I need help with this statement: s=DatosMateria.sum('inscritos',:conditions=> "datos_materia.ano=2005 AND materias.codigo=2394",:include=>"materias"). it generates : SELECT sum('datos_materia'.inscritos) AS  sum_inscritos FROM 'datos_materia' LEFT OUTER JOIN 'materias' ON materias.codigo=datos_materia.id WHERE.. I need it to join on materias.codigo=datos_materia.codigo_materia instead of materias_codigo=datos_materia.id How can I do that?

class DatosMateria < ActiveRecord::Base
	set_table_name "datos_materia"

	has_many :materias,:foreign_key => "codigo"
end

class Materia < ActiveRecord::Base

	set_primary_key "codigo"
	belongs_to :datosMateria, :foreign_key=> "materia_codigo"

end