data.PostingMeta: Add date property.

This is something reporting tools will want a lot. This will make it
easier for them to look at just postings without worrying about the
parent transaction.

conservancy_beancount/data.py

@@ -20,6 +20,7 @@ throughout Conservancy tools.
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 import collections
+import datetime
 import decimal
 import functools
 
@@ -231,6 +232,14 @@ class PostingMeta(Metadata):
         else:
             super().__delitem__(key)
 
+    # This is arguably cheating a litttle bit, but I'd argue the date of
+    # the parent transaction still qualifies as posting metadata, and
+    # it's something we want to access so often it's good to have it
+    # within easy reach.
+    @property
+    def date(self) -> datetime.date:
+        return self.txn.date
+
 
 class Posting(BasePosting):
     """Enhanced Posting objects

tests/test_data_posting_meta.py

@@ -112,6 +112,20 @@ def test_keyerror_when_no_entity_or_payee(simple_txn):
     with pytest.raises(KeyError):
         meta['entity']
 
+@pytest.mark.parametrize('date', [
+    testutil.FUTURE_DATE,
+    testutil.FY_START_DATE,
+    testutil.FY_MID_DATE,
+    testutil.PAST_DATE,
+])
+def test_date(date):
+    txn = testutil.Transaction(date=date, postings=[
+        ('Income:Donations', -15),
+        ('Assets:Cash', 15),
+    ])
+    for index, post in enumerate(txn.postings):
+        assert data.PostingMeta(txn, index, post).date == date
+
 # 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):