Rails nested_form for a recursive model association - ruby-on-rails

Question

It's based on two models: Formula and Operand. A formula can have many Operands. Each operand can have many or none operands.
I've established a polymorphic association in which Operand belongs to operation and also have many operands as operator. Formula have many operands as operation itself.
I'm trying to build a nested_form that can handle this, but i'm stuck. I'm using simple_nested_form. Any help would be appreciated.
class Formula < ActiveRecord::Base
validates_presence_of :name, :client_id
belongs_to :client
has_many :operands, as: :operation, dependent: :destroy
accepts_nested_attributes_for :operands, allow_destroy: true
end
class Operand < ActiveRecord::Base
validates_presence_of :operator
belongs_to :operation, polymorphic: true
has_many :operands, as: :operation, dependent: :destroy
accepts_nested_attributes_for :operands, allow_destroy: true
OPERATOR_TYPES = ["+", "-", "*", "/"]
end
UPDATE
Based in models above, I add in formulas_controller:
def new
#formula.operands.build
end
In the formula _form, I add fields_for :operands
= simple_nested_form_for #formula, :html => { :class => 'form-vertical' } do |f|
= f.error_notification
.form-inputs
.row
.span3= f.input :name
.well
= f.fields_for :operands
= f.link_to_add t('helpers.links.add'), :operands
And then, the partial _operands_fields, also rendering fields_for :operands
.well
.row
.span2= f.input :operator, collection: Operand::OPERATOR_TYPES
.span3= f.input :numeric_operand
.span3= f.link_to_remove t('helpers.links.remove')
.well
= f.fields_for :operands
= f.link_to_add t('helpers.links.add'), :operands
I think this should work, but it seems that when the formula form is rendered, it includes the fields_for the formula operand. Then, inside this partial, instead of including "Add", it renders another operand fields, and so on, ending in an infinite loop such this:
User Load (0.4ms) SELECT "users".* FROM "users" WHERE "users"."id" = 1 ORDER BY "users"."id" ASC LIMIT 1
Client Load (0.2ms) SELECT "clients".* FROM "clients" WHERE "clients"."id" = ? LIMIT 1 [["id", 1]]
Rendered formulas/_operand_fields.html.haml (48.4ms)
Rendered formulas/_operand_fields.html.haml (9.0ms)
Rendered formulas/_operand_fields.html.haml (7.5ms)
Rendered formulas/_operand_fields.html.haml (9.1ms)
Rendered formulas/_operand_fields.html.haml (8.8ms)
Rendered formulas/_operand_fields.html.haml (9.2ms)
Rendered formulas/_operand_fields.html.haml (9.3ms)
Rendered formulas/_operand_fields.html.haml (9.5ms)
Rendered formulas/_operand_fields.html.haml (17.7ms)
Rendered formulas/_operand_fields.html.haml (52.4ms)
Rendered formulas/_operand_fields.html.haml (10.4ms)
Rendered formulas/_operand_fields.html.haml (11.3ms)
Any ideas in how to solve this problem?