Support for indirect rates conversion (#428)

* Support for indirect rates conversion

* Fix

* Support for indirect rates conversion

* Add a comment about rates swap

* Update comment from feedback

* Fix lint

djmoney/contrib/exchange/models.py

@@ -35,15 +35,6 @@ def get_default_backend_name():
     return import_string(EXCHANGE_BACKEND).name
 
 
-def get_one():
-    """
-    For SQLite it is required to cast value to NUMERIC type, otherwise integer division will be used.
-    """
-    if settings.DATABASES['default']['ENGINE'] == 'django.db.backends.sqlite3':
-        return '1::NUMERIC'
-    return 1
-
-
 def get_rate(source, target, backend=None):
     """
     Returns an exchange rate between source and target currencies.
@@ -62,19 +53,52 @@ def get_rate(source, target, backend=None):
 
 
 def _get_rate(source, target, backend):
-    if text_type(source) == text_type(target):
+    source, target = text_type(source), text_type(target)
+    if text_type(source) == target:
         return 1
-    try:
-        forward = models.Q(currency=target, backend__base_currency=source)
-        reverse = models.Q(currency=source, backend__base_currency=target)
-        return Rate.objects.annotate(
-            rate=models.Case(
-                models.When(forward, then=models.F('value')),
-                models.When(reverse, then=models.Value(get_one()) / models.F('value')),
-            )
-        ).get(forward | reverse, backend=backend).rate
-    except Rate.DoesNotExist:
+    rates = Rate.objects.filter(currency__in=(source, target), backend=backend).select_related('backend')
+    if not rates:
         raise MissingRate('Rate %s -> %s does not exist' % (source, target))
+    if len(rates) == 1:
+        return _try_to_get_rate_directly(source, target, rates[0])
+    return _get_rate_via_base(rates, target)
+
+
+def _try_to_get_rate_directly(source, target, rate):
+    """
+    Either target or source equals to base currency of existing rate.
+    """
+    # Converting from base currency to target
+    if rate.backend.base_currency == source and rate.currency == target:
+        return rate.value
+    # Converting from target currency to base
+    elif rate.backend.base_currency == target and rate.currency == source:
+        return 1 / rate.value
+    # Case when target or source is not a base currency
+    raise MissingRate('Rate %s -> %s does not exist' % (source, target))
+
+
+def _get_rate_via_base(rates, target):
+    """
+    :param: rates: A set/tuple of two base Rate instances
+    :param: target: A string instance of the currency to convert to
+
+    Both target and source are not a base currency - actual rate could be calculated via their rates to base currency.
+    For example:
+
+    7.84 NOK = 1 USD = 8.37 SEK
+
+    7.84 NOK = 8.37 SEK
+
+    1 NOK = 8.37 / 7.84 SEK
+    """
+    first, second = rates
+    # Instead of expecting an explicit order in the `rates` iterable, that will put the
+    # source currency in the first place, we decided to add an extra check here and swap
+    # items if they are ordered not as expected
+    if first.currency == target:
+        first, second = second, first
+    return second.value / first.value
 
 
 def convert_money(value, currency):

docs/changes.rst

@@ -6,6 +6,13 @@ Changelog
 Unreleased
 ----------
 
+Added
+~~~~~
+
+- Support for indirect rates conversion through maximum 1 extra step (when there is no direct conversion rate:
+  converting by means of a third currency for which both source and target currency have conversion
+  rates). `#425`_ (`Stranger6667`_, `77cc33`_)
+
 Fixed
 ~~~~~
 
@@ -563,6 +570,7 @@ Added
 .. _0.2: https://github.com/django-money/django-money/compare/0.2...a6d90348085332a393abb40b86b5dd9505489b04
 
 .. _#427: https://github.com/django-money/django-money/pull/427
+.. _#425: https://github.com/django-money/django-money/issues/425
 .. _#412: https://github.com/django-money/django-money/issues/412
 .. _#410: https://github.com/django-money/django-money/issues/410
 .. _#402: https://github.com/django-money/django-money/issues/402

tests/contrib/exchange/conftest.py

@@ -1,4 +1,5 @@
 import json
+from contextlib import contextmanager
 from decimal import Decimal
 
 from django.core.cache import cache
@@ -36,6 +37,14 @@
 FIXER_EXPECTED = json.loads(FIXER_RESPONSE, parse_float=Decimal)['rates']
 
 
+@contextmanager
+def mock_backend(value):
+    response = Mock()
+    response.read.return_value = value
+    with patch('djmoney.contrib.exchange.backends.base.urlopen', return_value=response):
+        yield
+
+
 class ExchangeTest:
 
     @pytest.fixture(autouse=True, params=(
@@ -46,9 +55,7 @@ def setup(self, request):
         klass, response_value, expected = request.param
         self.backend = klass()
         self.expected = expected
-        response = Mock()
-        response.read.return_value = response_value
-        with patch('djmoney.contrib.exchange.backends.base.urlopen', return_value=response):
+        with mock_backend(response_value):
             yield
 
     def assert_rates(self):
@@ -83,6 +90,12 @@ def simple_rates(backend):
     Rate.objects.create(currency='EUR', value=2, backend=backend)
 
 
+@pytest.fixture
+def default_openexchange_rates():
+    with mock_backend(OPEN_EXCHANGE_RATES_RESPONSE):
+        OpenExchangeRatesBackend().update_rates()
+
+
 @pytest.fixture(autouse=True)
 def django_cache():
     cache.clear()

tests/contrib/exchange/test_model.py

@@ -14,21 +14,48 @@
 pytestmark = pytest.mark.django_db
 
 
-@pytest.mark.parametrize('source, target, expected', (
-    ('USD', 'USD', 1),
-    ('USD', 'EUR', 2),
-    ('EUR', 'USD', Decimal('0.5')),
-    (Currency('USD'), 'USD', 1),
-    ('USD', Currency('USD'), 1),
+@pytest.mark.parametrize('source, target, expected, queries', (
+    ('USD', 'USD', 1, 0),
+    ('USD', 'EUR', 2, 1),
+    ('EUR', 'USD', Decimal('0.5'), 1),
+    (Currency('USD'), 'USD', 1, 0),
+    ('USD', Currency('USD'), 1, 0),
 ))
 @pytest.mark.usefixtures('simple_rates')
-def test_get_rate(source, target, expected):
-    assert get_rate(source, target) == expected
+def test_get_rate(source, target, expected, django_assert_num_queries, queries):
+    with django_assert_num_queries(queries):
+        assert get_rate(source, target) == expected
+
+
+@pytest.mark.parametrize('source, target, expected', (
+    ('NOK', 'SEK', Decimal('1.067732610555839085161462146')),
+    ('SEK', 'NOK', Decimal('0.9365640705489186883886537319')),
+))
+@pytest.mark.usefixtures('default_openexchange_rates')
+def test_rates_via_base(source, target, expected, django_assert_num_queries):
+    with django_assert_num_queries(1):
+        assert get_rate(source, target) == expected
 
 
-def test_unknown_currency():
-    with pytest.raises(MissingRate, match='Rate USD \\-\\> EUR does not exist'):
-        get_rate('USD', 'EUR')
+@pytest.mark.parametrize('source, target', (
+    ('NOK', 'ZAR'),
+    ('ZAR', 'NOK'),
+    ('USD', 'ZAR'),
+    ('ZAR', 'USD'),
+))
+@pytest.mark.usefixtures('default_openexchange_rates')
+def test_unknown_currency_with_partially_exiting_currencies(source, target):
+    with pytest.raises(MissingRate, match='Rate %s \\-\\> %s does not exist' % (source, target)):
+        get_rate(source, target)
+
+
+@pytest.mark.parametrize('source, target', (
+    ('USD', 'EUR'),
+    ('SEK', 'ZWL')
+))
+def test_unknown_currency(source, target):
+    with pytest.raises(MissingRate, match='Rate %s \\-\\> %s does not exist' % (source, target)):
+        get_rate(source, target)
 
 
 def test_string_representation(backend):