Fixed #22210 -- Saving model instances to non-related fields.

Previously, saving a model instance to a non-related field (in
particular a FloatField) would silently convert the model to an Integer
(the pk) and save it. This is undesirable behaviour, and likely to cause
confusion so the validatio has been hardened.

Thanks to @PirosB3 for the patch and @jarshwah for the review.

4 files changed, 27 insertions, 2 deletions

diff --git a/django/db/models/sql/compiler.py b/django/db/models/sql/compiler.py
index 11c503cd5d..5636f344df 100644
--- a/django/db/models/sql/compiler.py
+++ b/django/db/models/sql/compiler.py
@@ -4,6 +4,7 @@ from django.conf import settings
 from django.core.exceptions import FieldError
 from django.db.backends.utils import truncate_name
 from django.db.models.constants import LOOKUP_SEP
+from django.db.models.expressions import ExpressionNode
 from django.db.models.query_utils import select_related_descend, QueryWrapper
 from django.db.models.sql.constants import (CURSOR, SINGLE, MULTI, NO_RESULTS,
         ORDER_DIR, GET_ITERATOR_CHUNK_SIZE, SelectInfo)
@@ -951,7 +952,14 @@ class SQLUpdateCompiler(SQLCompiler):
         values, update_params = [], []
         for field, model, val in self.query.values:
             if hasattr(val, 'prepare_database_save'):
-                val = val.prepare_database_save(field)
+                if field.rel or isinstance(val, ExpressionNode):
+                    val = val.prepare_database_save(field)
+                else:
+                    raise TypeError("Database is trying to update a relational field "
+                                     "of type %s with a value of type %s. Make sure "
+                                     "you are setting the correct relations" %
+                                     (field.__class__.__name__,
+                                     val.__class__.__name__))
             else:
                 val = field.get_db_prep_save(val, connection=self.connection)
 
diff --git a/docs/releases/1.7.txt b/docs/releases/1.7.txt
index 52007c98aa..2dd3931060 100644
--- a/docs/releases/1.7.txt
+++ b/docs/releases/1.7.txt
@@ -674,6 +674,10 @@ Models
   the new :attr:`ManyToManyField.through_fields <django.db.models.ManyToManyField.through_fields>`
   argument.
 
+* Assigning a model instance to a non-relation field will now throw an error.
+  Previously this used to work if the field accepted integers as input as it
+  took the primary key.
+
 Signals
 ^^^^^^^
 
diff --git a/tests/model_fields/models.py b/tests/model_fields/models.py
index e110578ce9..b16287a66c 100644
--- a/tests/model_fields/models.py
+++ b/tests/model_fields/models.py
@@ -48,6 +48,9 @@ class Whiz(models.Model):
 class BigD(models.Model):
     d = models.DecimalField(max_digits=38, decimal_places=30)
 
+class FloatModel(models.Model):
+    size = models.FloatField()
+
 
 class BigS(models.Model):
     s = models.SlugField(max_length=255)
diff --git a/tests/model_fields/tests.py b/tests/model_fields/tests.py
index 194219a114..a39ad81f64 100644
--- a/tests/model_fields/tests.py
+++ b/tests/model_fields/tests.py
@@ -23,7 +23,7 @@ from django.utils.functional import lazy
 from .models import (
     Foo, Bar, Whiz, BigD, BigS, BigInt, Post, NullBooleanModel,
     BooleanModel, PrimaryKeyCharModel, DataModel, Document, RenamedField,
-    VerboseNameField, FksToBooleans, FkToChar)
+    VerboseNameField, FksToBooleans, FkToChar, FloatModel)
 
 
 class BasicFieldTests(test.TestCase):
@@ -78,6 +78,16 @@ class BasicFieldTests(test.TestCase):
 
         self.assertEqual(m._meta.get_field('id').verbose_name, 'verbose pk')
 
+    def test_float_validates_object(self):
+        instance = FloatModel(size=2.5)
+        instance.save()
+        self.assertTrue(instance.id)
+
+        obj = FloatModel.objects.get(pk=1)
+        obj.size = obj
+        with self.assertRaises(TypeError):
+            obj.save()
+
     def test_choices_form_class(self):
         """Can supply a custom choices form class. Regression for #20999."""
         choices = [('a', 'a')]