Creating association with existing model using has_many :through - ruby-on-rails

Question

I'd like users to be able to create a User System with multiple existing Parts. That it. Currently when I edit or create a user system with a part select I get.
Processing by UserSystemsController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"LgudsjWoAROXi0DGpUaH0ie2GNAuooRGtsPN+fOg+Gtev6Q6QmJC4dYIwFW9evaBOa0TUWotrQec1o1ROCbwFQ==", "user_system"=>{"name"=>"adfa"}, "parts_user_systems"=>{"part_id"=>["", "2", "3"]}, "button"=>""}
User Load (0.3ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 ORDER BY "users"."id" ASC LIMIT 1 [["id", 101]]
(0.1ms) BEGIN
SQL (0.6ms) INSERT INTO "user_systems" ("name", "user_id", "created_at", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id" [["name", "adfa"], ["user_id", 101], ["created_at", "2015-07-10 17:10:39.564555"], ["updated_at", "2015-07-10 17:10:39.564555"]]
(0.2ms) COMMIT
Nothing get's submitted to the join table.
class UserSystem < ActiveRecord::Base
has_many :parts_user_systems, foreign_key: "part_id"
has_many :parts, through: :parts_user_systems, foreign_key: "part_id"
end
class Part < ActiveRecord::Base
has_many :parts_user_systems
has_many :user_systems, through: :parts_user_systems
end
class PartsUserSystem < ActiveRecord::Base
belongs_to :part
belongs_to :user_system
end
class UserSystemsController < ApplicationController
def user_system_params
params.require(:user_system).permit(:name, :user_id, part_ids: [:ids])
end
end
_form.html.erb
<%= form_for(#user_system) do |f| %>
<div class="field">
<%= f.label :part_id, "Part " %><br/>
<%= collection_select :parts_user_systems, :part_id, Part.all, :id, :name, {:selected => 1}, {:multiple => true} %>
</div>
<div class="form-actions">
<%= f.button :submit %>
</div>
</div>
<% end %>
show.html.erb
<%= #user_system.parts.each do |part| %>
<li><%= part.name%> by by <%= part.parts_user_system%></li></li>
<% end %>