diff --git a/conservancy_beancount/reports/__init__.py b/conservancy_beancount/reports/__init__.py
new file mode 100644
index 0000000000000000000000000000000000000000..e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
diff --git a/conservancy_beancount/reports/core.py b/conservancy_beancount/reports/core.py
new file mode 100644
index 0000000000000000000000000000000000000000..7c6debc4d67a17d195d4b91254577ab9325a6ff9
--- /dev/null
+++ b/conservancy_beancount/reports/core.py
@@ -0,0 +1,79 @@
+"""core.py - Common data classes for reporting functionality"""
+# Copyright © 2020 Brett Smith
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program. If not, see .
+
+import collections
+
+from decimal import Decimal
+
+from .. import data
+
+from typing import (
+ overload,
+ Dict,
+ List,
+ Sequence,
+ Union,
+)
+
+class RelatedPostings(Sequence[data.Posting]):
+ """Collect and query related postings
+
+ This class provides common functionality for collecting related postings
+ and running queries on them: iterating over them, tallying their balance,
+ etc.
+
+ This class doesn't know anything about how the postings are related. That's
+ entirely up to the caller.
+
+ A common pattern is to use this class with collections.defaultdict
+ to organize postings based on some key::
+
+ report = collections.defaultdict(RelatedPostings)
+ for txn in transactions:
+ for post in Posting.from_txn(txn):
+ if should_report(post):
+ key = post_key(post)
+ report[key].add(post)
+ """
+
+ def __init__(self) -> None:
+ self._postings: List[data.Posting] = []
+
+ @overload
+ def __getitem__(self, index: int) -> data.Posting: ...
+
+ @overload
+ def __getitem__(self, s: slice) -> Sequence[data.Posting]: ...
+
+ def __getitem__(self,
+ index: Union[int, slice],
+ ) -> Union[data.Posting, Sequence[data.Posting]]:
+ if isinstance(index, slice):
+ raise NotImplementedError("RelatedPostings[slice]")
+ else:
+ return self._postings[index]
+
+ def __len__(self) -> int:
+ return len(self._postings)
+
+ def add(self, post: data.Posting) -> None:
+ self._postings.append(post)
+
+ def balance(self) -> Sequence[data.Amount]:
+ currency_balance: Dict[str, Decimal] = collections.defaultdict(Decimal)
+ for post in self:
+ currency_balance[post.units.currency] += post.units.number
+ return [data.Amount(number, key) for key, number in currency_balance.items()]
diff --git a/tests/test_reports_related_postings.py b/tests/test_reports_related_postings.py
new file mode 100644
index 0000000000000000000000000000000000000000..9a70d8ebc3b4d58c55f63740787f53382adc1138
--- /dev/null
+++ b/tests/test_reports_related_postings.py
@@ -0,0 +1,87 @@
+"""test_reports_related_postings - Unit tests for RelatedPostings"""
+# Copyright © 2020 Brett Smith
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program. If not, see .
+
+import datetime
+import itertools
+
+import pytest
+
+from . import testutil
+
+from conservancy_beancount import data
+from conservancy_beancount.reports import core
+
+_day_counter = itertools.count(1)
+def next_date():
+ return testutil.FY_MID_DATE + datetime.timedelta(next(_day_counter))
+
+def txn_pair(acct, src_acct, dst_acct, amount, date=None, txn_meta={}, post_meta={}):
+ if date is None:
+ date = next_date()
+ src_txn = testutil.Transaction(date=date, **txn_meta, postings=[
+ (acct, amount, post_meta.copy()),
+ (src_acct, -amount),
+ ])
+ dst_date = date + datetime.timedelta(days=1)
+ dst_txn = testutil.Transaction(date=dst_date, **txn_meta, postings=[
+ (acct, -amount, post_meta.copy()),
+ (dst_acct, amount),
+ ])
+ return (src_txn, dst_txn)
+
+def donation(amount, currency='USD', date=None, other_acct='Assets:Cash', **meta):
+ if date is None:
+ date = next_date()
+ return testutil.Transaction(date=date, postings=[
+ ('Income:Donations', -amount, currency, meta),
+ (other_acct, amount, currency),
+ ])
+
+def test_balance():
+ related = core.RelatedPostings()
+ related.add(data.Posting.from_beancount(donation(10), 0))
+ assert related.balance() == [testutil.Amount(-10)]
+ related.add(data.Posting.from_beancount(donation(15), 0))
+ assert related.balance() == [testutil.Amount(-25)]
+ related.add(data.Posting.from_beancount(donation(20), 0))
+ assert related.balance() == [testutil.Amount(-45)]
+
+def test_balance_zero():
+ related = core.RelatedPostings()
+ related.add(data.Posting.from_beancount(donation(10), 0))
+ related.add(data.Posting.from_beancount(donation(-10), 0))
+ assert related.balance() == [testutil.Amount(0)]
+
+def test_balance_multiple_currencies():
+ related = core.RelatedPostings()
+ related.add(data.Posting.from_beancount(donation(10, 'GBP'), 0))
+ related.add(data.Posting.from_beancount(donation(15, 'GBP'), 0))
+ related.add(data.Posting.from_beancount(donation(20, 'EUR'), 0))
+ related.add(data.Posting.from_beancount(donation(25, 'EUR'), 0))
+ assert set(related.balance()) == {
+ testutil.Amount(-25, 'GBP'),
+ testutil.Amount(-45, 'EUR'),
+ }
+
+def test_balance_multiple_currencies_one_zero():
+ related = core.RelatedPostings()
+ related.add(data.Posting.from_beancount(donation(10, 'EUR'), 0))
+ related.add(data.Posting.from_beancount(donation(15, 'USD'), 0))
+ related.add(data.Posting.from_beancount(donation(-10, 'EUR'), 0))
+ assert set(related.balance()) == {
+ testutil.Amount(-15, 'USD'),
+ testutil.Amount(0, 'EUR'),
+ }