How to call a form from an element - ruby-on-rails

Question

I have Post with nested elements who also have different nested form (text for this example) according to a :cat variable.
To create the texts I'm using a post method create_element with a variable (cat = 1). And I would like to call an AJAX method to render the new text created into my post form.
My problem: I don't know how to call the text nested_form created directly into my AJAX method.
My code :
Post form (posts/new):
<%= simple_form_for #post do |f| %>
<div class="elements">
<%= f.fields_for :elements do |element| %>
<%= render 'posts/element_field', f: element %>
<% end %>
</div>
<div class="btn_add_element_area">
<a href="/posts/<%=#post.id%>/create_element?cat=1" data-method="patch" data-remote="true">
<i class="fa fa-font"></i>
</a>
</div>
<%= f.submit "Publish", class: "btn" %>
<% end %>
Post form (posts/_element_field):
<div class="element_form" id="element_form<%= f.object.id %>">
<div class="undisplay">
<%= f.number_field :cat %>
</div>
<%- if f.object.cat == 1 %>
<%= f.fields_for :post_texts do |text| %>
<%= render 'posts/elements/text_field', f: text %>
<% end %>
<% end %>
</div>
Post form (posts/_text_field):
<%= f.text_area :content %>
Post controller (create_element) :
def create_element
if params[:cat] == "1"
#element = Element.create(post: #post, cat: 1)
Text.create(element: #element)
end
respond_to do |format|
format.js {render 'elements/create'}
end
end
And the AJAX method (elements/create) :
$('.elements').append("<%= j render partial: 'posts/element_field', locals: {f: #element} %>");
Error = undefined method `object'
The problem is that in the AJAX file I call the #element created for my f variable instead of calling the form of this #element.
I also tried something like that for the AJAX method, but still not working :
<% form_for(#element) do |elm| %>
$('.elements').append("<%= j render partial: 'posts/element_field', locals: {f: elm} %>");
<% end %>
Error = undefined method `element_path'
Or
<%= simple_form_for(#post) do |p| %>
<%= p.fields_for :elements, #element do |elm| %>
$('.elements').append("<%= j render partial: 'posts/element_field', locals: {f: elm} %>");
<% end %>
<% end %>
No error but nothing happen
If you have any ideas to solve that, I would really appreciate !
PS: And ideas to improve the question title are welcome !!