On 04/08/2008, at 12:56 AM, Andrus Adamchik wrote:
On Aug 2, 2008, at 9:16 PM, Marcin Skladaniec wrote:
I'm going to add support for horizontal inheritance to cayenne.
I've already added functionality for an entity to be abstract (https://issues.apache.org/cayenne/browse/CAY-794
), now the second part.
BTW, I don't recall those patches being committed. Which patches out
of the 3 files there should go in? Is it just the latest one
Ari is taking care of this, sorry that I did so many files. i did not
know I could use the same file name to override the old ones.
Current implementation of inheritance allows only one of two
parameters set: "inheritance" or "table/view"
If inheritance is selected the ObjEntity does not have any own
DbEntity or DbAttributes, it gets them only from the parent
ObjEntity. (although it has all the ObjAttributes defined in model)
Table/view is a simple approach where the ObjEntity gets its
attributes from its definition in model, there is no parent
For horizontal inheritance ObjEnity.getAttributes() should return
both the attributes defined in model as well as from parent
Yet having both parameters accepted at the same time might now do
it well enough, therefore I would like to propose adding yet
another parameter to the model: "inheritance db relationship". On
database level the horizontal inheritance is nothing more than a
one-to-one relationship between two tables, and reflecting this in
the model will be required to route the queries and inserts
Did I miss something ? Is that the right approach ?
I think you are talking about VERTICAL inheritance, not horizontal
here. In case of horizontal you wouldn't have a relationship between
subclass/superclass tables. Anyways, assuming we are discussing
vertical inheritance, yes, there is a relationship involved. Per JPA
approach, this relationship can be figured automatically : it is a
1..1 relationship between the PK's of two tables. So we don't have
to store it in XML, as we can find it in runtime. Still internally
there should be a concept of a relationship joining parent and child
tables, so you are right about the big picture.
I'm now confused with vertical-horizontal naming. You say vertical,
I guess we talk about the same thing anyway, but just to be sure: I'm
trying to model a database schema where there are 3 tables: Artwork,
Painting and Sculpture.
Artwork is an abstract entity with its own table
Painting and Sculpture are extending Artwork, but have their own
tables as well.
You are suggesting that both superclass and subclass should have PK,
this means yet another table in required to link the two tables. This
seems odd to me as the relationship is simple 1..1, is that a
requirement of JPA ?
I'm reading up on JPA, but I cannot find it.