add support for select_for_update

Author: michiya

sql_server/pyodbc/base.py

@@ -52,6 +52,8 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     can_return_id_from_insert = True
     can_use_chunked_reads = False
     has_bulk_insert = True
+    has_select_for_update = _DJANGO_VERSION >= 14
+    has_select_for_update_nowait = _DJANGO_VERSION >= 14
     ignores_nulls_in_unique_constraints = False
     supports_1000_query_parameters = False
     supports_microsecond_precision = True

sql_server/pyodbc/compiler.py

@@ -92,6 +92,12 @@ def as_sql(self, with_limits=True, with_col_aliases=False):
         result.extend(from_)
         params.extend(f_params)
 
+        if self.query.select_for_update and self.connection.features.has_select_for_update:
+            # If we've been asked for a NOWAIT query but the backend does not support it,
+            # raise a DatabaseError otherwise we could get an unexpected deadlock.
+            nowait = self.query.select_for_update_nowait
+            result.append(self.connection.ops.for_update_sql(nowait=nowait))
+
         if where:
             result.append('WHERE %s' % where)
             params.extend(w_params)

sql_server/pyodbc/operations.py

@@ -144,6 +144,15 @@ def field_cast_sql(self, db_type):
         """
         return '%s'
 
+    def for_update_sql(self, nowait=False):
+        """
+        Returns the FOR UPDATE SQL clause to lock rows for an update operation.
+        """
+        if nowait:
+            return 'WITH (NOWAIT, ROWLOCK, UPDLOCK)'
+        else:
+            return 'WITH (ROWLOCK, UPDLOCK)'
+
     def fulltext_search_sql(self, field_name):
         """
         Returns the SQL WHERE clause to use in order to perform a full-text