Fix invoice item issues

- Fixed invoice getting saved on InvoiceItem as a string of the object
- Added missing fields (fixes #15)

lib/fabric/app/models/fabric/invoice_item.rb

@@ -6,22 +6,54 @@ class InvoiceItem
 
     belongs_to :customer, class_name: 'Fabric::Customer',
       primary_key: :stripe_id, touch: true
+    belongs_to :invoice, class_name: 'Fabric::Invoice', primary_key: :stripe_id
+    belongs_to :subscription, class_name: 'Fabric::Subscription',
+      primary_key: :stripe_id
 
     field :stripe_id, type: String
     field :amount, type: Integer
-    field :invoice, type: String
     field :currency, type: String
+    field :date, type: Time
+    field :description, type: String
+    field :discountable, type: Boolean
+    field :discounts, type: Array
+    field :livemode, type: Boolean
+    field :metadata, type: Hash
+    field :period, type: Hash
+    field :price, type: Hash
+    field :proration, type: Boolean
+    field :quantity, type: Integer
+    field :subscription_item, type: String
+    field :tax_rates, type: Array
+    field :unit_amount, type: Integer
+    field :unit_amount_decimal, type: String
 
-    validates_uniqueness_of :stripe_id
+    validates :stripe_id, presence: true, uniqueness: true
 
     index({ stripe_id: 1 }, { background: true, unique: true })
 
     def sync_with(invoice_item)
       self.stripe_id = invoice_item.id
       self.amount = invoice_item.amount
-      self.invoice = invoice_item.invoice
       self.currency = invoice_item.currency
       self.customer_id = handle_expanded(invoice_item.customer)
+      self.date = invoice_item.date
+      self.description = invoice_item.description
+      self.discountable = invoice_item.discountable
+      self.discounts = invoice_item.discounts
+      self.invoice_id = handle_expanded(invoice_item.invoice)
+      self.livemode = invoice_item.livemode
+      self.metadata = convert_metadata(invoice_item.metadata)
+      self.period = handle_hash(invoice_item.period)
+      self.price = handle_hash(invoice_item.price)
+      self.proration = invoice_item.proration
+      self.quantity = invoice_item.quantity
+      self.subscription_id = handle_expanded(invoice_item.subscription)
+      self.tax_rates = invoice_item.tax_rates.map do |tr|
+        handle_hash(tr)
+      end
+      self.unit_amount = invoice_item.unit_amount
+      self.unit_amount_decimal = invoice_item.unit_amount_decimal
     end
   end
 end