Rails associations in a form - ruby-on-rails

Question

I am trying to figure out how to register certain values to their respective table using the form, check the order form to better understand which values need to be registering to which table.
I have also displayed the table entries, models and controller related to this question. If someone can guide me to where I can obtain further understanding on coding the associations in forms that would be great.
order form
<%= simple_form_for(#order) do |f| %>
<%= f.association :items, collection: Item.all, label_method: :name, value_method: :id %>
<%= [need to display the price of the item selected] %>
<%= f.input :quantity ???? [need to register in the order_items table] %>
<%= [register sub total to orders table] %>
<%= f.submit %>
<% end %>
tables
create_table "order_items", force: true do |t|
t.integer "item_id"
t.integer "order_id"
t.integer "quantity"
end
create_table "orders", force: true do |t|
t.integer "user_id"
t.integer "client_id"
t.boolean "status"
t.decimal "sub_total"
end
create_table "items", force: true do |t|
t.string "name"
t.decimal "price"
t.integer "stock"
end
models
class Order < ActiveRecord::Base
...
has_many :order_items
has_many :items, :through => :order_items
end
class Item < ActiveRecord::Base
...
has_many :order_items
has_many :orders, :through => :order_items
end
class OrderItem < ActiveRecord::Base
belongs_to :item
belongs_to :order
end
orders controller
def create
#order = Order.new(order_params)
#order.user_id = current_user.id
#order.status = TRUE
end
def order_params
params.require(:order).permit(:code, :client_id, :user_id, :memo, :status, item_ids: [])
end