Rails 4 form to set has_many through additional column - ruby-on-rails

Question

I have a has_many association between Items and their Components through a table called ComponentItems. ComponentItems contains a column quantity in addition to item_id and component_id. How is it possible to add a number_field to my form that shows the quantity of each component required for an item? The form must contain a number_field for each Item in the database, even if no relationship exists (i.e. #item.component_ids.empty? == true).
class Item < ActiveRecord::Base
has_many :components, through: :component_items
has_many :component_items
end
class Component < Item
has_many :items, through: :component_items
has_many :component_items
end
class ComponentItem < ActiveRecord::Base
belongs_to :item
belongs_to :component
end
I believe I've tried every permutation of model, controller and form_builder possible, except the correct one.
In response to the answer below, here's a form that shows a checkbox and the item code for component items that make up one particular item;
<%= form_for [#item] do |f| %>
<%= f.collection_check_boxes :component_items, Item.active.where.not(sku: #item.sku).sort_by{|n| n.sku}, :id, :sku do |b| %>
<%= b.check_box %> <%= b.label %><br/>
<% end %>
<% end %>
So, ideally I'd replace the check_box with a number_field for quantity. How?

Answer 1

So it seems what I wanted is not so straightforward after all. In the end I opted for using some jQuery for adding extra Components to Items via a separate form. Trying to add/remove components and adjust the quantities was beyond me, so choosing to use separate forms for each user action seemed simpler. It may not be the most user-friendly way of working but it's the best I have.
To edit the quantities I did the following;
<% #item.component_items.each do |x| %>
<%= hidden_field_tag "item[component_items_attributes][][id]", x.id%>
<%= label_tag x.component.sku, x.component.sku.upcase, :class=>"col-md-3 control-label" %>
<%= number_field_tag "item[component_items_attributes][][quantity]", x.quantity, :class=>"col-md-5"%>
<%end %>
and ensured the Item model accepted nested attributes for component_items. Finally, add the nested params array for multiple component_items to items_controller.rb...
def item_params
params.require(:item).permit(
:component_items_attributes =>[:component_id, :item_id, :quantity, :id]
)
end
Note I didn't use fields_for which seemed to generate an extra component_items_attributes array that didn't make any sense at all.

Answer 2

This should work:
#item.components.to_a.sum(&:quantity)
This will throw an error if quantity on some component is nil, so you may try like this to avoid errors:
#item.components.to_a.map(&:quantity).compact.sum
UPDATE
<% #item.component_items.each do |component_item| %>
<%= form_for(component_item) do |f| %>
<div class="field">
<%= f.label :quantity, 'Quantity' %><br />
<%= f.number_field :quantity %>
</div>
<% end %>
<% end %>