[Cayenne-user] Auto-creation of reverse relationships

This comes from EntityResolver.applyDBLayerDefaults(). The commit where
this was introduced (Revision 606720) says:

"JPA: adding extra step - connecting reverse relationships on Cayenne end
for consistent runtime..."

So is it OK to turn this off? Or will things blow up?

Thanks,
John

Hi John,

I don't know the answer to your question offhand, but what is the harm
that is being caused from your perspective (aside from extra logs on
startup)?

Thanks,

mrg

My concern with the automatic reverse relationships is if adding an object fires a fault. Firing a fault on a small dimension table could result in attempting to load millions of records from the detail table into memory. As long as the automatic reverse relationship doesn't do that, I don't care.

Joe

Hi Joe,

EOF had a problem with unintentionally (or unexpectedly) fired faults
loading in extra records (yes, potentially millions), especially when
using addObjectToBothSidesOfRelationshipWithKey, but I've not seen
this happen in Cayenne and I believe I recall Andrus saying it doesn't
happen unless you explicitly trigger a fault -- such as
state.getAddresses(). If you have an Address and do
address.setState(state) you should be fine, even if you have an
explicit or complimentary relationship going the other way (State ->>
Address).

mrg

My entity classes use explicit fields to store data instead of a Map, so
when it tries to access the "runtimeRelationshipX" fields it blows up
because these don't exist in the generated java classes. I've dealt with
it so far by just modeling all the relationships so that the fields are
there to hold the data. But now I'm using more introspection and having
these unnecessary relationships is requiring workaround that I would prefer
not to have.

I did try turning this off today and thus far everything seems ok, but
there is a lot more to test to be sure.

The purpose of "runtime" relationships is to allow you to skip certain relationships from your data model. E.g. artist.paintings is an explicit to-many, but there may be no painting.artist. With runtime relationships there, adding a new painting to an artist would correctly update painting ARTIST_ID on save.

In a default CayenneDataObject situation auto-reverse relationships should do no harm (such as triggering fault fetches ... if they do, please report it, let's take a look). But generally they are an implementation detail, and if we come up with an alternative and less invasive algorithm to handle one-way relationships, I'll be happy to dump runtime rels.

Not using a "generic" object (CayenneDataObject) may cause an issue with runtime rels... Just turn them off, but be aware of consequences for one-way to-many relationships as mentioned above.

Andrus

So, just to follow up on this... I did run into some problems - one was
when adding some prefetches to a query it would look for the reverse
relationship and blow up. There was another occurrence elsewhere too, but
I don't recall where. So I turned back on the creation of the reverse
DbRelationships only and that seems to work well. These "unneeded"
DbRelationships don't hurt anything and still keep my entity classes free
of accessors for relationships that I don't want.

So my question now is, it is ok if I submit a patch to make it optional to
create the reverse ObjRelationships? I would probably use a property to
configure it, but maybe there is a better way.

Thanks,
John