Added a stealth option to flush to allow cascades.

This allows using flush on a subset of the tables without having to
manually cascade to all tables with foreign keys to the tables being
truncated, when they're known to be empty.

On databases where truncate is implemented with DELETE FROM, this
doesn't make a difference. The cascade is allowed, not mandatory.

django/core/management/commands/flush.py

@@ -32,8 +32,9 @@ def handle_noargs(self, **options):
         connection = connections[db]
         verbosity = int(options.get('verbosity'))
         interactive = options.get('interactive')
-        # 'reset_sequences' is a stealth option
+        # 'reset_sequences' and 'allow_cascade' are stealth options
         reset_sequences = options.get('reset_sequences', True)
+        allow_cascade = options.get('allow_cascade', False)
 
         self.style = no_style()
 
@@ -45,7 +46,9 @@ def handle_noargs(self, **options):
             except ImportError:
                 pass
 
-        sql_list = sql_flush(self.style, connection, only_django=True, reset_sequences=reset_sequences)
+        sql_list = sql_flush(self.style, connection, only_django=True,
+                             reset_sequences=reset_sequences,
+                             allow_cascade=allow_cascade)
 
         if interactive:
             confirm = input("""You have requested a flush of the database.

django/core/management/sql.py

@@ -102,7 +102,7 @@ def sql_delete(app, style, connection):
     return output[::-1]  # Reverse it, to deal with table dependencies.
 
 
-def sql_flush(style, connection, only_django=False, reset_sequences=True):
+def sql_flush(style, connection, only_django=False, reset_sequences=True, allow_cascade=False):
     """
     Returns a list of the SQL statements used to flush the database.
 
@@ -114,7 +114,7 @@ def sql_flush(style, connection, only_django=False, reset_sequences=True):
     else:
         tables = connection.introspection.table_names()
     seqs = connection.introspection.sequence_list() if reset_sequences else ()
-    statements = connection.ops.sql_flush(style, tables, seqs)
+    statements = connection.ops.sql_flush(style, tables, seqs, allow_cascade)
     return statements
 
 

django/db/backends/__init__.py

@@ -390,7 +390,7 @@ def constraint_checks_disabled(self):
     def disable_constraint_checking(self):
         """
         Backends can implement as needed to temporarily disable foreign key
-        constraint checking. Should return True if the constraints were 
+        constraint checking. Should return True if the constraints were
         disabled and will need to be reenabled.
         """
         return False
@@ -947,7 +947,7 @@ def set_time_zone_sql(self):
         """
         return ''
 
-    def sql_flush(self, style, tables, sequences):
+    def sql_flush(self, style, tables, sequences, allow_cascade=False):
         """
         Returns a list of SQL statements required to remove all data from
         the given database tables (without actually removing the tables
@@ -958,6 +958,10 @@ def sql_flush(self, style, tables, sequences):
 
         The `style` argument is a Style object as returned by either
         color_style() or no_style() in django.core.management.color.
+
+        The `allow_cascade` argument determines whether truncation may cascade
+        to tables with foreign keys pointing the tables being truncated.
+        PostgreSQL requires a cascade even if these tables are empty.
         """
         raise NotImplementedError()
 

django/db/backends/mysql/base.py

@@ -298,14 +298,17 @@ def quote_name(self, name):
     def random_function_sql(self):
         return 'RAND()'
 
-    def sql_flush(self, style, tables, sequences):
+    def sql_flush(self, style, tables, sequences, allow_cascade=False):
         # NB: The generated SQL below is specific to MySQL
         # 'TRUNCATE x;', 'TRUNCATE y;', 'TRUNCATE z;'... style SQL statements
         # to clear all tables of all data
         if tables:
             sql = ['SET FOREIGN_KEY_CHECKS = 0;']
             for table in tables:
-                sql.append('%s %s;' % (style.SQL_KEYWORD('TRUNCATE'), style.SQL_FIELD(self.quote_name(table))))
+                sql.append('%s %s;' % (
+                    style.SQL_KEYWORD('TRUNCATE'),
+                    style.SQL_FIELD(self.quote_name(table)),
+                ))
             sql.append('SET FOREIGN_KEY_CHECKS = 1;')
             sql.extend(self.sequence_reset_by_name_sql(style, sequences))
             return sql

django/db/backends/oracle/base.py

@@ -339,17 +339,17 @@ def savepoint_create_sql(self, sid):
     def savepoint_rollback_sql(self, sid):
         return convert_unicode("ROLLBACK TO SAVEPOINT " + self.quote_name(sid))
 
-    def sql_flush(self, style, tables, sequences):
+    def sql_flush(self, style, tables, sequences, allow_cascade=False):
         # Return a list of 'TRUNCATE x;', 'TRUNCATE y;',
         # 'TRUNCATE z;'... style SQL statements
         if tables:
             # Oracle does support TRUNCATE, but it seems to get us into
             # FK referential trouble, whereas DELETE FROM table works.
-            sql = ['%s %s %s;' % \
-                    (style.SQL_KEYWORD('DELETE'),
-                     style.SQL_KEYWORD('FROM'),
-                     style.SQL_FIELD(self.quote_name(table)))
-                    for table in tables]
+            sql = ['%s %s %s;' % (
+                style.SQL_KEYWORD('DELETE'),
+                style.SQL_KEYWORD('FROM'),
+                style.SQL_FIELD(self.quote_name(table))
+            ) for table in tables]
             # Since we've just deleted all the rows, running our sequence
             # ALTER code will reset the sequence to 0.
             sql.extend(self.sequence_reset_by_name_sql(style, sequences))

django/db/backends/postgresql_psycopg2/operations.py

@@ -101,15 +101,24 @@ def quote_name(self, name):
     def set_time_zone_sql(self):
         return "SET TIME ZONE %s"
 
-    def sql_flush(self, style, tables, sequences):
+    def sql_flush(self, style, tables, sequences, allow_cascade=False):
         if tables:
             # Perform a single SQL 'TRUNCATE x, y, z...;' statement.  It allows
             # us to truncate tables referenced by a foreign key in any other
             # table.
-            sql = ['%s %s;' % \
-                (style.SQL_KEYWORD('TRUNCATE'),
-                    style.SQL_FIELD(', '.join([self.quote_name(table) for table in tables]))
-            )]
+            tables_sql = ', '.join(
+                style.SQL_FIELD(self.quote_name(table)) for table in tables)
+            if allow_cascade:
+                sql = ['%s %s %s;' % (
+                    style.SQL_KEYWORD('TRUNCATE'),
+                    tables_sql,
+                    style.SQL_KEYWORD('CASCADE'),
+                )]
+            else:
+                sql = ['%s %s;' % (
+                    style.SQL_KEYWORD('TRUNCATE'),
+                    tables_sql,
+                )]
             sql.extend(self.sequence_reset_by_name_sql(style, sequences))
             return sql
         else:

django/db/backends/sqlite3/base.py

@@ -209,15 +209,15 @@ def quote_name(self, name):
     def no_limit_value(self):
         return -1
 
-    def sql_flush(self, style, tables, sequences):
+    def sql_flush(self, style, tables, sequences, allow_cascade=False):
         # NB: The generated SQL below is specific to SQLite
         # Note: The DELETE FROM... SQL generated below works for SQLite databases
         # because constraints don't exist
-        sql = ['%s %s %s;' % \
-                (style.SQL_KEYWORD('DELETE'),
-                 style.SQL_KEYWORD('FROM'),
-                 style.SQL_FIELD(self.quote_name(table))
-                 ) for table in tables]
+        sql = ['%s %s %s;' % (
+            style.SQL_KEYWORD('DELETE'),
+            style.SQL_KEYWORD('FROM'),
+            style.SQL_FIELD(self.quote_name(table))
+        ) for table in tables]
         # Note: No requirement for reset of auto-incremented indices (cf. other
         # sql_flush() implementations). Just return SQL at this point
         return sql