[Flow] Persist Classes that extends another Class

[Stephen Bungert]

One thing I noticed abut the base class is that if you use identity 
annotation (for the route) then it should be in the extending classes.

In my base class there is a title property as every record need one, then 
when I started setting up my route I set title to be the identity field... 
this meant that the title needed to be unique across  every class that 
extended the base class. The way round this is to overwrite the title field 
definition (in my case) in the extending classes and add the identity 
anotation there. The getters and setters can remain in the base class.