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Re: Core Data: unidirectional relationships

Subject: Re: Core Data: unidirectional relationships
From: Adam P Jenkins <email@hidden>
Date: Wed, 13 Feb 2008 13:32:42 -0500
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This was my first idea also on seeing the original post. I didn't post it though because the problem is fetched properties are modeled as to-many relationships. That is, when you access the fetched property you get back a set or array, not a single object. It seems kind of messy that when you access a Department's headOfDepartment attribute, you'd get back an NSSet containing the department head rather than getting an Employee object. If you ignore that flaw though, then it would work nicely. You could always hide this messiness with some additional wrapper properties which pull the object out of the set.

On Feb 13, 2008, at 12:52 PM, William Turner wrote:

Hi Ruotger,
Another alternative that might work for you is to add a "title" (string) attribute to Employee and fetched properties to the Department for each of headOfDepartment, viceHeadOfDepartment, pointyHairedBoss, and dilbert, with their predicates being title=="Head", title=="ViceHead", etc.

You can read more about using fetched properties to represent one- way relationships near the end of http://developer.apple.com/documentation/Cocoa/Conceptual/CoreData/Articles/cdRelationships.html
Wil
On Feb 13, 2008, at 9:29 AM, Brian Williams wrote:
I have had to deal with this also. I think that the best solution is to just have those empty back links and accept the clutter. You could also have subclasses of employee, but i am sure that would cause other problems.

On 2/13/08, Ruotger Skupin <email@hidden> wrote:
Hi,
consider the following example (that what I would like to model):
Entity: Department
Attributes: ...whatever...
Relationships:
headOfDepartment --> Employee
viceHeadOfDepartment --> Employee
pointyHairedBoss -->
dilbert -->
other Employees -->>
Entity: Employee
Attributes: ...whatever...
Relationships:
department --> Department
Every Employee is only in one Department. The Department has (and
needs) four special Employees (head, vice head, pointy haired boss,
dilbert). Turns out that it is impossible to model this with
bidirectional relationships if you don't want to model it like that:
Entity: Employee
Attributes: ...whatever...
Relationships:
departmentImHeadOf --> Department
departmentImViceHeadOf --> Department
departmentImPointyHairedBossOf --> Department
departmentImDilbertOf --> Department
departmentOtherEmployeeOf --> Department
The other alternative is to model it with unidirectional relationship, which is discouraged.
Can anyone gime some advice on this?
Ruotger
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References:
	>Core Data: unidirectional relationships (From: Ruotger Skupin <email@hidden>)
	>Re: Core Data: unidirectional relationships (From: "Brian Williams" <email@hidden>)
	>Re: Core Data: unidirectional relationships (From: William Turner <email@hidden>)

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