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Posted to dev@cayenne.apache.org by Andrus Adamchik <an...@objectstyle.org> on 2007/06/01 11:32:58 UTC
Re: vertical inheritance joins
This was a long thread, and I may have missed some points already
made. Let me try to summarize this discussion and add my comments to
it. There are two aspects of the "vertical inheritance relationship":
1. Specifying the join semantics for non-trivial PK-to-PK mapping
(such as compound keys).
2. Specifying delete rules.
I am with Ari on (1) - it is in the Cayenne philosophy to map things
explicitly, so that various weird legacy schemas can still be mapped.
I am with Craig on (2) - if a user expects to delete a record from a
subclass table without deleting a record in a superclass table, this
situation should not be mapped as inheritance, as it breaks the ORM
model at a fundamental level (an object is deleted, but some of its
properties still remain persistent - BAD).
With this in mind we do not need an inheritance ObjRelationship, but
we may use a DbRelationship to store join semantics (anybody
mentioned that already??). Another nice side effect of it is that
such relationship is not a part of the object model (ObjRelationship
would've been an object property). So there is nothing artificial
about such mapping, and no new concepts are needed in Cayenne to map it.
Andrus
Re: vertical inheritance joins
Posted by Lachlan Deck <la...@gmail.com>.
Hi Andrus,
On 03/06/2007, at 7:12 PM, Andrus Adamchik wrote:
> On Jun 1, 2007, at 3:24 PM, Lachlan Deck wrote:
>> On 01/06/2007, at 7:32 PM, Andrus Adamchik wrote:
>>
>>> With this in mind we do not need an inheritance ObjRelationship,
>>> but we may use a DbRelationship to store join semantics (anybody
>>> mentioned that already??).
>>
>> Well the original suggestion I made was for <obj-entity
>> superRelationship="nameOfRelationship" ...> to allow for
>> specifically specifying vertical inheritance (which would have the
>> effect of not listing this relationship along with those returned
>> from objEntity.getRelationships() but via
>> objEntity.getSuperRelationship().
>
> This is what I was trying to avoid - introducing relationships with
> special behavior.
Okay. It'd be perhaps less obvious what type of inheritance was in
play in the modeller but then again perhaps a simple dynamic label
could display something to the same effect along with the below
validation.
>> Are you suggesting that you'd prefer <db-entity
>> superRelationship="nameOfRelationship" ...>
>
> No, as DbEntities (tables) do not have inheritance among
> themselves. It would be more of <obj-entity
> superDbRelationship="nameOfDBRelationship" ...>, but I do believe
> that this can be made implicit, as arguably there can be only one
> DbRelationship between the primary keys of two tables. E.g:
>
> SuperOE -> DB1
> SubOE1 -> DB2
> SubOE2 -> DB3
>
> In this case Cayenne can easily figure out the name of DB2->DB1 and
> DB3->DB1 relationships based on relationship semantics.
Okay great. I suppose there'd need to be validation to that effect to
ensure that such relations exist when choosing a different DbEntity
from the parent entity.
>>> Another nice side effect of it is that such relationship is not a
>>> part of the object model (ObjRelationship would've been an object
>>> property). So there is nothing artificial about such mapping, and
>>> no new concepts are needed in Cayenne to map it.
>>
>> Can you clarify this a bit more.
>
> See above - I don't want to redefine what ObjRelationship is. The
> difference between Cayenne and EOF is that Cayenne splits DB and
> Java mapping in two separate layers of metadata. EOF allows to mark
> a relationship as "not included in the object model", while Cayenne
> allows to map the DB relationship without mapping corresponding
> ObjRelationship, essentially achieving the same thing in a
> different way.
Thanks. Just a slight mind-shift ;-)
with regards,
--
Lachlan Deck
Re: vertical inheritance joins
Posted by Andrus Adamchik <an...@objectstyle.org>.
Hi Lachlan,
On Jun 1, 2007, at 3:24 PM, Lachlan Deck wrote:
> On 01/06/2007, at 7:32 PM, Andrus Adamchik wrote:
>
>> With this in mind we do not need an inheritance ObjRelationship,
>> but we may use a DbRelationship to store join semantics (anybody
>> mentioned that already??).
>
> Well the original suggestion I made was for <obj-entity
> superRelationship="nameOfRelationship" ...> to allow for
> specifically specifying vertical inheritance (which would have the
> effect of not listing this relationship along with those returned
> from objEntity.getRelationships() but via
> objEntity.getSuperRelationship().
This is what I was trying to avoid - introducing relationships with
special behavior.
> Are you suggesting that you'd prefer <db-entity
> superRelationship="nameOfRelationship" ...>
No, as DbEntities (tables) do not have inheritance among themselves.
It would be more of <obj-entity
superDbRelationship="nameOfDBRelationship" ...>, but I do believe
that this can be made implicit, as arguably there can be only one
DbRelationship between the primary keys of two tables. E.g:
SuperOE -> DB1
SubOE1 -> DB2
SubOE2 -> DB3
In this case Cayenne can easily figure out the name of DB2->DB1 and
DB3->DB1 relationships based on relationship semantics.
>> Another nice side effect of it is that such relationship is not a
>> part of the object model (ObjRelationship would've been an object
>> property). So there is nothing artificial about such mapping, and
>> no new concepts are needed in Cayenne to map it.
>
> Can you clarify this a bit more.
See above - I don't want to redefine what ObjRelationship is. The
difference between Cayenne and EOF is that Cayenne splits DB and Java
mapping in two separate layers of metadata. EOF allows to mark a
relationship as "not included in the object model", while Cayenne
allows to map the DB relationship without mapping corresponding
ObjRelationship, essentially achieving the same thing in a different
way.
Andrus
Re: vertical inheritance joins
Posted by Lachlan Deck <la...@gmail.com>.
On 01/06/2007, at 7:32 PM, Andrus Adamchik wrote:
> With this in mind we do not need an inheritance ObjRelationship,
> but we may use a DbRelationship to store join semantics (anybody
> mentioned that already??).
Well the original suggestion I made was for <obj-entity
superRelationship="nameOfRelationship" ...> to allow for specifically
specifying vertical inheritance (which would have the effect of not
listing this relationship along with those returned from
objEntity.getRelationships() but via objEntity.getSuperRelationship().
Are you suggesting that you'd prefer <db-entity
superRelationship="nameOfRelationship" ...> or just figuring out the
relationship by finding the one that happens to map to the parent as
a toOne?
The nice thing I thought about having a superRelationship definition
in the modeler is that you can then make your inheritance choice in
one place and it's clear of the choice. i.e., the above would just
require an additional popup next to the popup for which DbEntity an
ObjEntity maps to to choose a relationship. Otherwise perhaps it
wouldn't be as clear whether such a relationship exists.
The other option I considered initially was having superRelationship
as a boolean flag with validation to ensure there was an accompanying
relationship called 'super' but thought this unnecessarily
restrictive in the end.
> Another nice side effect of it is that such relationship is not a
> part of the object model (ObjRelationship would've been an object
> property). So there is nothing artificial about such mapping, and
> no new concepts are needed in Cayenne to map it.
Can you clarify this a bit more.
Thanks.
with regards,
--
Lachlan Deck