Enumerate and remove DEFAULT constraints

sql_server/pyodbc/introspection.py

@@ -258,6 +258,7 @@ def get_constraints(self, cursor, table_name):
                     "foreign_key": (ref_table, ref_column) if kind.lower() == "foreign key" else None,
                     "check": False,
                     "index": False,
+                    "default": False,
                 }
             # Record the details
             constraints[constraint]['columns'].append(column)
@@ -283,6 +284,7 @@ def get_constraints(self, cursor, table_name):
                     "foreign_key": None,
                     "check": True,
                     "index": False,
+                    "default": False,
                 }
             # Record the details
             constraints[constraint]['columns'].append(column)
@@ -321,9 +323,32 @@ def get_constraints(self, cursor, table_name):
                     "foreign_key": None,
                     "check": False,
                     "index": True,
+                    "default": False,
                 }
             indexes[index]["columns"].append(column)
         for index, constraint in indexes.items():
             if index not in constraints:
                 constraints[index] = constraint
+        # Now get DEFAULT constraint columns
+        cursor.execute("""
+            SELECT
+                [name],
+                COL_NAME([parent_object_id], [parent_column_id])
+            FROM
+                [sys].[default_constraints]
+            WHERE
+                OBJECT_NAME([parent_object_id]) = %s
+        """, [table_name])
+        for constraint, column in cursor.fetchall():
+            if constraint not in constraints:
+                constraints[constraint] = {
+                    "columns": [],
+                    "primary_key": False,
+                    "unique": False,
+                    "foreign_key": None,
+                    "check": False,
+                    "index": False,
+                    "default": True,
+                }
+            constraints[constraint]['columns'].append(column)
         return constraints

sql_server/pyodbc/schema.py

@@ -535,6 +535,15 @@ def remove_field(self, model, field):
                     "name": self.quote_name(pk_name),
                 }
             )
+        # Drop default, SQL Server treats defaults as constraints requiring explicit deletion
+        for default_name in self._default_constraint_names(model, [field.column]):
+            sql = self.sql_alter_column % {
+                "table": self.quote_name(model._meta.db_table),
+                "changes": self.sql_alter_column_no_default % {
+                    "name": self.quote_name(default_name),
+                }
+            }
+            self.execute(sql)
         # Delete the column
         sql = self.sql_delete_column % {
             "table": self.quote_name(model._meta.db_table),
@@ -544,3 +553,13 @@ def remove_field(self, model, field):
         # Reset connection if required
         if self.connection.features.connection_persists_old_columns:
             self.connection.close()
+
+    def _default_constraint_names(self, model, column_names):
+        """Returns default constraint names matching the columns"""
+        # As BaseDatabaseSchemaEditor._constraint_names doesn't know about defaults
+        with self.connection.cursor() as cursor:
+            constraints = self.connection.introspection.get_constraints(cursor, model._meta.db_table)
+        return [
+            name for name, infodict in constraints.items()
+            if infodict["columns"] == column_names and infodict.get("default")
+        ]