CoreData model design: Is excessive use of NSFetchRequest a symptom of a poorly designed model?

Question

Core Data objects can be retrieved by using an NSFetchRequest or by following relationships with other objects in the graph. Is it fair to say that in a well designed model will contain sufficient relationships (and fetched properties) such that the use of NSFetchRequests are kept to a minimum?

The counter argument is that in iOS there exists NSFetchedRequestController. Presumably if Apple believed that relationships and fetched properties provided satisfactory performance with the existing faulting/caching they would not have created NSFetchedRequestController.

There are instances when using an NSFetchRequest are superior as Core Data can do all the work within SQLite. An example would be fetching aggregate values.

Any thoughts on this? I've had a look at the Core Data Programming Guide. There's relevant advice in the 'Fetching Managed Objects' and 'Core Data Performance' sections, but nothing to strongly suggest relationships over fetch requests or vice versa.

Answer 1

I would say that relationships and NSFetchRequest each have their strengths and weaknesses. As a developer you should know when it is appropriate to use one or the other. For example take this model:

--------------  1           * ------------
| Department |<------------->>| Employee |
--------------                ------------
| name       |                | name     |
--------------                | age      |
                              | salary   |
                              ------------

If you want to retrieve all the employees belonging to a particular department then it is appropriate to follow the relationship 'department.employees'. It is not appropriate to create an NSFetchRequest for the Employee entity with predicate 'department == xxxx'.

Conversely, if you want to find employees for a particular department with a salary > x then it is appropriate to use an NSFetchRequest with a predicate like 'department == xxxxx AND salary > x'. It is not appropriate to retrieve all the employees using the department.employees relationship and then iterate over them in a loop to find the high-earners.

In summary, relationships or NSFetchRequests are not inherently 'good' or 'bad'. Just use them appropriately. Use relationships to navigate your object graph. Use NSFetchRequest to perform subset searches or where you need the flexibility of returning results as dictionaries or need to use NSExpressions.

EDIT TO ADD:

A lot of NSFetchRequests littered all over your code is a sign of bad design. I always encapsulate data requests in custom NSManagedObject classes and try not to have any Core Data calls in view/view controller code.

Fetched properties are, I suppose, a way of achieving the same thing. I prefer to create an NSFetchRequest in code rather than use the Core Data editor to do it. It is far more flexible but both ways amount to the same thing really.