app/models/forum.rb
class Forum < ActiveRecord::Base
# Associations
has_many :topics, :dependent => :destroy
has_many :posts, :through => :topics
belongs_to :forum_group
# Accessors
attr_accessible :title, :description, :state, :position, :forum_group_id
# Scopes
default_scope :order => 'position ASC'
# Validations
validates :title, :presence => true
validates :description, :presence => true
validates :forum_group_id, :presence => true
end