Q:

Form for each user

I have an ordinary form to create a Package object at /packages/new: 
<%= form_for @package do |f| %>
    <%= f.text_field :name %>
    <%= f.text_field :address %>
    ...
<% end %>

The package model belongs_to :partner.
I am looking for a way to associate a new package form to a specific partner, preferably without any input from the user filling it in. 
For example, if partner A sends a link to the form, I want the form to include partner_id:A.id.
How can I connect forms to partners?

A:

You can send partner_id param with the link which your partner will send.
Something like
http://website.com/packages/new?partner_id=3

And use the param as hidden_field in the form
<%= form_for @package do |f| %>
  <%= f.text_field :name %> 
  <%= f.text_field :address %> 
  <%= f.hidden_field :partner_id, value: params[:partner_id] %> 
  ... 
<% end %>

Alternatively you can also make use of Nested Resources

