meta_receipt: Correctly fall back to other metadata on zero-value postings.

RT#10633.

conservancy_beancount/plugin/meta_receipt.py

@@ -59,23 +59,16 @@ class MetaReceipt(core._RequireLinksPostingMetadataHook):
         else:
             return
 
-        if not post.units.number:
-            post_amount = 0
-        elif post.units.number > 0:
-            post_amount = 1
-        else:
-            post_amount = -1
-
         check_method: _CheckMethod = self._check_links
         if post.account.is_checking():
-            if post_amount == -1:
+            if post.is_debit():
                 check_method = self._check_check_id
                 fallback_key = 'check-id'
             else:
                 fallback_key = 'check'
-        elif post.account.is_credit_card() and post_amount == -1:
+        elif post.account.is_credit_card() and not post.is_credit():
             fallback_key = 'invoice'
-        elif post.account.is_under('Assets:PayPal') and post_amount == 1:
+        elif post.account.is_under('Assets:PayPal') and not post.is_debit():
             fallback_key = 'paypal-id'
         else:
             yield receipt_error

tests/test_meta_receipt.py

@@ -326,3 +326,20 @@ def test_bad_type_check_id_on_txn(hook, test_acct, other_acct, value):
 def test_fallback_not_accepted_on_other_accounts(hook, test_acct, other_acct, key, value):
     check(hook, test_acct, other_acct, {test_acct.missing_message()},
           post_meta={key: value})
+
+
+@pytest.mark.parametrize('test_acct,other_acct,value', testutil.combine_values(
+    ACCOUNTS_WITH_LINK_FALLBACK,
+    NOT_REQUIRED_ACCOUNTS,
+    testutil.LINK_METADATA_STRINGS,
+))
+def test_fallback_on_zero_amount_postings(hook, test_acct, other_acct, value):
+    # Unfortunately it does happen that we get donations that go 100% to
+    # banking fees, and our importer writes a zero-amount posting to the
+    # Assets account.
+    txn = testutil.Transaction(postings=[
+        ('Income:Donations', '-.1'),
+        ('Expenses:BankingFees', '.1'),
+        (test_acct.name, 0, {test_acct.fallback_meta: value}),
+    ])
+    assert not list(hook.run(txn))