Question

I have a string attribute in a Core Data entity whose Max Length value is 40. I’d like to use this value in code and not have to re-type the value “40.” Is this possible?

Answer 1

As @K.Steff says in the comments above, you are better off validating in your code and not setting a max length in your core data model. To add on to that comment, I would also advise you to look at using a custom NSManagedObject subclass for this entity type, and within that subclass overriding the validateValue:forKey:error: or implementing a key-specific validation method for this property.

The value of this approach is that you can do things like "coerce" the validation by truncating strings at validation time. From the NSManagedObject documentation:

This method is responsible for two things: coercing the value into an appropriate
type for the object, and validating it according to the object’s rules.

The default implementation provided by NSManagedObject consults the object’s entity
description to coerce the value and to check for basic errors, such as a null value when
that isn’t allowed and the length of strings when a field width is specified for the
attribute. It then searches for a method of the form validate< Key >:error: and invokes it
if it exists.

You can implement methods of the form validate< Key >:error: to perform validation that is
not possible using the constraints available in the property description. If it finds an
unacceptable value, your validation method should return NO and in error an NSError object
that describes the problem. For more details, see “Model Object Validation”. For
inter-property validation (to check for combinations of values that are invalid), see
validateForUpdate: and related methods.

So you can implement this method to both validate that the string is not too long and, if necessary, truncate it when it is too long.