Currently, the foreignKey attribute in a <joinedHierarchy> is mandatory. Consider this example:

<mappingConfig ...>

    <entityTable table="LIFEFORM">
        <joinedHierarchy>
            <rootEntity discriminatorColumn='KIND' discriminator='LF'/>
            <entityTable table="MAMMAL" discriminator="MA" foreignKey="FK_MAMMAL_LIFEFORM"/>
            <entityTable table="INSECT" discriminator="IN" foreignKey="FK_INSECT_LIFEFORM"/>
        </joinedHierarchy>
    </entityTable>

</mappingConfig>

However, in most cases we can determine which foreign key to use for an <entityTable> automatically: We only need to look for the foreign referring to the root table. The only time we need a distinction is when a table has more than one foreign key referring to the root table and the default does not suffice.

The proposal:

• if foreignKey is specified, it determines the foreign key to use for joins with the root table
• If foreignKey is absent, derive the foreign key like this:
  • iterate over all foreign keys
  • stop iteration at the first foreign key which refers to the root table and use that
  • if no foreign key is found, it is an error.