categories.rb

class Category < ActiveRecord::Base
     has_and_belongs_to_many	:users
     attr_accessible :name
end

HABTM.rb

1.9.2p0 :002 > f  = User.first
  User Load (0.4ms)  SELECT "users".* FROM "users" LIMIT 1
 => #<User id: 1, email: "[email protected]", encrypted_password: "$2a$10$LTc6zq5VorMZzsDSv7nD2eZpQkV30BaylB2Q6oY4yYjJ...", reset_password_token: nil, reset_password_sent_at: nil, remember_created_at: nil, sign_in_count: 2, current_sign_in_at: "2012-06-13 08:12:53", last_sign_in_at: "2012-06-13 08:09:08", current_sign_in_ip: "127.0.0.1", last_sign_in_ip: "127.0.0.1", name: "Ayy Bee Cee", created_at: "2012-06-13 08:08:01", updated_at: "2012-06-13 08:12:53", confirmation_token: nil, confirmed_at: "2012-06-13 08:09:08", confirmation_sent_at: "2012-06-13 08:08:01", unconfirmed_email: nil, authentication_token: nil> 
1.9.2p0 :003 > f.categories
  Category Load (8.1ms)  SELECT "categories".* FROM "categories" INNER JOIN "categories_users" ON "categories"."id" = "categories_users"."category_id" WHERE "categories_users"."user_id" = 1
 => []
1.9.2p0 :003 > f.categories
 => [] 
1.9.2p0 :007 > f.categories.create({:id => 1})
   (0.1ms)  begin transaction
  SQL (0.6ms)  INSERT INTO "categories" ("created_at", "name", "updated_at") VALUES (?, ?, ?)  [["created_at", Fri, 15 Jun 2012 00:02:37 UTC +00:00], ["name", nil], ["updated_at", Fri, 15 Jun 2012 00:02:37 UTC +00:00]]
   (12.3ms)  INSERT INTO "categories_users" ("user_id", "category_id") VALUES (1, 10)
   (67.1ms)  rollback transaction
ActiveRecord::StatementInvalid: SQLite3::ConstraintException: constraint failed: INSERT INTO "categories_users" ("user_id", "category_id") VALUES (1, 10)

mass-assign-error

1.9.2p0 :008 > f.categories.create({:user_id => 1, :category_id => 1})
   (16.9ms)  begin transaction
   (0.1ms)  rollback transaction
ActiveModel::MassAssignmentSecurity::Error: Can't mass-assign protected attributes: user_id, category_id

schema.rb

CREATE TABLE "categories_users" ("category_id" integer, "user_id" integer, "created_at" datetime NOT NULL, "updated_at" datetime NOT NULL);

User.rb

class User < ActiveRecord::Base
	has_and_belongs_to_many	:categories

  attr_accessible :email, :password, :password_confirmation, :remember_me, :name, :confirmed_at, :confirmation_token, :categories

end