I have the following mapping for classes A and B where I want to have an instance of B
represented within A.

Class A does not have the primary key of B directly, but wishes to derive it from the query:
select b.primaryKey from B b where b.month=A.month and b.year=A.year

When i use the formula tag I get the runtime error: "ORA-00936: Missing Expression"
What am I not understanding????

<class name="A" table="A">
<id name="name">....</id>
<property name="month">....</property>
<property name="year">....</property>
<one-to-one name="b" class="B">
<formula>select b.primaryKey from B b where b.month=A.month and b.year=A.year</formula>
</one-to-one>
</class>

<class name="B" table="B">
<id name="primaryKey">....</id>
<property name="month">....</property>
<property name="year">....</property>

</class>