Trouble while loading data in Combobox

I'm having the following problem:

I'm triying to get some data from my database and show it in a combobox in a template, and I created a function in the model class that represents that table to retrieve that data

here is the model class code:

class TipoPago < ActiveRecord::Base

def self.tipos_de_pago

@tipos = find(:all, :order => :display )

@desplegar = []

for tipo in @tipos

#agregar a lo que se mostrara en el select

@desplegar << [ tipo.display , tipo.valor ]

end

@desplegar

end

end

in one view template I have this line:

<%= forma.select :tipo_pago, TipoPago.tipos_de_pago, :prompt => "Seleccione el tipo de pago" %>

forma is an user form

I have tree records in my TipoPagos table:

id | display | valor

----------------------------------

1 | Cheque | chq

2 | Tarjeta de Credito | tc

3 | Orden de compra | oc

The bad think was that when I test It, in my browser the combobox showed:

"Seleccione el tipo de pago"

""

"Orden de compra"

"Tarjeta de Credito"

I didn't understand why it didn't showed "Cheque", the first option in the array

I know it looks like I'm doing something bad somewhere, but the reality was that when I just made a little change to my function, It worked WELL:

class TipoPago < ActiveRecord::Base

def self.tipos_de_pago

@tipos = find(:all, :order => :display )

@desplegar = []

for tipo in @tipos

#

tipo.valor #THIS LINE DOESN'T HAVE ANY SENSE

#

#agregar a lo que se mostrara en el select

@desplegar << [ tipo.display , tipo.valor ]

end

@desplegar

end

end

then, my combobox showed:

"Seleccione el tipo de pago"

"Cheque"

"Orden de compra"

"Tarjeta de Credito"

I took me a little while to get to that change, and for me it doesn't have any sense, that line of code shouldn't be there, but I remove it, It stop working well.

I hope you find a way to fix that if it's a bug o please tell what I'm doing bad

Thank you