Попробуйте это:
class User
has_many :posts do
def today
where(:created_at => (Time.zone.now.beginning_of_day..Time.zone.now))
end
def this_week
where(:created_at => (Time.zone.now.beginning_of_week..Time.zone.now))
end
end
end
class Post
belongs_to :user
validate :user_quota, :on => :create
private
def user_quota
if user.posts.today.count >= 2
errors.add(:base, "Exceeds daily limit")
elsif user.posts.this_week.count >= 5
errors.add(:base, "Exceeds weekly limit")
end
end
end