Fixed #24595 -- Prevented loss of null info in MySQL field alteration

Thanks Simon Percivall for the report, and Simon Charette and Tim
Graham for the reviews.

django/db/backends/base/schema.py

@@ -9,9 +9,13 @@ from django.utils.encoding import force_bytes
 logger = logging.getLogger('django.db.backends.schema')
 
 
-def _related_non_m2m_objects(opts):
-    # filters out m2m objects from reverse relations.
-    return (obj for obj in opts.related_objects if not obj.field.many_to_many)
+def _related_non_m2m_objects(old_field, new_field):
+    # Filters out m2m objects from reverse relations.
+    # Returns (old_relation, new_relation) tuples.
+    return zip(
+        (obj for obj in old_field.model._meta.related_objects if not obj.field.many_to_many),
+        (obj for obj in new_field.model._meta.related_objects if not obj.field.many_to_many)
+    )
 
 
 class BaseDatabaseSchemaEditor(object):
@@ -454,7 +458,8 @@ class BaseDatabaseSchemaEditor(object):
         old_type = old_db_params['type']
         new_db_params = new_field.db_parameters(connection=self.connection)
         new_type = new_db_params['type']
-        if (old_type is None and old_field.remote_field is None) or (new_type is None and new_field.remote_field is None):
+        if ((old_type is None and old_field.remote_field is None) or
+                (new_type is None and new_field.remote_field is None)):
             raise ValueError(
                 "Cannot alter field %s into %s - they do not properly define "
                 "db_type (are you using PostGIS 1.5 or badly-written custom "
@@ -515,10 +520,12 @@ class BaseDatabaseSchemaEditor(object):
         if old_field.primary_key and new_field.primary_key and old_type != new_type:
             # '_meta.related_field' also contains M2M reverse fields, these
             # will be filtered out
-            for rel in _related_non_m2m_objects(new_field.model._meta):
-                rel_fk_names = self._constraint_names(rel.related_model, [rel.field.column], foreign_key=True)
+            for _old_rel, new_rel in _related_non_m2m_objects(old_field, new_field):
+                rel_fk_names = self._constraint_names(
+                    new_rel.related_model, [new_rel.field.column], foreign_key=True
+                )
                 for fk_name in rel_fk_names:
-                    self.execute(self._delete_constraint_sql(self.sql_delete_fk, rel.related_model, fk_name))
+                    self.execute(self._delete_constraint_sql(self.sql_delete_fk, new_rel.related_model, fk_name))
         # Removed an index? (no strict check, as multiple indexes are possible)
         if (old_field.db_index and not new_field.db_index and
                 not old_field.unique and not
@@ -552,7 +559,9 @@ class BaseDatabaseSchemaEditor(object):
         post_actions = []
         # Type change?
         if old_type != new_type:
-            fragment, other_actions = self._alter_column_type_sql(model._meta.db_table, new_field.column, new_type)
+            fragment, other_actions = self._alter_column_type_sql(
+                model._meta.db_table, old_field, new_field, new_type
+            )
             actions.append(fragment)
             post_actions.extend(other_actions)
         # When changing a column NULL constraint to NOT NULL with a given
@@ -669,7 +678,7 @@ class BaseDatabaseSchemaEditor(object):
         # referring to us.
         rels_to_update = []
         if old_field.primary_key and new_field.primary_key and old_type != new_type:
-            rels_to_update.extend(_related_non_m2m_objects(new_field.model._meta))
+            rels_to_update.extend(_related_non_m2m_objects(old_field, new_field))
         # Changed to become primary key?
         # Note that we don't detect unsetting of a PK, as we assume another field
         # will always come along and replace it.
@@ -692,20 +701,23 @@ class BaseDatabaseSchemaEditor(object):
                 }
             )
             # Update all referencing columns
-            rels_to_update.extend(_related_non_m2m_objects(new_field.model._meta))
+            rels_to_update.extend(_related_non_m2m_objects(old_field, new_field))
         # Handle our type alters on the other end of rels from the PK stuff above
-        for rel in rels_to_update:
-            rel_db_params = rel.field.db_parameters(connection=self.connection)
+        for old_rel, new_rel in rels_to_update:
+            rel_db_params = new_rel.field.db_parameters(connection=self.connection)
             rel_type = rel_db_params['type']
+            fragment, other_actions = self._alter_column_type_sql(
+                new_rel.related_model._meta.db_table, old_rel.field, new_rel.field, rel_type
+            )
             self.execute(
                 self.sql_alter_column % {
-                    "table": self.quote_name(rel.related_model._meta.db_table),
-                    "changes": self.sql_alter_column_type % {
-                        "column": self.quote_name(rel.field.column),
-                        "type": rel_type,
-                    }
-                }
+                    "table": self.quote_name(new_rel.related_model._meta.db_table),
+                    "changes": fragment[0],
+                },
+                fragment[1],
             )
+            for sql, params in other_actions:
+                self.execute(sql, params)
         # Does it have a foreign key?
         if (new_field.remote_field and
                 (fks_dropped or not old_field.remote_field or not old_field.db_constraint) and
@@ -740,7 +752,7 @@ class BaseDatabaseSchemaEditor(object):
         if self.connection.features.connection_persists_old_columns:
             self.connection.close()
 
-    def _alter_column_type_sql(self, table, column, type):
+    def _alter_column_type_sql(self, table, old_field, new_field, new_type):
         """
         Hook to specialize column type alteration for different backends,
         for cases when a creation type is different to an alteration type
@@ -753,8 +765,8 @@ class BaseDatabaseSchemaEditor(object):
         return (
             (
                 self.sql_alter_column_type % {
-                    "column": self.quote_name(column),
-                    "type": type,
+                    "column": self.quote_name(new_field.column),
+                    "type": new_type,
                 },
                 [],
             ),

django/db/backends/mysql/schema.py

@@ -61,3 +61,11 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
                     # index creation for FKs (index automatically created by MySQL)
                     field.db_index = False
         return super(DatabaseSchemaEditor, self)._model_indexes_sql(model)
+
+    def _alter_column_type_sql(self, table, old_field, new_field, new_type):
+        # Keep null property of old field, if it has changed, it will be handled separately
+        if old_field.null:
+            new_type += " NULL"
+        else:
+            new_type += " NOT NULL"
+        return super(DatabaseSchemaEditor, self)._alter_column_type_sql(table, old_field, new_field, new_type)

django/db/backends/postgresql_psycopg2/schema.py

@@ -34,11 +34,12 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
                         model, [field], suffix='_like', sql=self.sql_create_text_index))
         return output
 
-    def _alter_column_type_sql(self, table, column, type):
+    def _alter_column_type_sql(self, table, old_field, new_field, new_type):
         """
         Makes ALTER TYPE with SERIAL make sense.
         """
-        if type.lower() == "serial":
+        if new_type.lower() == "serial":
+            column = new_field.column
             sequence_name = "%s_%s_seq" % (table, column)
             return (
                 (
@@ -82,4 +83,6 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
                 ],
             )
         else:
-            return super(DatabaseSchemaEditor, self)._alter_column_type_sql(table, column, type)
+            return super(DatabaseSchemaEditor, self)._alter_column_type_sql(
+                table, old_field, new_field, new_type
+            )