Re: Question about Core Data Inverse Relationships

Well, I'm not sure plain logic would tell me this is impossible.  
Clearly it is impossible using just a single relationship, but in many  
OO programs I have written there are objects that will live in  
different collections based on their current application state. This  
is precisely what the Attribute is in my problem. When a new Attribute  
begins life the process would be as follows in a pure OO  
implementation sense:

1. Locate any existing Attribute in the "currentAttributes" collection  
that matches the attribute name.
2. If an object was found in step 1 move it to the "oldAttributes"  
collection so we can keep around knowledge that is once existed.
3. Insert the new Attribute into the "currentAttributes" collection  
and set the dateRecorded date to "now".

This seems completely reasonable in an application, but clearly there  
is more work to do before it can be translated into something Core  
Data is happy with. That is why I posted the question.

Since Attribute contains the same attributes whether it is in  
"currentAttributes" or "oldAttributes" it seems kind of silly to  
maintain two relationships on every Attribute. If one choose to go  
this route how would one represent that either the relationship for  
"currentAttributesUser" or "oldAttributesUser" must always be nil  
since an attribute cannot be in both relationships at the same time?

I was looking at the possibility of using a Core Data abstract entity  
as a "base class" for two different attribute types. I would put all  
of the shared attributes in the abstract entity and only have the  
relationships in the derived entities. I tried finding more  
information in the Core Data book by Marcus Zarra, but nothing  
immediately jumped out at me.

Could someone familiar with the use of Abstract Entities in Core Data  
please comment on whether this approach is worth considering?

Many thanks.

-- Bennett

One possibility
On Sep 20, 2009, at 3:07 AM, Christiaan Hofman wrote:

>
> On Sep 20, 2009, at 6:18, Bennett Smith wrote:
>
>> Well, I tried to attach the diagram, but guess the mailing list  
>> strips it out.  The jist of the diagram was that I have the  
>> following entities:
>>
>> User
>> name
>> currentAttributes (to-many relationship with Attribute)
>> oldAttributes (to-many relationship with Attribute)
>>
>> Attribute
>> dateRecorded
>> name
>> value
>> user (relationship with User)
>>
>> The problem I'm having is with setting up the inverse relationship  
>> in the Attribute entity. I would to specify an inverse relationship  
>> for both currentAttributes and oldAttributes, but can't see how to  
>> do this.
>>
>> Many thanks, and sorry for the confusion about posting the diagram.
>>
>> Cheers
>>
>> -- Bennett
>
> Plain logic tells you this is impossible. An attribute can be both  
> an oldAttribute and a currentAttribute,so which user should be set  
> in the user property? You should not have to ask this. So you'll  
> have to change your model, and add another user relationship to use  
> as the inverse of oldAttributes.
>
> Christiaan
>
>>
>> On Sep 19, 2009, at 9:14 PM, Bennett Smith wrote:
>>
>>> Forgot to attach the diagram:
>>>
>>>
>>>
>>> On Sep 19, 2009, at 9:09 PM, Bennett Smith wrote:
>>>
>>>> Hello
>>>>
>>>> I am stuck on a problem that I just can't figure out.  I am very  
>>>> new to Core Data so it might just be that I am going about this  
>>>> wrong. I want to setup a model for something like this:
>>>>
>>>>
>>>> The currentAttributes and oldAttributes relationships in User are  
>>>> configured as "to-many relationships." My problem is in figuring  
>>>> out how to set an inverse relationship in the Attribute class. It  
>>>> seems like I cannot have an inverse relationship that points back  
>>>> to User for all Attribute entities in this case. (At least I  
>>>> can't figure out how to do it in Xcode.)
>>>>
>>>> The purpose for this model is to allow me to track a set of  
>>>> attributes associated with a user and to keep track of any  
>>>> changes in the attribute values over time. A change is defined as  
>>>> replacing the value associated with the attribute or deleting an  
>>>> attribute. I am open to other suggestions on how to achieve this  
>>>> goal using Core Data.
>>>>
>>>> Thanks
>>>>
>>>> -- Bennett
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>>>
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