has_many through consistently causes me to re-think what it all means. Here's a useful example plus accompanying rspec tests

gistfile1.txt

English: a User manages many Listings, and a Listing is managed by many Users. The association is handled by ListingManager via listing_id and manager_id as foreign keys.  

In the examples, below notice how one can create willy-nilly method names for the association as long as one defines how to get from one side of the association to the other. Another take away: the info for class_name and foreign_key and source are handled as literal strings for the purposes of the method constructing its SQL behind the scenes; it's only where Rails cannot infer the naming that one has to help out a bit

class ListingManager < ActiveRecord::Base
  belongs_to :listing
  belongs_to :manager, class_name:"User"
  belongs_to :cato, class_name:"User", foreign_key: "manager_id"

end

class Listing < ActiveRecord::Base
  has_many :listing_managers
  has_many :managers, through: :listing_managers
  has_many :pepperonis, through: :listing_managers, source: :cato
end

class User < ActiveRecord::Base

  attr_accessible :first_name, :last_name

  has_many :listing_managers
  has_many :listings, through: :listing_managers
  has_many :managed_listings, through: :listing_managers, source: :listing

end

gistfile2.txt

rspec tests:

describe Listing do
  it "has a valid factory" do
    expect(FactoryGirl.create(:listing)).to be_valid
  end

  it "has many listing managers"  do
    m1 = FactoryGirl.build(:user)
    m2 = FactoryGirl.build(:user)
    l = FactoryGirl.build(:listing)
    l.managers << m1
    l.managers << m2

    expect( l.managers.size ).to eq 2

  end

  it "should have many listing managers as pepperonis" do
    m1 = FactoryGirl.build(:user)
    m2 = FactoryGirl.build(:user)
    l = FactoryGirl.build(:listing)
    l.pepperonis << m1
    l.pepperonis << m2

    expect( l.pepperonis.size ).to eq 2
  end
end

describe User do
  before :each do
    @user = FactoryGirl.build(:user)
  end

  it "returns the first name when given a User"  do
    expect( @user.first_name ).to eq "Cato"
  end

  it "manages many listings"  do
    l1 = FactoryGirl.build(:listing)
    l2 = FactoryGirl.build(:listing)
    @user.listings << l1
    @user.listings << l2

    expect( @user.listings.size ).to eq 2
  end

  it "manages many managed_listings"  do
    l1 = FactoryGirl.build(:listing)
    l2 = FactoryGirl.build(:listing)

    @user.managed_listings << l1
    @user.managed_listings << l2

    expect( @user.managed_listings.size ).to eq 2
  end


end

describe ListingManager do
  it "should have a user as a manager referenced via cato" do
    user = FactoryGirl.create(:user, first_name: "Cato")
    lm = FactoryGirl.create(:listing_manager)

    lm.cato << user
    expect(lm.cato.first_name).to eq("Cato")
  end
end

gistfile3.txt

Factories:

FactoryGirl.define do
  factory :listing_manager do
    listing_id 1
    manager_id 1
    added_at "2013-08-09 10:44:30"
    removed_at "2013-08-09 10:44:30"
    added_by 1
    removed_by 1
  end
end

FactoryGirl.define do
  factory :listing do
    type ""
    unit_number "MyString"
    space "9.99"
    space_units "MyString"
    token "MyString"
    description "MyString"
    title "MyString"
    lease_rate "9.99"
    lease_rate_units "MyString"
    building_id 1
    decider_id 1
  end
end


FactoryGirl.define do
  factory :user do
    first_name "Cato"
    last_name "delGato"
    email {Faker::Internet.email}
    username "cato_delgato"
    password "1234567890"
    password_confirmation "1234567890"
    confirmed_at 7.days.ago
  end
end