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ManyToMany: Why is no exception thrown? [message #379033]

Mon, 11 August 2008 09:57

Christian Reiter

Messages: 4
Registered: July 2009

Junior Member

Hi!

I've defined two Entity Objects named Person and City. They have a
ManyToMany JPA relationship.

The relationship is established through a relation table in the DB named
CITY_PERSON.

My Problem is that when i try to remove a City which is "in
use"/referenced by some Person i do not get an exception, instead the City
as well as the row in CITY_PERSON gets deleted without a warning.

When i try to remove a Person i do get a RollbackException as expected and
the Person is not removed.

Why do i not get an exception when removing a used/referenced City?

Is there any way to check if any entity has references prior to removing
it.

Best regards,
christian

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Re: ManyToMany: Why is no exception thrown? [Code Example] [message #379034 is a reply to message #379033]

Mon, 11 August 2008 09:58

Christian Reiter

Messages: 4
Registered: July 2009

Junior Member

PS:
You can review my code here:
http://web.percom.at/ManyToMany.html

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Re: ManyToMany: Why is no exception thrown? [message #379036 is a reply to message #379033]

Mon, 11 August 2008 13:21

James

Messages: 272
Registered: July 2009

Senior Member

JPA does not maintain bi-directional relationships for you. When you
remove an object or one side of a relationship you are responsible for
maintaining the other side of the relationship.

You do not get the error from one side because it owns the relationship
and removes the entries from the join table when being deleted, the other
side is read-only so does not, and gets a database constraint error. In
both cases your application or model code is responsible for removing all
references to the object being deleted.

You could potentially use a preRemove event to traverse your object's
relationships and remove its' references. Although if the user is
deleting an object that has references you may consider this application
logic.

-- James

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Re: ManyToMany: Why is no exception thrown? [message #379037 is a reply to message #379036]

Mon, 11 August 2008 13:39

Christian Reiter

Messages: 4
Registered: July 2009

Junior Member

Thanks James for your fast response!

Is there any JPA- or EclipseLink-specific way to check if an Entity has
any refrences? Or do i have to implement this myself?

best regards,
christian

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Re: ManyToMany: Why is no exception thrown? [message #379038 is a reply to message #379037]

Mon, 11 August 2008 14:33

Tom Eugelink

Messages: 817
Registered: July 2009

Senior Member

> Is there any JPA- or EclipseLink-specific way to check if an Entity has
> any refrences? Or do i have to implement this myself?

I prefer reverse engineering the BM from the database: you then know for sure that all relations have the appropriate handling.

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Re: ManyToMany: Why is no exception thrown? [message #379263 is a reply to message #379038]

Tue, 12 August 2008 06:44

Christian Reiter

Messages: 4
Registered: July 2009

Junior Member

Hi tbee!

What means "BM"?

thanks,
christian

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Re: ManyToMany: Why is no exception thrown? [message #379265 is a reply to message #379037]

Tue, 12 August 2008 13:26

James

Messages: 272
Registered: July 2009

Senior Member

There is no specific way. If you make the relationships bi-directional,
then you can at least navigate to the references from the object.
Otherwise you would need to perform a query to find the references.

-- James

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