[2.0.x] Fixed #28915 -- Prevented SQLite from truncating trailing zeros in the fractional part of DecimalField.

This reverts commit a146b65628 and adds
a test for the regression.

Backport of 6fd6d8383f from master

django/db/backends/sqlite3/operations.py

@@ -1,12 +1,12 @@
 import datetime
 import uuid
+from decimal import Decimal
 
 from django.conf import settings
 from django.core.exceptions import FieldError
 from django.db import utils
 from django.db.backends.base.operations import BaseDatabaseOperations
 from django.db.models import aggregates, fields
-from django.db.models.expressions import Col
 from django.utils import timezone
 from django.utils.dateparse import parse_date, parse_datetime, parse_time
 from django.utils.duration import duration_string
@@ -208,9 +208,7 @@ class DatabaseOperations(BaseDatabaseOperations):
             converters.append(self.convert_datefield_value)
         elif internal_type == 'TimeField':
             converters.append(self.convert_timefield_value)
-        # Converter for Col is added with Database.register_converter()
+        elif internal_type == 'DecimalField':
-        # in base.py.
-        elif internal_type == 'DecimalField' and not isinstance(expression, Col):
             converters.append(self.convert_decimalfield_value)
         elif internal_type == 'UUIDField':
             converters.append(self.convert_uuidfield_value)
@@ -241,8 +239,7 @@ class DatabaseOperations(BaseDatabaseOperations):
     def convert_decimalfield_value(self, value, expression, connection):
         if value is not None:
             value = expression.output_field.format_number(value)
-            # Value is not converted to Decimal here as it will be converted
+            value = Decimal(value)
-            # later in BaseExpression.convert_value().
         return value
 
     def convert_uuidfield_value(self, value, expression, connection):

docs/releases/2.0.1.txt

@@ -26,3 +26,6 @@ Bugfixes
 
 * Corrected admin check to allow a ``OneToOneField`` in
   ``ModelAdmin.autocomplete_fields`` (:ticket:`28898`).
+
+* Fixed a regression on SQLite where ``DecimalField`` returned a result with
+  trailing zeros in the fractional part truncated (:ticket:`28915`).

tests/model_fields/test_decimalfield.py

@@ -81,3 +81,9 @@ class DecimalFieldTests(TestCase):
         expected_message = validators.DecimalValidator.messages['max_whole_digits'] % {'max': 2}
         with self.assertRaisesMessage(ValidationError, expected_message):
             field.clean(Decimal('999'), None)
+
+    def test_roundtrip_with_trailing_zeros(self):
+        """Trailing zeros in the fractional part aren't truncated."""
+        obj = Foo.objects.create(a='bar', d=Decimal('8.320'))
+        obj.refresh_from_db()
+        self.assertEqual(obj.d.compare_total(Decimal('8.320')), Decimal('0'))