Added introspection.table_exists() method to db instrospection; this is used all over the place in django.db.management and will break under database backends that don't support LIMIT.

git-svn-id: http://code.djangoproject.com/svn/django/branches/magic-removal@2461 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/backends/ado_mssql/introspection.py

@@ -10,4 +10,7 @@ def get_relations(cursor, table_name):
 def get_indexes(cursor, table_name):
     raise NotImplementedError
 
+def table_exists(cursor, table_name):
+    raise NotImplementedError
+
 DATA_TYPES_REVERSE = {}

django/db/backends/dummy/introspection.py

@@ -4,5 +4,6 @@ get_table_list = complain
 get_table_description = complain
 get_relations = complain
 get_indexes = complain
+table_exists = complain
 
 DATA_TYPES_REVERSE = {}

django/db/backends/mysql/introspection.py

@@ -1,3 +1,4 @@
+from django.db import transaction
 from django.db.backends.mysql.base import quote_name
 from MySQLdb.constants import FIELD_TYPE
 
@@ -27,6 +28,16 @@ def get_indexes(cursor, table_name):
         indexes[row[4]] = {'primary_key': (row[2] == 'PRIMARY'), 'unique': not bool(row[1])}
     return indexes
     
+def table_exists(cursor, table_name):
+    """Returns True if the given table exists."""
+    try:
+        cursor.execute("SELECT 1 FROM %s LIMIT 1" % quote_name(table_name))
+    except:
+        transaction.rollback_unless_managed()
+        return False
+    else:
+        return True
+        
 DATA_TYPES_REVERSE = {
     FIELD_TYPE.BLOB: 'TextField',
     FIELD_TYPE.CHAR: 'CharField',

django/db/backends/postgresql/introspection.py

@@ -1,3 +1,4 @@
+from django.db import transaction
 from django.db.backends.postgresql.base import quote_name
 
 def get_table_list(cursor):
@@ -67,6 +68,16 @@ def get_indexes(cursor, table_name):
         indexes[col_name] = {'primary_key': row[2], 'unique': row[1]}
     return indexes
     
+def table_exists(cursor, table_name):
+    """Returns True if the given table exists."""
+    try:
+        cursor.execute("SELECT 1 FROM %s LIMIT 1" % quote_name(table_name))
+    except:
+        transaction.rollback_unless_managed()
+        return False
+    else:
+        return True
+
 # Maps type codes to Django Field types.
 DATA_TYPES_REVERSE = {
     16: 'BooleanField',

django/db/backends/sqlite3/introspection.py

@@ -1,3 +1,4 @@
+from django.db import transaction
 from django.db.backends.sqlite3.base import quote_name
 
 def get_table_list(cursor):
@@ -14,6 +15,16 @@ def get_relations(cursor, table_name):
 def get_indexes(cursor, table_name):
     raise NotImplementedError
 
+def table_exists(cursor, table_name):
+    """Returns True if the given table exists."""
+    try:
+        cursor.execute("SELECT 1 FROM %s LIMIT 1" % quote_name(table_name))
+    except:
+        transaction.rollback_unless_managed()
+        return False
+    else:
+        return True
+
 # Maps SQL types to Django Field types. Some of the SQL types have multiple
 # entries here because SQLite allows for anything and doesn't normalize the
 # field type; it uses whatever was given.