fix(1.11): Drop defaults before removing field

Hand merge of michiya#38 :

When removing columns make sure we remove defaults (which Microsoft has
deemed to be constraints).

Author: Melvyn Sopacua

sql_server/pyodbc/introspection.py

@@ -302,6 +302,7 @@ 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)
@@ -328,6 +329,7 @@ def get_constraints(self, cursor, table_name):
                     "foreign_key": None,
                     "check": True,
                     "index": False,
+                    "default": False,
                 }
             # Record the details
             constraints[constraint]['columns'].append(column)
@@ -370,6 +372,7 @@ def get_constraints(self, cursor, table_name):
                     "foreign_key": None,
                     "check": False,
                     "index": True,
+                    "default": False,
                     "orders": [],
                     "type": Index.suffix if type_ in (1,2) else desc.lower(),
                 }
@@ -378,4 +381,26 @@ def get_constraints(self, cursor, table_name):
         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_contraints]
+            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

@@ -6,7 +6,6 @@
 )
 from django.db.models import Index
 from django.db.models.fields import AutoField, BigAutoField
-from django.db.models.fields.related import ManyToManyField
 from django.db.transaction import TransactionManagementError
 from django.utils import six
 from django.utils.text import force_text
@@ -716,6 +715,18 @@ def remove_field(self, model, field):
                     "table": self.quote_name(model._meta.db_table),
                     "name": self.quote_name(name),
                 })
+        # Drop default constraints, SQL Server treats defaults as constraints requiring explicit
+        # deletion.
+        for name, infodict in constraints.items():
+            if field.column in infodict['columns'] and infodict['default']:
+                self.execute(
+                    self.sql_alter_column % {
+                        "table": self.quote_name(model._meta.db_table),
+                        "changes": self.sql_alter_column_no_default % {
+                            "column": self.quote_name(name)
+                        }
+                    }
+                )
         # Delete the column
         sql = self.sql_delete_column % {
             "table": self.quote_name(model._meta.db_table),