Can I only use method described in '15.4. Associations'? Maybe there is another better way to handle this? So I can get my objects with filtered sub objects without any maps, etc.

I have:

A1{b1, b2}
A2{b3, b4, b5}

I want to get somethink like this:

A1{b1}
A2{b3}

Using method described in 15.4 I have:

A1{b1,b2}
b1

A2{b3,b4,b5}
b3

:(