reports: Add RelatedPostings.group_by_account() classmethod.

conservancy_beancount/reports/core.py

@@ -274,6 +274,12 @@ class RelatedPostings(Sequence[data.Posting]):
         for value, posts in mapping.items():
             yield value, cls(posts, _can_own=True)
 
+    @classmethod
+    def group_by_account(cls: Type[RelatedType],
+                         postings: Iterable[data.Posting],
+    ) -> Iterator[Tuple[data.Account, RelatedType]]:
+        return cls._group_by(postings, operator.attrgetter('account'))
+
     @classmethod
     def group_by_meta(cls: Type[RelatedType],
                       postings: Iterable[data.Posting],

tests/test_reports_related_postings.py

@@ -398,3 +398,21 @@ def test_group_by_first_meta_link(link_swap_posts):
         actual = actual_all.get(key, '')
         assert len(actual) == 2
         assert all(post.account == expect_account for post in actual)
+
+def test_group_by_account():
+    entries = [
+        testutil.Transaction(postings=[
+            ('Income:Donations', -10),
+            ('Assets:Cash', 10),
+        ]),
+        testutil.Transaction(postings=[
+            ('Income:Donations', -20),
+            ('Assets:Cash', 20),
+        ]),
+    ]
+    postings = data.Posting.from_entries(entries)
+    actual = dict(core.RelatedPostings.group_by_account(postings))
+    assert len(actual) == 2
+    for key, related in actual.items():
+        assert len(related) == 2
+        assert all(post.account == key for post in related)