can t apply any method on nested form fields attributes

I am using cocoon from github to deal with accepts_nested_attributes_for. In my partial i am trying to manipulate nested attributes but it is not working. My code is in my partial :

  = f.inputs do
    = f.object.myarrayfield.length

I have an error message :

undefined method `length' for nil:NilClass

If I do only that

  = f.inputs do
    = f.object.myarrayfield

There is no error message and the page give me the value of the field :


The length method should give me 2 not nil, do you know why ?

It is not the length method that is giving nil, read the error
carefully. It says that you cannot call length on something that is
nil. It is f.object.myarrayfield that is nil.