Thanks for your detailed answer! I think what I'm trying to achieve is a bit more difficult than this. I'm already using polymorphic joins and it works really well. However, in the scenario I try to model here I don't want to use single table inheritance for performance reasons.
I'm implementing some kind of history for my entities. If a relation between two entities is changed, the old relation is written to a history table and then gets replaced by the new one. The historized relation still references the two entities in the normal tables. If now one of these entities is deleted, it is also put in a separate history table. The historized relation from before now references one normal entity and one historized entity. So what is referenced by the relation changed without this information being explicitly available anywhere. The IDs for all these entities and relations are permanent, so the normal and the corresponding historized entities always have the same ID and only differ by their revision (and the changed values, of course). The normal and the history tables should be split to improve query performance if the information from the history is not needed. It might be possible to model this as vertical inheritance with NormalEntity and HistorizedEntity being subclasses to a common superclass. But this would complicate my model and introduce additional tables which is what I am trying to avoid. So I want to select only the IDs from the relationship table and then do two additional queries for the normal and the historized entities identified by these. But I have not found a possible to model this. Cheers, Joerg -----Ursprüngliche Nachricht----- Von: Aristedes Maniatis [mailto:a...@maniatis.org] Gesendet: Dienstag, 12. Juni 2012 15:53 An: user@cayenne.apache.org Betreff: Re: Mapping a relationship with type List<Long> On 12/06/12 7:01pm, Schönfisch, Jörg wrote: > Hi, > > I have two tables, one relates to an object entity, the other is a join > table. A FK relationship exists between those two. However the "other" IDs in > the join table may be present in several different tables so I cannot define > a FK relationship for them. Thus I directly want to read a list of IDs and > not convert them into object entities. The mapping of IDs to objects will > later take place in the business code. > Any hints on how to do this or if it's possible at all? What you are describing is a polymorphic join. In Rails it looks like this: http://guides.rubyonrails.org/association_basics.html#polymorphic-associations There is no built-in construction in Cayenne, but it isn't hard to map. Let's pretend you want to have the ability to add "attachments" to various other entities. Contacts, Artists, Paintings. These attachments might be a picture or other information. So you create: [Attachment] id other columns [AttachmentRelation] foreignRecordId (int) foreignTable (string or enum) [ContactAttachmentRelation] [PaintingAttachmentRelation] [ArtistAttachmentRelation] These last three classes you create in Cayenne model and you make them a subclass of AttachmentRelation. There is no database table that goes with them, so effectively you are creating them as single table inheritance. Three subclasses, and only table table in the database. Put a method like this in the AttachmentRelation: public abstract void setAttachedRelation(Attachable attachable); And then code like this goes in the ContactAttachmentRelation subclass: public void setAttachedRelation(Attachable attachable) { setAttachedContact((Contact) attachable); } The model will have a qualifier like this: <obj-entity name="ContactAttachmentRelation" superEntityName="AttachmentRelation" className="x.y.z.ContactAttachmentRelation" clientClassName="x.y.z.ContactAttachmentRelation"> <qualifier><![CDATA[entityIdentifier like "Contact"]]></qualifier> </obj-entity> There is a bit more to do, but it is pretty simple. The main trick is to use the Cayenne inheritance and let it handle all your SQL generation. It works really nicely and we've had something like this in production for many years. Ari -- --------------------------> Aristedes Maniatis GPG fingerprint CBFB 84B4 738D 4E87 5E5C 5EFA EF6A 7D2E 3E49 102A
