Split model_fields tests into different files.

tests/model_fields/test_floatfield.py

@@ -0,0 +1,34 @@
+from django.db import transaction
+from django.test import TestCase
+
+from .models import FloatModel
+
+
+class TestFloatField(TestCase):
+
+    def test_float_validates_object(self):
+        instance = FloatModel(size=2.5)
+        # Try setting float field to unsaved object
+        instance.size = instance
+        with transaction.atomic():
+            with self.assertRaises(TypeError):
+                instance.save()
+        # Set value to valid and save
+        instance.size = 2.5
+        instance.save()
+        self.assertTrue(instance.id)
+        # Set field to object on saved instance
+        instance.size = instance
+        msg = (
+            'Tried to update field model_fields.FloatModel.size with a model '
+            'instance, <FloatModel: FloatModel object>. Use a value '
+            'compatible with FloatField.'
+        )
+        with transaction.atomic():
+            with self.assertRaisesMessage(TypeError, msg):
+                instance.save()
+        # Try setting field to object on retrieved object
+        obj = FloatModel.objects.get(pk=instance.id)
+        obj.size = obj
+        with self.assertRaises(TypeError):
+            obj.save()