I was implementing the Strategy pattern presented here to avoid the limitations of Single-Type Inheritance but I wanted to implement some conditional validations based on the type:

• If it’s a “MonthDay-based” recurring event (e.g. 23rd of every month), then the monthday field must be present
• If it’s a “Date-based” recurring event (e.g. 1st August, repeating every year), then the frequency and unit fiels must be present

It turns out that the validates_XXX methods can be an ‘if’ parameter but it didn’t seem to work as expected. You can’t just do

validates_presence_of :count, 
  :if => recurring_strategy == "WeekDay"

but a quick Google codesearch revealed a more standard pattern:

validates_numericality_of :count, 
  :if => Proc.new{|s| s.recurring_strategy == "WeekDay" }

A more experienced ruby’er could tell you exactly what this is doing but I’ll hazard a guess that Proc.new creates a new procedure that’s called with the parameter s to test the if condition. Anyway, it seems to work. The only hassle I had it that in the strategy example they overrode the payment_strategy accessor which isn’t really necessary and makes accessing the ‘type’ of strategy quite difficult.