gistfile1.rb

require "active_record" # 4.0.0.rc2
require "minitest/autorun"

class Migrate < ActiveRecord::Migration
  self.verbose = false

  def up
    create_table(:profiles)
    create_table(:networks)
    create_table(:memberships) do |t|
      t.references :network, null: false
      t.references :profile, null: false
    end
  end
end

class Profile < ActiveRecord::Base
  has_many :memberships
  has_many :networks, through: :memberships

  def shared_memberships(profile)
    memberships.shared(profile)
  end 
end

class Network < ActiveRecord::Base
end

class Membership < ActiveRecord::Base
  belongs_to :profile
  belongs_to :network

  def self.shared(other_profile)
    # seems to be loading other_profile's memberships association using self.membership's scope
    where(:network_id => other_profile.networks).map(&:network)
  end
end

class TestSharedMemberships < MiniTest::Unit::TestCase
  def setup
    ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
    Migrate.new.up

    @n = Network.create!
    @p1 = Profile.create!
    @p2 = Profile.create!
  end

  def test_share_profiles_with_network
    Membership.create!(profile: @p1, network: @n)
    Membership.create!(profile: @p2, network: @n)

    # passes
    assert_equal [@n], @p1.shared_memberships(@p2)
  end

  def test_share_profiles_with_network_2
    Membership.create!(profile: @p1, network: @n)

    # fails
    assert_equal [], @p1.shared_memberships(@p2)
  end

  def test_share_profiles_with_network_3
    Membership.create!(profile: @p1, network: @n)
 
    # pre-load association outside of p1->memberships scope
    @p2.networks

    # passes
    assert_equal [], @p1.shared_memberships(@p2)
  end
end