Ruby on Rails

Composing scopes is completely normal - they are clear, easy to test and can serve as building blocks. What you need to keep in mind is to keep your code DRY.

For example, if you have an e-commerce site selling guitars and from the data you see that high-end left-handed guitars are most often bough by male left-handed adults then you don't want to litter the code with multiple occurrences of `Person.male.adult.left_handed`. This is an implementation detail. You should define a scope (or a class method) `Person.best_high_end_left_handed_buyers` and define it to equal `Person.male.adult.left_handed`. If requirements change at a later point in time you can just change the definition. (Alternatively you can define a private method that returns the scope in your controller).

--