[hibernate-dev] Secondary table support 5.0

Secondary table support is pretty much done on the metamodel branch. One thing we should discuss is support for using property-ref in conjunction with <key/> there in hbm.xml. Hibernate has never supported this. But I wanted to look at adding support for it in 5.0 with these metamodel changes. What I found is that we *could* support it, but that would mean a few changes and "limitations". First, the limitations come down to: 1) property-refs would have to be limited to naming properties whose columns are part of the primary table. I don't think this is a huge deal in practice. 2) hbm.xml does not support "nested" secondary tables. Basically it is assumed that all foreign keys across all secondary tables reference back to the primary table. We could somewhat alleviate these limitations by allowing the nesting of joins. This starts to get complicated to support in-and-of-itself though. But in this approach, property-refs could name properties defined as part of any <join/> mappings "above" the one being defined or the primary table. The major change this requires in processing comes in Binder. Currently, when processing an entity, we: 1) bind the primary table, 2) bind its identifier, 3) bind its (if any) secondary tables, 4) bind attributes. Essentially we follow the JPA model here where the "column sources" name the table to which they belong (and adapt the hbm.xml model to that case). However, to support hbm.xml property-refs at all we will need to change that process. In the approach of not allowing nested secondary tables, the process would become: 1) bind the primary table 2) bind its identifier 3) *** bind any attributes mapped to the primary table ***, 4) bind its (if any) secondary tables, 5) bind attributes coming from secondary tables And in terms of allowing the nesting of secondary tables, the process then needs to become essentially the same as above, but we would need to process attributes from each secondary table as we process the secondary table. Interestingly, these approaches more naturally follow the hbm.xml model. I guess the first thing to discuss is whether we want to add support for <key property-ref/> in regards to <join/> for hbm.xml. We kind of want to steer people to use annotations anyway, so maybe we just decide this is not worthwhile. WDYT? -- steve(a)hibernate.org http://hibernate.org