meta_receipt: Accept paypal-id in lieu of receipt for PayPal credits.

conservancy_beancount/plugin/meta_receipt.py

@@ -42,14 +42,24 @@ class MetaReceipt(core._RequireLinksPostingMetadataHook):
             receipt_error = error
         else:
             return
+
+        if not post.units.number:
+            post_amount = 0
+        elif post.units.number > 0:
+            post_amount = 1
+        else:
+            post_amount = -1
+
         if post.account.is_checking():
             fallback_key = 'check'
-        elif (post.account.is_under('Liabilities:CreditCard')
-              and (post.units.number or 0) < 0):
+        elif post.account.is_under('Liabilities:CreditCard') and post_amount == -1:
             fallback_key = 'invoice'
+        elif post.account.is_under('Assets:PayPal') and post_amount == 1:
+            fallback_key = 'paypal-id'
         else:
             yield receipt_error
             return
+
         try:
             self._check_links(txn, post, fallback_key)
         except errormod.Error as fallback_error:

tests/test_meta_receipt.py

@@ -59,6 +59,8 @@ ACCOUNTS = [AccountForTesting._make(t) for t in [
     ('Assets:Bank:CheckCard', PostType.BOTH, 'check'),
     ('Assets:Cash', PostType.BOTH, None),
     ('Assets:Checking', PostType.BOTH, 'check'),
+    ('Assets:PayPal', PostType.CREDIT, 'paypal-id'),
+    ('Assets:PayPal', PostType.DEBIT, None),
     ('Assets:Savings', PostType.BOTH, None),
     ('Liabilities:CreditCard', PostType.CREDIT, None),
     ('Liabilities:CreditCard', PostType.DEBIT, 'invoice'),