Backward operation for RemoveField does not allow a default value in case the field is not null.

[Gagaro]

If a field does not allow NULL and has no default, and this field is removed using RemoveField, the backward operation will break if the database is populated because the field will be created with no value.

Replication:

models.py

class TestModel(Model):
    field = CharField(max_length=255)

$ python manage.py makemigrations
$ python manage.py migrate
$ python manage.py shell
>>> TestModel.objects.create(field='test')

models.py

class TestModel(Model):
    pass

$ python manage.py makemigrations
$ python manage migrate
$ python manage migrate app 0001

POC Patch made by apollo13 in IRC:

diff --git a/django/db/migrations/operations/fields.py b/django/db/migrations/operations/fields.py
index 041f8a6..5604e36 100644
--- a/django/db/migrations/operations/fields.py
+++ b/django/db/migrations/operations/fields.py
@@ -122,6 +122,10 @@ class RemoveField(FieldOperation):
     Removes a field from a model.
     """
 
+    def __init__(self, *args, **kwargs):
+        self.default = kwargs.pop('default', None)
+        super(RemoveField, self).__init__(*args, **kwargs)
+
     def deconstruct(self):
         kwargs = {
             'model_name': self.model_name,
@@ -150,7 +154,12 @@ class RemoveField(FieldOperation):
         to_model = to_state.apps.get_model(app_label, self.model_name)
         if self.allow_migrate_model(schema_editor.connection.alias, to_model):
             from_model = from_state.apps.get_model(app_label, self.model_name)
-            schema_editor.add_field(from_model, to_model._meta.get_field(self.name))
+            to_field = to_model._meta.get_field(self.name)
+            old_default = to_field.default
+            if self.default is not None:
+                to_field.default = self.default
+            schema_editor.add_field(from_model, to_field)
+            to_field.default = old_default
 
     def describe(self):
         return "Remove field %s from %s" % (self.name, self.model_name)

[Florian Apolloner]

The autodetector should be updated to ask for a new default, tests and docs missing.

[Simon Charette]

It would be great if we could also rely on the transient default of AddField (preserve_default=False) if it has been provided.

e.g. default should not be required in this case

operations = [
    AddField('model', 'field', models.TextField(default='foo'), preserve_default=False),
    RemoveField('model', 'field'),
]

[Markus Holtermann]

PR: ​https://github.com/django/django/pull/6870

[Bhuvnesh]

​WIP PR