fields_for block sends a form builder object nil

[dfabreguette]

When I use the "fields_for" helper, the form builder object is nil.

I have two models AppDomain and AppFeature having a one to many relationship.

Considering the form below :

= form_for [:admin, @admin_app_domain] do |f|
    = fields_for :app_features do |app_feature_form|
        - raise app_feature_form.object.inspect #=> IS NIL !!

Even though I have app_features in @admin_app_domain.app_features, app_feature_form.object is nil.

As a workaround, I tried to pass on the collection (as mentionned in rails doc http://apidock.com/rails/ActionView/Helpers/FormHelper/fields_for) and then I got the whole collection in "object" which leads me to an error in my form as I try to call an attribute method on a collection.

= form_for [:admin, @admin_app_domain] do |f|
    = fields_for : app_features, @admin_app_domain.app_features do |app_feature_form|
        - raise app_feature_form.object.inspect #=> IS A COLLECTION

Is it a normal behavior and in that case how are we supposed to use "fields_for" method ?