Supports nullable unique constraints

README.rst

@@ -193,6 +193,11 @@ Dictionary. Current available keys are:
    Integer. Sets the back off time in seconds for reries of
    the database connection process. Default value is ``5``.
 
+-  supports_nullable_unique_constraints
+
+   Boolean. Supports nullable unique constraints.
+   Default value is ``False``.
+
 backend-specific settings
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 

sql_server/pyodbc/base.py

@@ -201,7 +201,7 @@ def __init__(self, *args, **kwargs):
                     ops[op] = '%s COLLATE %s' % (sql, collation)
             self.operators.update(ops)
 
-        self.features = DatabaseFeatures(self)
+        self.features = DatabaseFeatures(self, opts.get('supports_nullable_unique_constraints', False))
         self.ops = DatabaseOperations(self)
         self.client = DatabaseClient(self)
         self.creation = DatabaseCreation(self)

sql_server/pyodbc/features.py

@@ -30,3 +30,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_timezones = False
     supports_transactions = True
     uses_savepoints = True
+
+    def __init__(self, connection, supports_nullable_unique_constraints=False):
+        BaseDatabaseFeatures.__init__(self, connection)
+        self.supports_nullable_unique_constraints = supports_nullable_unique_constraints

sql_server/pyodbc/schema.py

@@ -51,6 +51,8 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
     sql_delete_table = "DROP TABLE %(table)s"
     sql_rename_column = "EXEC sp_rename '%(table)s.%(old_column)s', %(new_column)s, 'COLUMN'"
     sql_rename_table = "EXEC sp_rename %(old_table)s, %(new_table)s"
+    sql_create_unique_null = "CREATE UNIQUE INDEX %(name)s ON %(table)s(%(columns)s) " \
+                             "WHERE %(columns)s IS NOT NULL"
 
     def _alter_column_type_sql(self, table, old_field, new_field, new_type):
         new_type = self._set_field_new_type_null_status(old_field, new_type)
@@ -269,7 +271,14 @@ def _alter_field(self, model, old_field, new_field, old_type, new_type,
                 self.execute(sql, params)
         # Added a unique?
         if not old_field.unique and new_field.unique:
-            self.execute(self._create_unique_sql(model, [new_field.column]))
+            if new_field.null and self.connection.features.supports_nullable_unique_constraints:
+                self.execute(
+                    self._create_index_sql(
+                        model, [new_field], sql=self.sql_create_unique_null, suffix="_uniq"
+                    )
+                )
+            else:
+                self.execute(self._create_unique_sql(model, [new_field.column]))
         # Added an index?
         if (not old_field.db_index and new_field.db_index and
                 not new_field.unique and not
@@ -398,6 +407,13 @@ def add_field(self, model, field):
         # It might not actually have a column behind it
         if definition is None:
             return
+
+        if field.null and field.unique and self.connection.features.supports_nullable_unique_constraints:
+            definition = definition.replace(' UNIQUE', '')
+            self.deferred_sql.append(self._create_index_sql(
+                model, [field], sql=self.sql_create_unique_null, suffix="_uniq"
+            ))
+
         # Check constraints can go on the column SQL here
         db_params = field.db_parameters(connection=self.connection)
         if db_params['check']:
@@ -452,6 +468,13 @@ def create_model(self, model):
             definition, extra_params = self.column_sql(model, field)
             if definition is None:
                 continue
+
+            if field.null and field.unique and self.connection.features.supports_nullable_unique_constraints:
+                definition = definition.replace(' UNIQUE', '')
+                self.deferred_sql.append(self._create_index_sql(
+                    model, [field], sql=self.sql_create_unique_null, suffix="_uniq"
+                ))
+
             # Check constraints can go on the column SQL here
             db_params = field.db_parameters(connection=self.connection)
             if db_params['check']: