Added DatabaseFeatures.supports_microsecond_precision.

This reverts commit a80903b.

Author: timgraham

django/db/backends/base/features.py

@@ -87,6 +87,9 @@ class BaseDatabaseFeatures:
     # by returning the type used to store duration field?
     supports_temporal_subtraction = False
 
+    # Do time/datetime fields have microsecond precision?
+    supports_microsecond_precision = True
+
     # Does the __regex lookup support backreferencing and grouping?
     supports_regex_backreferencing = True
 

tests/basic/tests.py

@@ -4,13 +4,14 @@
 from unittest import mock
 
 from django.core.exceptions import MultipleObjectsReturned, ObjectDoesNotExist
-from django.db import DEFAULT_DB_ALIAS, DatabaseError, connections, models
+from django.db import DEFAULT_DB_ALIAS, DatabaseError, connection, connections, models
 from django.db.models.manager import BaseManager
 from django.db.models.query import MAX_GET_RESULTS, EmptyQuerySet
 from django.test import (
     SimpleTestCase,
     TestCase,
     TransactionTestCase,
+    skipIfDBFeature,
     skipUnlessDBFeature,
 )
 from django.utils.translation import gettext_lazy
@@ -224,6 +225,7 @@ def test_not_equal_and_equal_operators_behave_as_expected_on_instances(self):
             Article.objects.get(id__exact=a1.id), Article.objects.get(id__exact=a2.id)
         )
 
+    @skipUnlessDBFeature("supports_microsecond_precision")
     def test_microsecond_precision(self):
         a9 = Article(
             headline="Article 9",
@@ -235,6 +237,33 @@ def test_microsecond_precision(self):
             datetime(2005, 7, 31, 12, 30, 45, 180),
         )
 
+    @skipIfDBFeature("supports_microsecond_precision")
+    def test_microsecond_precision_not_supported(self):
+        # In MySQL, microsecond-level precision isn't always available. You'll
+        # lose microsecond-level precision once the data is saved.
+        a9 = Article(
+            headline="Article 9",
+            pub_date=datetime(2005, 7, 31, 12, 30, 45, 180),
+        )
+        a9.save()
+        self.assertEqual(
+            Article.objects.get(id__exact=a9.id).pub_date,
+            datetime(2005, 7, 31, 12, 30, 45),
+        )
+
+    @skipIfDBFeature("supports_microsecond_precision")
+    def test_microsecond_precision_not_supported_edge_case(self):
+        # If microsecond-level precision isn't available, you'll lose
+        # microsecond-level precision once the data is saved.
+        a = Article.objects.create(
+            headline="Article",
+            pub_date=datetime(2008, 12, 31, 23, 59, 59, 999999),
+        )
+        self.assertEqual(
+            Article.objects.get(pk=a.pk).pub_date,
+            datetime(2008, 12, 31, 23, 59, 59, 999000),
+        )
+
     def test_manually_specify_primary_key(self):
         # You can manually specify the primary key when creating a new object.
         a101 = Article(
@@ -818,6 +847,13 @@ def _update(self, *args, **kwargs):
 
 
 class ModelRefreshTests(TestCase):
+    def _truncate_ms(self, val):
+        # Some databases don't support microseconds in datetimes which causes
+        # problems when comparing the original value to that loaded from the DB.
+        if connection.features.supports_microsecond_precision:
+            return val
+        return val - timedelta(microseconds=val.microsecond)
+
     def test_refresh(self):
         a = Article.objects.create(pub_date=datetime.now())
         Article.objects.create(pub_date=datetime.now())
@@ -877,7 +913,7 @@ def test_refresh_null_fk(self):
         self.assertEqual(s2.selfref, s1)
 
     def test_refresh_unsaved(self):
-        pub_date = datetime.now()
+        pub_date = self._truncate_ms(datetime.now())
         a = Article.objects.create(pub_date=pub_date)
         a2 = Article(id=a.pk)
         with self.assertNumQueries(1):

tests/db_functions/comparison/test_greatest.py

@@ -11,9 +11,17 @@
 from ..models import Article, Author, DecimalModel, Fan
 
 
+def microsecond_support(value):
+    return (
+        value
+        if connection.features.supports_microsecond_precision
+        else value.replace(microsecond=0)
+    )
+
+
 class GreatestTests(TestCase):
     def test_basic(self):
-        now = timezone.now()
+        now = microsecond_support(timezone.now())
         before = now - timedelta(hours=1)
         Article.objects.create(
             title="Testing with Django", written=before, published=now
@@ -25,7 +33,7 @@ def test_basic(self):
 
     @skipUnlessDBFeature("greatest_least_ignores_nulls")
     def test_ignores_null(self):
-        now = timezone.now()
+        now = microsecond_support(timezone.now())
         Article.objects.create(title="Testing with Django", written=now)
         articles = Article.objects.annotate(
             last_updated=Greatest("written", "published")
@@ -42,7 +50,7 @@ def test_propagates_null(self):
 
     def test_coalesce_workaround(self):
         past = datetime(1900, 1, 1)
-        now = timezone.now()
+        now = microsecond_support(timezone.now())
         Article.objects.create(title="Testing with Django", written=now)
         articles = Article.objects.annotate(
             last_updated=Greatest(

tests/db_functions/comparison/test_least.py

@@ -11,9 +11,17 @@
 from ..models import Article, Author, DecimalModel, Fan
 
 
+def microsecond_support(value):
+    return (
+        value
+        if connection.features.supports_microsecond_precision
+        else value.replace(microsecond=0)
+    )
+
+
 class LeastTests(TestCase):
     def test_basic(self):
-        now = timezone.now()
+        now = microsecond_support(timezone.now())
         before = now - timedelta(hours=1)
         Article.objects.create(
             title="Testing with Django", written=before, published=now
@@ -23,7 +31,7 @@ def test_basic(self):
 
     @skipUnlessDBFeature("greatest_least_ignores_nulls")
     def test_ignores_null(self):
-        now = timezone.now()
+        now = microsecond_support(timezone.now())
         Article.objects.create(title="Testing with Django", written=now)
         articles = Article.objects.annotate(
             first_updated=Least("written", "published"),
@@ -38,7 +46,7 @@ def test_propagates_null(self):
 
     def test_coalesce_workaround(self):
         future = datetime(2100, 1, 1)
-        now = timezone.now()
+        now = microsecond_support(timezone.now())
         Article.objects.create(title="Testing with Django", written=now)
         articles = Article.objects.annotate(
             last_updated=Least(