gistfile1.rb

# User class with #is_member_of? method
class User < ActiveRecord::Base
  has_many :memberships
  has_many :groups, :through => :memberships

  def is_member_of?(group)
    memberships.exists?(:membership_group_id => group.id)
  end
end

# Test for #is_member_of? method
class UserSpec
  let(:user) { User.create(:name => 'Howard') }

  describe '#is_member_of?' do
    let(:group) { Group.create(:name => 'Awesome People') }
  
    context 'the user is a member' do
      before { Membership.create(:user => subject, :group => group) }
      specify { user.is_a_member_of?(group).should be_true }
    end
    
    context 'the user is not a member' do
      specify { user.is_a_member_of?(group).should be_false }
    end
  end
end