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Re: Delete cascade?
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Re: Delete cascade?


  • Subject: Re: Delete cascade?
  • From: Chuck Hill <email@hidden>
  • Date: Mon, 19 Mar 2007 16:48:35 -0700


On Mar 19, 2007, at 4:20 PM, Ian Joyner wrote:
On 20/03/2007, at 2:57 AM, Chuck Hill wrote:
On Mar 18, 2007, at 10:11 PM, Ian Joyner wrote:

What you say makes sense. Maybe, because my dependent entity does not have any further cascades, I'm not seeing anything else being brought into memory (maybe it's already there).

I think what I would expect is to see is:

SELECT * FROM <owned_table> WHERE OWNER_KEY = <PRIMARY_KEY of owner>

<processing of any cascades from those records>

DELETE FROM <owned_table> WHERE OWNER_KEY = <PRIMARY_KEY of owner>

That is not safe. What if records were added to <owned_table> with an OWNER_KEY = <PRIMARY_KEY of owner> while you were processing the cascades?

My unease is growing here, but then again I suspect it must be right because it has been in operation for such a long time.

I suspect you might be misunderstanding the sequence of events.


I would guess that WO is handling the cascades due to underlying DBs handling them in different ways.

No. EOF is handling them that way as it needs to be able to call validateForDelete on the objects and may need information they possess to process other delete rules.



I think records could be added to the <owned_table> by another transaction from another user in any situation, unless maybe the whole table is locked, or at least the parent record is locked in such a way that owned records cannot be added. So you want the DELETE to be atomic.

The delete _is_ atomic. Maybe hundreds of single DELETE statements, but all within the context of a single, atomic DB transaction. The DB transaction spans a shorter time time (read DB, process rules, update DB) than the single, atomic EOF transaction. It is important to keep in mind that the DB transaction is "nested" within the EOF one.


What you _don't_ want is to successfully delete the parent if someone has just added children. Your suggested delete would do just that. That might be acceptable in some situations, but not in all. EOF pointedly deletes what it knows about and nothing else. If there are other records, the DB will return an error when the transaction is committed. This will inform the application which can then take appropriate action.



I suspect that is why the designers of EOF elected to not use this "optimization". Generally, they seem to have chosen safe over fast in implementing EOF.

I agree with that, but I have a feeling that expanding the transaction over time makes for more chance for untoward things to happen. However, limiting the possibility for a race condition to occur does not mean that it won't occur and have very nasty consequences.

I don't think there is any race condition as EOF is implemented. One process or the other will receive the correct error notification.



Thanks for your and Peter's answers, I don't want to waste anymore of people's time on it, since the answer seems to be that it's working as intended (and hopefully correctly) so it's nothing I'm doing that's stupid to cause seemingly non-optimum (and I hope safe) behaviour. I don't believe this is covered in the wikibook yet.

Learning more about how EOF works is rarely a waste of time. I doubt that is in the Wiki.



Chuck



However, all I see is the DELETEs a single one at a time. Maybe I just have an simple case?


Thanks
Ian

On 19/03/2007, at 2:08 PM, Peter Vandoros wrote:

Hi Ian,

WebObjects does this because it needs to bring all the EO's into memory to process the delete rules specified in your EOModel.

Regards

Peter

Ian Joyner wrote:
I just noticed in my system that if I have the cascade delete rule set on a to-many relationship in EOModeler that the deletes happen one by one.

That is if I have a master record with three detail records, WO issues three SQL DELETEs.

The setup is, each record has a primary key PRIMARY_KEY, which relates to the owned records which have OWNER_KEY. To do the delete cascade, I would think that DELETE FROM <owned table> WHERE OWNER_KEY = <PRIMARY_KEY of owner> would suffice.

Have I got something set up wrong here, or am I missing something?

Thanks
Ian Joyner
Sportstec

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  • Follow-Ups:
    • Re: Delete cascade?
      • From: Ian Joyner <email@hidden>
References: 
 >Delete cascade? (From: Ian Joyner <email@hidden>)
 >Re: Delete cascade? (From: Peter Vandoros <email@hidden>)
 >Re: Delete cascade? (From: Ian Joyner <email@hidden>)
 >Re: Delete cascade? (From: Chuck Hill <email@hidden>)
 >Re: Delete cascade? (From: Ian Joyner <email@hidden>)

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