my-habtm.rb

#{PATH_TO_APP}$ rails g model Person name:string email:string
#{PATH_TO_APP}$ rails g model Community name:string description:text

class Person < ActiveRecord::Base
  has_and_belongs_to_many :communities
end

class Community < ActiveRecord::Base
  has_and_belongs_to_many :persons
end

class CreatePeople < ActiveRecord::Migration
  def change
    create_table :people do |t|
      t.string :name, null: false
      t.string :email, null: false
      t.timestamps
    end
  end
end

class CreateCommunities < ActiveRecord::Migration
  def change
    create_table :communities do |t|
      t.string :name, null: false
      t.text :description
      t.timestamps
    end
  end
end

# {PATH_TO_APP}$ rails g migration CreateJoinTablePersonCommunity person community
# invoke  active_record
# create    db/migrate/20140708122358_create_join_table_person_community.rb

class CreateJoinTablePersonCommunity < ActiveRecord::Migration
  def change
    create_join_table :people, :communities, column_options: {null: true} do |t|
      # t.index [:person_id, :community_id]
      # t.index [:community_id, :person_id]
    end
  end
end