Pseudo code

promos.rb

## Promocode controller/generate endpoint controller

def generate
  begin
    @promocode.is_valid?
  rescue e #An array of errors 
    render json: e
  end
  
  if @promocode.save
    blah
  end
end

## Promocode model

def is_valid?
  # I wan't to map over the constraints and run their validate function which should either return nil or throw a
  # ConstraintError, but I want this method to swallow that throw and append it to an array of errors
  # and then throw those errors up. But I don't really know how to do that.
  errors = []
  begin
    self.promotion.constraints.map{|con| con.validate}
  rescue e
    errors << e
    next
  end
  
  if errors
    throw errors
  else
    true
  end
end

## A constraint

class Constraint
  def validate
    # if some criteria isn't met, throw a Constraint error
  end
end