Fixed #28967 -- Prevented Cast to FloatField from rounding to integer on MySQL.

django/db/backends/mysql/operations.py

@@ -20,7 +20,6 @@ class DatabaseOperations(BaseDatabaseOperations):
         'IntegerField': 'signed integer',
         'BigIntegerField': 'signed integer',
         'SmallIntegerField': 'signed integer',
-        'FloatField': 'signed',
         'PositiveIntegerField': 'unsigned integer',
         'PositiveSmallIntegerField': 'unsigned integer',
     }

django/db/models/functions/comparison.py

@@ -14,6 +14,11 @@ class Cast(Func):
         extra_context['db_type'] = self.output_field.cast_db_type(connection)
         return super().as_sql(compiler, connection, **extra_context)
 
+    def as_mysql(self, compiler, connection):
+        # MySQL doesn't support explicit cast to float.
+        template = '(%(expressions)s + 0.0)' if self.output_field.get_internal_type() == 'FloatField' else None
+        return self.as_sql(compiler, connection, template=template)
+
     def as_postgresql(self, compiler, connection):
         # CAST would be valid too, but the :: shortcut syntax is more readable.
         return self.as_sql(compiler, connection, template='%(expressions)s::%(db_type)s')

tests/db_functions/test_cast.py

@@ -1,4 +1,5 @@
 import datetime
+import decimal
 
 from django.db import models
 from django.db.models.expressions import Value
@@ -55,5 +56,7 @@ class CastTests(TestCase):
         self.assertEqual(dates.get().cast_datetime, now)
 
     def test_cast_from_python(self):
-        numbers = Author.objects.annotate(cast_float=Cast(0, models.FloatField()))
-        self.assertEqual(numbers.get().cast_float, 0.0)
+        numbers = Author.objects.annotate(cast_float=Cast(decimal.Decimal(0.125), models.FloatField()))
+        cast_float = numbers.get().cast_float
+        self.assertIsInstance(cast_float, float)
+        self.assertEqual(cast_float, 0.125)