Re: Foreign keys on inherited attributes

On Mon, 2004-01-26 at 19:23, Shawn Harrison wrote:

I'm using pg 7.3.5 and playing with table inheritance, and I've run into the
fact that foreign keys cannot be defined on inherited attributes. (As much
is stated in the documentation, but it didn't sink in until I ran into the
fact.)

I have a similar problem and have two inelegant workarounds.

1) Use triggers which does the key check. I don't know of any simple way to cascade deletes or updates.

2) Use inheritance to define the FK-containing tables as well, one for each PK-containing table. The supertable of these is provides the abstraction you sought.

Example:

I wanted a hierarchy of models of different types (each with table-specific data) and heterogeneous sets of models, roughly like this:
(if the word model is distracting, think instead of jobs and sets of related job families, or some such analogy)

        model      modelA      modelB      modelC     (model{A,B,C} ISA model)
        mid(PK)    mid(PK)     mid(PK)     mid(PK)
                   colA1       colB1       colC1
        setmodel
        mid(FK)
        sid(FK)

        set
        sid(PK) 
        name

As you noted, making modelsetmodel.mid a FK of model.mid doesn't work because the PK index is NOT inherited.
Instead, I have this:

        model      modelA      modelB      modelC     (model{A,B,C} ISA model)
        mid(PK)    mid(PK)     mid(PK)     mid(PK)
                   colA1       colB1       colC1
        setmodel   setmodelA   setmodelB   setmodelC  (setmodel{A,B,C} ISA setmodel)
        mid(FK)    mid(FK)     mid(FK)     mid(FK)
        sid(FK)    sid(FK)     sid(FK)     sid(FK)

        set
        sid(PK) 
        name

Of course, setmodelA.mid is now a FK of modelA.mid, and so on for B and C. I can still select from setmodel to get the heterogeneous sets I originally sought.

I hope that helps,
Reece

-- 
Reece Hart, http://www.in-machina.com/~reece/, GPG:0x25EC91A0