Rails Admin and dealing with Calculated Totals,Subtotals and Discount fields - ruby-on-rails

Question

Ok so I have a table orders, order_items, and items. Everything works fine except for 1 thing.
class CreateOrders < ActiveRecord::Migration[5.0]
def change
create_table :orders do |t|
t.integer :seller_id
t.integer :customer_id
t.string :shipping_method
t.string :shipping_terms
t.string :payment_method
t.date :delivery_date
t.date :due_date
t.decimal :total_discount, :precision =>8, :scale => 2
t.decimal :subtotal, :precision =>8, :scale => 2
t.decimal :sales_tax, :precision =>8, :scale => 2
t.decimal :total, :precision =>8, :scale => 2
t.timestamps
end
end
end
class CreateOrderItems < ActiveRecord::Migration[5.0]
def change
create_table :order_items do |t|
t.integer :order_id
t.integer :item_id
t.integer :quantity
t.decimal :discount, :precision =>8, :scale => 2
t.decimal :line_total, :precision =>8, :scale => 2
t.boolean :packed
t.string :packed_by
t.string :ffr
t.timestamps
end
end
end
class CreateItems < ActiveRecord::Migration[5.0]
def change
create_table :items do |t|
t.integer :sku
t.string :name
t.decimal :price, :precision =>8, :scale => 2
t.string :description
t.decimal :weight, :precision =>8, :scale => 2
t.timestamps
end
end
end
Basically there needs to be calculations done either as the items are being added to the order or before everything is saved.
On the Orders table the following needs to happen
Calculate : total_discount(dollar amount), sub_total, :sales_tax, :total
On OrderItems table the following needs to happen
Calculate : line_total, discount(dollar amount)
There has to be a sane way to do this but I have yet to figure it out. Either I really do not know what I am looking for or I am googling all the wrong things.