Best practice for limiting the number of associations within a has_many relationship?

belongs_to :account

has_many :users

@account.users << someuser

@account.users.create

class User
  belongs_to :account
end

class User
  validates_each :account do |user, attr, value|
    user.errors.add attr, "too much users for account" if user.account.users.size >= 3
  end
end