Rails: has_many :through _ids nested? - ruby-on-rails

Question

On a single page I might have a single Video and then checkboxes to add multiple dogs to it. Easy enough (as follows)...
View:
<%= check_box_tag "video[dog_ids][]", dog.id %>
Controller:
params[:video][:dog_ids] ||= []
But what I can't figure out how to do is have multiple videos, each with multiple dogs.
I currently have this:
<% #videos.each do |video| %>
<%= fields_for "item[]", video do |f| %>
<%= f.hidden_field :id, :index => nil %>
<%= f.text_field :title, :index => nil%>
<%= f.text_area :body, :index => nil %>
<% video.dogs.each do |dog| %>
<%= check_box_tag "item[][video[dog_ids][]]", dog.id %>
<% end %>
<% end %>
<% end %>
But when I do that, dogs_ids is always nil when it's submitted.
Any idea what I'm doing wrong?

Answer 1

Such a setup with fields_for "item[]" and f.text_field :title, :index => nil produces:
<input id="item__name" name="item[][title]" size="30" type="text" value="vid1">
This indicates that the checkbox name should be item[][dog_ids][].
A working example:
<% #videos.each do |video| %>
<%= fields_for "item[]", video do |f| %>
<%= f.text_field :title, :index => nil %>
<% video.dogs.each do |dog| %>
<%= check_box_tag "item[][dog_ids][]", dog.id %> <%= Dog.name %>
<% end %>
<br>
<% end %>
<% end %>
This produces result in params:
{"item"=> [
{"title"=>"vid1", "dog_ids"=>["1", "2"]},
{"title"=>"vid2", "dog_ids"=>["2"]},
{"title"=>"vid3", "dog_ids"=>["2", "3"]}
]}