Tengo el siguiente código (algo simplificado ...

create_table :signatures do |t|
  t.integer :signer_id
  t.integer :card_id

  t.timestamps
end

Con los modelos parecidos a ...

class Signature < ActiveRecord::Base
    belongs_to :card
    belongs_to :user
end

class Card < ActiveRecord::Base
    has_many :signatures
    has_many :signers, :through => :signatures, :foreign_key => "card_id"
end


class User < ActiveRecord::Base

    has_many :sent_cards, :class_name => "Card", :foreign_key => "sender_id"
    has_many :received_cards, :class_name => "Card", :foreign_key => "recipient_id"

    has_many :signatures
    has_many :signed_cards, :through => :signatures, :foreign_key => "signer_id"

end

Veo el siguiente error al usar la consola de rails ...

ruby-1.9.2-p0 > u15.signed_cards
ActiveRecord::HasManyThroughSourceAssociationNotFoundError: Could not find the source association(s) :signed_card or :signed_cards in model Signature. Try 'has_many :signed_cards, :through => :signatures, :source => <name>'. Is it one of :card or :user?
    from /home/slabounty/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.1.0/lib/active_record/reflection.rb:517:in `check_validity!'
    from /home/slabounty/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.1.0/lib/active_record/associations/association.rb:27:in `initialize'
    from /home/slabounty/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.1.0/lib/active_record/associations/collection_association.rb:24:in `initialize'
    from /home/slabounty/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.1.0/lib/active_record/associations.rb:164:in `new'
    from /home/slabounty/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.1.0/lib/active_record/associations.rb:164:in `association'
    from /home/slabounty/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.1.0/lib/active_record/associations/builder/association.rb:41:in `block in define_readers'
    from (irb):11
    from /home/slabounty/.rvm/gems/ruby-1.9.2-p0/gems/railties-3.1.0/lib/rails/commands/console.rb:45:in `start'
    from /home/slabounty/.rvm/gems/ruby-1.9.2-p0/gems/railties-3.1.0/lib/rails/commands/console.rb:8:in `start'
    from /home/slabounty/.rvm/gems/ruby-1.9.2-p0/gems/railties-3.1.0/lib/rails/commands.rb:40:in `<top (required)>'
    from script/rails:6:in `require'
    from script/rails:6:in `<main>'

Obtengo lo mismo cuando agrego elsource => :card/:user (debería ser: tarjeta en este caso, creo).

¿Alguna idea de lo que estoy haciendo mal aquí?

Mostrar una solución parcial porque quería limpiar algunas cosas. La migración se mantuvo igual que la versión anterior. Ahora veo un error de SQL (ver más abajo) donde no puede encontrar user_id en Signature. Odio decirlo, pero sobre todo he estado poniendo: foreign_key donde sea que creo que podrían ayudar en vano.

    class Signature < ActiveRecord::Base
        belongs_to :card
        belongs_to :signer, :class_name => "User"
    end


    class Card < ActiveRecord::Base
        # Correct
        has_many :signatures
        has_many :signers, :through => :signatures, :source => :user

    end

    class User < ActiveRecord::Base
        # Wrong!
        has_many :signatures, :foreign_key => "signer_id"
        has_many :signed_cards, :through => :signatures, :source => :card
    end

Con el error (menos seguimiento de pila)

ruby-1.9.2-p0 >   u15.signed_cards
  Card Load (0.5ms)  SELECT "cards".* FROM "cards" INNER JOIN "signatures" ON "cards"."id" = "signatures"."card_id" WHERE "signatures"."user_id" = 15 ORDER BY cards.created_at DESC
SQLite3::SQLException: no such column: signatures.user_id: SELECT "cards".* FROM "cards" INNER JOIN "signatures" ON "cards"."id" = "signatures"."card_id" WHERE "signatures"."user_id" = 15 ORDER BY cards.created_at DESC
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: signatures.user_id: SELECT "cards".* FROM "cards" INNER JOIN "signatures" ON "cards"."id" = "signatures"."card_id" WHERE "signatures"."user_id" = 15 ORDER BY cards.created_at DESC

Card.signers devuelve una matriz vacía como se esperaba.

Todavía estoy buscando ayuda en este caso. No he podido localizar mucho en el camino de explicaciones simples y directas de esto donde no estás usando los mismos nombres (es decir, necesitas una clave y fuente extranjeras.