.. index::
single: thread safety; transactions
+.. _autocommit:
Understanding Autocommit
========================
The previous transaction example illustrates how to use :class:`.Transaction`
so that several executions can take part in the same transaction. What happens
when we issue an INSERT, UPDATE or DELETE call without using
-:class:`.Transaction`? The answer is **autocommit**. While many DBAPI
-implementation provide various special "non-transactional" modes, the current
-SQLAlchemy behavior is such that it implements its own "autocommit" which
+:class:`.Transaction`? While some DBAPI
+implementations provide various special "non-transactional" modes, the core
+behavior of DBAPI per PEP-0249 is that a *transaction is always in progress*,
+providing only ``rollback()`` and ``commit()`` methods but no ``begin()``.
+SQLAlchemy assumes this is the case for any given DBAPI.
+
+Given this requirement, SQLAlchemy implements its own "autocommit" feature which
works completely consistently across all backends. This is achieved by
detecting statements which represent data-changing operations, i.e. INSERT,
UPDATE, DELETE, as well as data definition language (DDL) statements such as
CREATE TABLE, ALTER TABLE, and then issuing a COMMIT automatically if no
-transaction is in progress. The detection is based on compiled statement
-attributes, or in the case of a text-only statement via regular expressions::
+transaction is in progress. The detection is based on the presence of the
+``autocommit=True`` execution option on the statement. If the statement
+is a text-only statement and the flag is not set, a regular expression is used
+to detect INSERT, UPDATE, DELETE, as well as a variety of other commands
+for a particular backend::
conn = engine.connect()
conn.execute("INSERT INTO users VALUES (1, 'john')") # autocommits
+The "autocommit" feature is only in effect when no :class:`.Transaction` has
+otherwise been declared. This means the feature is not generally used with
+the ORM, as the :class:`.Session` object by default always maintains an
+ongoing :class:`.Transaction`.
+
Full control of the "autocommit" behavior is available using the generative
:meth:`.Connection.execution_options` method provided on :class:`.Connection`,
:class:`.Engine`, :class:`.Executable`, using the "autocommit" flag which will
:members: where, values
:show-inheritance:
+.. autoclass:: UpdateBase
+ :members: params, bind, returning
+ :show-inheritance:
+
.. autoclass:: ValuesBase
:members:
:show-inheritance:
seq_column = tbl._autoincrement_column
insert_has_sequence = seq_column is not None
- if insert_has_sequence:
+ if getattr(self.compiled._mssql_requires_identity_insert, False):
+ self._enable_identity_insert = True
+ elif insert_has_sequence:
self._enable_identity_insert = \
seq_column.key in self.compiled_parameters[0]
else:
"INSERT INTO mytable (SELECT mytable.x, mytable.y, mytable.z FROM mytable WHERE mytable.x > :x_1)"
+.. note::
+
+ The above ``InsertFromSelect`` construct probably wants to have "autocommit"
+ enabled. See :ref:`enabling_compiled_autocommit` for this step.
+
Cross Compiling between SQL and DDL compilers
---------------------------------------------
ddlcompiler.sql_compiler.process(constraint.expression)
)
+.. _enabling_compiled_autocommit:
+
+Enabling Autocommit on a Construct
+==================================
+
+Recall from the section :ref:`autocommit` that the :class:`.Engine`, when asked to execute
+a construct in the absence of a user-defined transaction, detects if the given
+construct represents DML or DDL, that is, a data modification or data definition statement, which
+requires (or may require, in the case of DDL) that the transaction generated by the DBAPI be committed
+(recall that DBAPI always has a transaction going on regardless of what SQLAlchemy does). Checking
+for this is actually accomplished
+by checking for the "autocommit" execution option on the construct. When building a construct like
+an INSERT derivation, a new DDL type, or perhaps a stored procedure that alters data, the "autocommit"
+option needs to be set in order for the statement to function with "connectionless" execution
+(as described in :ref:`dbengine_implicit`).
+
+Currently a quick way to do this is to subclass :class:`.Executable`, then add the "autocommit" flag
+to the ``_execution_options`` dictionary (note this is a "frozen" dictionary which supplies a generative
+``union()`` method)::
+
+ from sqlalchemy.sql.expression import Executable, ClauseElement
+
+ class MyInsertThing(Executable, ClauseElement):
+ _execution_options = \\
+ Executable._execution_options.union({'autocommit': True})
+
+More succinctly, if the construct is truly similar to an INSERT, UPDATE, or DELETE, :class:`.UpdateBase`
+can be used, which already is a subclass of :class:`.Executable`, :class:`.ClauseElement` and includes the
+``autocommit`` flag::
+
+ from sqlalchemy.sql.expression import UpdateBase
+
+ class MyInsertThing(UpdateBase):
+ def __init__(self, ...):
+ ...
+
+
+
+
+DDL elements that subclass :class:`.DDLElement` already have the "autocommit" flag turned on.
+
+
+
+
Changing the default compilation of existing constructs
=======================================================
bind = property(bind, _set_bind)
class UpdateBase(Executable, ClauseElement):
- """Form the base for ``INSERT``, ``UPDATE``, and ``DELETE`` statements."""
+ """Form the base for ``INSERT``, ``UPDATE``, and ``DELETE`` statements.
+
+ """
__visit_name__ = 'update_base'
return parameters
def params(self, *arg, **kw):
+ """Set the parameters for the statement.
+
+ This method raises ``NotImplementedError`` on the base class,
+ and is overridden by :class:`.ValuesBase` to provide the
+ SET/VALUES clause of UPDATE and INSERT.
+
+ """
raise NotImplementedError(
"params() is not supported for INSERT/UPDATE/DELETE statements."
" To set the values for an INSERT or UPDATE statement, use"
" stmt.values(**parameters).")
def bind(self):
+ """Return a 'bind' linked to this :class:`.UpdateBase`
+ or a :class:`.Table` associated with it.
+
+ """
return self._bind or self.table.bind
def _set_bind(self, bind):
projects / thirdparty / sqlalchemy / sqlalchemy.git / commitdiff
