Hi Aaron,
The nullify of the FK when a parent is deleted can be handled with Foreign Key
Constraint in the database if the relationship is not an attribute.
This works if you use a "real" engine that support deferred constraint checking
like Oracle, Sql Server, FrontBase or PostgreSql at least. This is not
supported in MySQL.
ALTER TABLE "Employee" ADD CONSTRAINT
"FOREIGN_KEY_Employee_CompanyID_Company_id" FOREIGN KEY ("CompanyID")
REFERENCES "Company" ("ID") on delete set null DEFERRABLE INITIALLY DEFERRED;
You have basically the same option as in EOF, you can have the DB react to
delete of parent with these options:
- deny (default behaviour)
- set null (set the FK to null)
- cascade (cascade te delete to children)
The same options re allowed for update with the "on update xxx"
Regards,
Samuel
> Le 28 juill. 2019 à 18:08, Aaron Rosenzweig via Webobjects-dev
> <webobjects-dev@lists.apple.com> a écrit :
>
> Hi Robert,
>
> Alright it’s coming together but let’s make it concrete. Let’s make a
> complete story shall we?
>
> “Employee” table has an FK to “Company”
>
> “Company” has a conceptual “toMany” to “Employee.” You could model it, or
> not. If you model it, you could make it visible, or not (class property).
> This is the big part of the question, how to model this “convenience”
> relationship since it isn’t real, it raises questions.
>
> Given the above, we now delete a company object, what should happen?
>
> If you model the “Company.employees” to-many relationship and make it a class
> property you have a choice for the delete rule:
>
> 1) Deny - if it finds at least one employee, it refuses the delete of the
> company
>
> 2) Nullify - it goes out to find all the 5,000 employees and suddenly breaks
> their bond the company so that they are now without a job.
>
> 3) Cascade - it goes out and terminates, lethally, all 5,000 employees before
> destroying the company.
>
> I’m willing to bet, dollars to donuts, that 1/2/3 will be ignored if the
> “employees” to-many relationship is not a class property. It’s gotta be
> visible for it to do either of those things. That makes sense right? If the
> idea of making it invisible is to not take the hit for faulting in 5k
> employee objects, how could it possibly “nullify” (for example) without
> faulting them in? That’s why it would HAVE to be a class property if you want
> it to do that bookkeeping.
>
> Generally, you’d never delete a company unless you manually, through a clever
> UI, allowed the user to re-home all the employees. In this story line, I
> would not model the “Company.employees” to-many relationship at all. If I
> ever needed that info, I would fetch “Employee.fetch(ec, Employee.COMPANY.is
> <https://ving.apple.com/proxy?t2=dE3O0r2E9w&o=http%3A%2F%2FEmployee.COMPANY.is>(appleComputer).”
> That way I’m taking the hit only when it’s needed. I would also make a true
> FK constraint in the DB that would prevent Apple from being deleted so long
> as there was at least one employee.
>
> I realize your case is not Employee and Company… but any story that has so
> many objects that you feel bad about modeling the to-many I’d feel the same
> about. I wouldn’t want the deletion of the Company to automatically nullify
> the 5k places. That said, it appears you need this… and for that the best
> course of action would be either:
>
> A) Manually fetch the Employee’s where their company relationship is equal to
> the one you are about to delete. Nullify all their relationships to company.
> Delete the company. Save changes. This will take a while if there is a
> plethora of employees. Might need a long running task so that the app doesn’t
> timeout or block other users.
>
> B) Let SQL nullify the FK and then delete the company. This would be fast and
> use little java memory. There are various helper methods to achieve this but
> here is one: ERXEOAccesUtilities.updateRowsDescribedByQualifier()
>
> A and B could be encapsulated in a method
> “Company.takeCareOfDependentsThenDelete()” that you create on Company.
>
>
> AARON ROSENZWEIG / Chat 'n Bike
> <https://ving.apple.com/proxy?t2=dE0W8n0x5N&o=http%3A%2F%2Fwww.chatnbike.com>
> e: aa...@chatnbike.com <mailto:aa...@chatnbike.com> t: (301) 956-2319
>
>
>> On Jul 28, 2019, at 4:42 PM, Robert Hanviriyapunt <robertha...@gmail.com
>> <mailto:robertha...@gmail.com>> wrote:
>>
>> Ok the root problem is that deleting records is leaving bad foreign keys.
>>
>> The reason for the problem is that I made a decision long ago that in
>> certain circumstances I would model to-one relationships with a “hidden”
>> to-many reverse relationship, hopefully to help save memory or something.
>> The “hiding” is done by turning off the “class property” on the reverse
>> to-many relationship but keep the nullify rule. Now when I delete the
>> to-one relationship destination EO, it does not nullify, leaving bad foreign
>> keys.
>
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