Here is my scheme:
I have
parcelas belongs_to pagamento
pagamento belongs_to :pagavel, polymorphic: true
pagavel belongs_to :agencia
Now I want to create a scope in Parcela that would look like
Parcela.from_agencia(agencia_id) and that would query only the Parcelas that belong to pagamento that belong to pagavel that belong to this agencia_id.