gistfile1.rb

class PageCategory < ActiveRecord::Base
  belongs_to :page
  belongs_to :category
  validates_presence_of :page_id
  validates_presence_of :category_id
end

class Category < ActiveRecord::Base
  has_many :page_categories, :dependent => :destroy
  has_many :pages, :through => :page_categories
end

class Page < ActiveRecord::Base
  has_many :page_categories, :dependent => :destroy
  has_many :categories, :through => :page_categories
end

# example 1

c = Category.new(:name => "blah")
c.save

p = Page.new(:title => "my page", :categories => [c])
p.save

# this yields []
c.pages.inspect

################

# example 2

c = Category.new(:name => "blah")
c.save

p = Page.new(:title => "my page")
p.categories << c
p.save

# this yields []
c.pages.inspect