data: PostingMeta.detached() can be chain-called.

conservancy_beancount/data.py

@@ -567,7 +567,7 @@ class PostingMeta(Metadata):
         that may want to "split" and manipulate the metadata multiple times.
         """
         retval = type(self)(self.txn, self.index, self.post)
-        retval.meta = retval.meta.new_child()
+        retval.meta = self.meta.new_child()
         return retval
 
 

tests/test_data_posting_meta.py

@@ -144,6 +144,26 @@ def test_mutable_copy():
     assert all(post.meta is None for post in txn.postings)
     assert meta.date == txn.date
 
+def test_double_detached():
+    txn = testutil.Transaction(filename='f', lineno=140, postings=[
+        ('Income:Donations', -19),
+    ])
+    meta1 = data.PostingMeta(txn, 0).detached()
+    meta1['metakey'] = 'meta'
+    meta1['layerkey'] = 'one'
+    meta2 = meta1.detached()
+    meta2['layerkey'] = 'two'
+    expected = {
+        'filename': 'f',
+        'lineno': 140,
+        'metakey': 'meta',
+        'layerkey': 'two',
+    }
+    assert dict(meta2) == expected
+    expected['layerkey'] = 'one'
+    assert dict(meta1) == expected
+    assert not any(post.meta for post in txn.postings)
+
 # The .get() tests are arguably testing the stdlib, but they're short and
 # they confirm that we're using the stdlib as we intend.
 def test_get_with_meta_value(simple_txn):