Rails fields_for returning nil - ruby-on-rails

Question

Think I've solved this in the process of writing it. I'll post my solution below.
tl;dr Rails fields_for not generating the expected params.
I have a User class, with the following code:
class User < ActiveRecord::Base
has_one :woojit, dependent: :destroy, validate: true
accepts_nested_attributes_for :woojit
# ... more
end
and a Woojit class
class Woojit < ActiveRecord::Base
belongs_to :user
end
Our UsersController includes this method (we're using the gem Administrate, but it's a subclass of Rails' ApplicationController):
class WoojitsController < Admin::ApplicationController
def new
# ... other stuff
#woojit = #user.build_woojit
end
end
And our user _form.html.erb partial looks like this:
<%= form_for([namespace, page.resource], html: { class: "form" }) do |f| %>
<!-- User fields -->
<fieldset>
<legend>Woojit fields</legend>
<%= f.fields_for #woojit do |ff| %>
<%= render 'woojit_fields', f: ff %>
<% end %>
</fieldset>
<% end %>
So when I submit the form, what I'm hoping to see is a params hash with this subset:
{
"user" => {
"woojit_attributes" => {
"attr_1"=>"Foo", # etc
}
}
}
But what I'm actually getting in the create action is a params hash with this subset:
{
"user" => {
"woojit" => {
"attr_1"=>"Foo", # etc
}
}
}
Ie 'woojit' instead of 'woojit_attributes'. I could hack this in the create action, but that seems like a horrible way of resolving the problem. I want to know why the key is getting mis-named to begin with.
I originally tried the alternative of using the line <%= f.fields_for :woojit do |ff| %> (ie symbol instead of instance var), and not building a Woojit object in the #new action.
The #fields_for guide suggests in the One-To-One section that this should work, but the code in the block was never executed, so the line returned nil - and the woojit params never made it to the create action in any form.

Answer 1

So the problem was apparently that #fields_for behaves differently when given an instance of a model (#woojit), rather than a symbol (:woojit).
In the former instance it generates fields that reference (model name), eg:
<input type="text" name="user[woojit][woojit_attr]" id="user_pledge_pledgor_name" />
In the latter instance it generates fields fields that reference the model attributes, eg:
<input type="text" name="user[woojit_attributes][woojit_attr]" id="user_pledge_attributes_pledgor_name" />
Hope that saves someone some of the frustration I've just been through!