CF9 computed properties

I wanted to make a computed property in CF9 just like you can on a sql table (computed column). Dan Vega had a nice how-to awhile back but it didn't show a complex example.

In my project I had a "project" class that had many ratings on it (Example: rating of 1-5, 1 being bad and 5 being great) and I wanted to have a computed property that gave me the average rating of a project. Here is what my hib file looks like for the "project" class.

<class entity-name="Project" lazy="true" name="cfc:myproject.app.model.Project" table="`Project`">
        <id name="id" type="int">
                <column length="10" name="ID" />
         <generator class="identity" />
        </id>
        <property name="name" type="string">
                <column name="Name" sql-type="varchar(max)" />
        </property>
  <property name="description" type="string">
        <column name="Description" sql-type="varchar(max)" />
        </property>
        <property type="float" 
                formula="(select (sum(r.rank)*100.0) / (count(pr.id) *100.0) from project as p inner join project_rating as pr on pr.project_id = p.id inner join rating as r on r.id = pr.rating_id where pr.project_id = id)" 
                name="rating"/>  
</class>

Some key things to point out:
1. The formula is a sql query not a hql query.
2. I wrapped my entire statement in (), don't know why but it worked.
3. You always start the sql statement with the table of the class you are working on. Reason being is when you get to the "where" clause notice this piece "pr.project_id = id". "id" is not aliased because it grabs the current id of the object that is running the formula against. This is how the query will only run for single object and not all of the other db records in the project table.
4. Alias every thing except the "where id" part.

I struggled through these above. Hope this helps someone.