bthm.rb

##Class

class User
  belongs_to :group, inverse_of: :member
end

class Group
  # inverse_of because we are adding options to the relationship that prevent
  # automatic guessing of the inverse
  has_many :members, class_name: "User", inverse_of: :user
end

##Migration

class AddGroupRefToUsers < ActiveRecord::Migration
  def change
    add_reference :user, :group, index: true # adds group_id to user table
  end
end

user.group
group.members