In a finance application, I have a single table called ProductDetails. From this table, I need to first determine a subclass based on a field in the database ( i.e. productType, which can be like BOND or OPTION ). I've set up a discriminator to do this, and it works quite well. I have a ProductAbstract class, and then ProductBond and ProductOption classes which extend it. Works like a charm.

However, for BOND products, I need to further be able to subclass the data based on a different column in the ProductDetails table. For simplicity say we have a bondType field, which can be either CORPORATE, MUNI, or GOVERNMENT. I can't base my first discriminator on this column, because it's only populated for BOND types (not the best data model, but we're stuck with it at this point).

I thought I'd just be able to set up another discriminator at the ProductBond subclass level, but they can't be defined at the subclass level.

Basically, I need to set up a discriminator based on two fields. I've thought of concatenating the two fields into a new database column and basing my discriminator on the new column, but think that's a bit ugly.

I've done some digging, and have seen that I might be able to do this using a <formula> tag, or an EnhancedUserType. I've not been able to find any examples on setting these up. Could someone please get me pointed in the right direction? Alternative approaches and/or example code would of course be very appreciated. Thanks very much.

I've had a similar situation where I also needed sub-discriminators. However I was able to get the discriminator formula working. Here's an example of the formula:

case when column1='Val' and column2='A' then 'SubType-A'
when column1='Val' and column2='B' then 'SubType-B'
when column1='Val' and column2='C' then 'SubType-C'
when column1='Val' and column2='D' then 'SubType-D'
when column1='Val' and column2='E' then 'SubType-E'
else column1 end

In this example the column1 is intended to be the mainDiscriminator, and the column2 is the sub-discriminator.

So in each of your objects, you'll set up the discriminator value mapping as SubType-* or any other value found in column1.

There is one drawback to this approach. When I create a new object, the discriminator column won't be automatically populated, as it would if you didn't use a formula. Therefore, in each subclass, you must "force" the value of column1 (and column2 in some cases). We did this in the default constructor by using abstract methods we defined.

In the end, it wasn't the most straightfoward approach, but it was easier and more natural to treat these objects as a hierarchy.