class User < ActiveRecord::Base
  has_many :user_clients, :dependent => true
  has_many :clients, :through => :user_client

end

class UserClient < ActiveRecord::Base

  belongs_to :user
  belongs_to :client

  # user_client join table contains :primary column

  after_create :init_primary
  before_destroy :preserve_primary

  def init_primary
    # first association for a client is always primary
    if self.client.user_clients.length == 1
      self.primary = true
      self.save
    end
  end

  def preserve_primary
    if self.primary
      #unless this is the last association, make soemone else primary
      unless self.client.user_clients.length == 1
        # there's gotta be a more concise way...
        if self.client.user_clients[0].equal? self
          self.client.user_clients[1].primary = true
        else
          self.client.user_clients[0].primary = true
        end
      end
    end
  end

end

class Client < ActiveRecord::Base
  has_many :user_clients, :dependent => true
  has_many :users, :through => :user_client

end

class Client < AR::B
  has_many :users, :dependent => :destroy
  has_one  :primary_contact, :class_name => "User",
                             :conditions => {:primary_contact => true},
                             :dependent  => :destroy
end

class User < AR::B
  belongs_to :client

  after_save    :ensure_only_primary
  before_create :ensure_at_least_one_primary
  after_destroy :select_another_primary

  private
  # We always want one primary contact, so find another one when I'm being
  # deleted
  def select_another_primary
    return unless primary_contact?
    u = self.client.users.first
    u.update_attribute(:primary_contact, true) if u
  end

  def ensure_at_least_one_primary
    return if self.client.users.count(:primary_contact).nonzero?
    self.primary_contact = true
  end

  # We want only 1 primary contact, so if I am the primary contact, all other
  # ones have to be secondary
  def ensure_only_primary
    return unless primary_contact?
    self.client.users.update_all(["primary_contact = ?", false], ["id <> ?", self.id])
  end
end