Cat
Attribute, Age: 12

Entity:Pet (Currently in database)
Attribute, Name: Brian
Attribute, Type: Cat
Attribute, Age: 7

In this example can I compare [Brian, Cat, 12] with [Brian, Cat, 7] or do I need to go through each attribute one by one to ascertain a full match?

Answer 1

Unique identifiers are often used to search for objects by only having to match the one field. As you note, matching on multiple fields could be annoying and inefficient, but it’s perhaps not as bad as you think: you can construct an NSPredicate to quite easily match all the required fields on objects in Core Data.

Use of NSPredicate aside: suppose you just want to match one field. If you don’t have a suitable unique identifier in the data as provided, you could derive one. The obvious way is to construct a hash code for everything you store, based on each field you want to match on. Then when you wish to check if an ‘incoming’ object is already in core data, compute the hash code for the new object, then just look for an object in core data with that same hash code. (Note: if you find an object that already exists with the same hash code, you might want to then compare all the fields to check that it really does represent the same object — there’s a tiny chance it might be a ‘different’ object, A.K.A. a hash collision).

A very naive hash code implementation for an object X would be something like:

hashcode(X) = hashcode(X.name) + hashcode(X.type) + hashcode(X.age)

To see a more realistic example of writing a hashcode function, see the accepted answer here.

By the way, I’m assuming that you don’t want to load all your objects from core data into memory at once. If however that is acceptable (suppose you have quite a limited amount of items), an alternative is to implement isEqual and hash on your class, and then use regular foundation class methods like NSArray indexOfObject: (or, even better, NSDictionary objectForKey:) to locate objects of interest.