20131017231102_categories_units_table.rb

class CategoriesUnitsTable < ActiveRecord::Migration
    def self.up
      create_table :categories_units, :id => false do |t|
        t.references :category
        t.references :unit
      end
      add_index :categories_units, [:category_id, :unit_id]
    end

    def self.down
      drop_table :categories_units
    end
end

category.rb

# == Schema Information
#
# Table name: categories
#
#  id                 :integer          not null, primary key
#  title              :string(255)
#  image_file_name    :string(255)
#  image_content_type :string(255)
#  image_file_size    :integer
#  image_updated_at   :datetime
#

class Category < ActiveRecord::Base
  has_and_belongs_to_many :units
  has_attached_file :image, :styles => { :medium => "275x55>" }
end

gistfile1.rb

# Truncated result set for clarity
Unit.all.includes(:categories)

<Unit id: 8, code: 1, title: "asdfasfd", phone: "33333333", active: true, category_id: nil>]>

## Raw: 
Unit.all.includes(:categories)
  Unit Load (35.9ms)  SELECT "units".* FROM "units"
  SQL (0.4ms)  SELECT "categories".*, "t0"."unit_id" AS ar_association_key_name FROM "categories" INNER JOIN "categories_units" "t0" ON "categories"."id" = "t0"."category_id" WHERE "t0"."unit_id" IN (1, 2, 3, 5, 7, 8)
 => #<ActiveRecord::Relation [#<Unit id: 1, code: 1234, title: "Store Name Here", phone: "(916) 367 9977", active: true, category_id: nil>, #<Unit id: 2, code: 123, title: "Unit Test", phone: "11231421345", active: true, category_id: nil>, #<Unit id: 3, code: 123414, title: "test", phone: "11231421345", active: false, category_id: nil>, #<Unit id: 5, code: nil, title: "asdgasdfg", phone: "", active: true, category_id: nil>, #<Unit id: 7, code: nil, title: "asdfas", phone: "222222222", active: true, category_id: nil>, #<Unit id: 8, code: 1, title: "asdfasfd", phone: "33333333", active: true, category_id: nil>]>

unit.rb

# == Schema Information
#
# Table name: units
#
#  id          :integer          not null, primary key
#  code        :integer
#  title       :string(255)
#  phone       :string(255)
#  active      :boolean
#  category_id :integer
#

class Unit < ActiveRecord::Base
  has_and_belongs_to_many :categories, :join_table => :categories_units

  accepts_nested_attributes_for :categories
end