document autocommit when using the compiler extension, update the "understanding autocommit" section

5 files changed, 88 insertions, 7 deletions

diff --git a/doc/build/core/connections.rst b/doc/build/core/connections.rst
index 3706e34b5..15b87cbae 100644
--- a/doc/build/core/connections.rst
+++ b/doc/build/core/connections.rst
@@ -177,6 +177,7 @@ one exists.
 .. index::
    single: thread safety; transactions
 
+.. _autocommit:
 
 Understanding Autocommit
 ========================
@@ -184,19 +185,31 @@ 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
diff --git a/doc/build/core/expression_api.rst b/doc/build/core/expression_api.rst
index fee9c9501..ac6aa9e8b 100644
--- a/doc/build/core/expression_api.rst
+++ b/doc/build/core/expression_api.rst
@@ -219,6 +219,10 @@ Classes
   :members: where, values
   :show-inheritance:
 
+.. autoclass:: UpdateBase
+  :members: params, bind, returning
+  :show-inheritance:
+
 .. autoclass:: ValuesBase
     :members:
     :show-inheritance:
diff --git a/lib/sqlalchemy/dialects/mssql/base.py b/lib/sqlalchemy/dialects/mssql/base.py
index 015c346eb..c523dbfa5 100644
--- a/lib/sqlalchemy/dialects/mssql/base.py
+++ b/lib/sqlalchemy/dialects/mssql/base.py
@@ -655,7 +655,9 @@ class MSExecutionContext(default.DefaultExecutionContext):
             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:
diff --git a/lib/sqlalchemy/ext/compiler.py b/lib/sqlalchemy/ext/compiler.py
index 7b0837741..cb126c374 100644
--- a/lib/sqlalchemy/ext/compiler.py
+++ b/lib/sqlalchemy/ext/compiler.py
@@ -91,6 +91,11 @@ Produces::
 
     "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
 ---------------------------------------------
 
@@ -106,6 +111,50 @@ constraint that embeds a SQL expression::
             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
 =======================================================
 
diff --git a/lib/sqlalchemy/sql/expression.py b/lib/sqlalchemy/sql/expression.py
index dc8714371..151a321f5 100644
--- a/lib/sqlalchemy/sql/expression.py
+++ b/lib/sqlalchemy/sql/expression.py
@@ -4782,7 +4782,9 @@ class Select(_SelectBase):
     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'
 
@@ -4800,12 +4802,23 @@ class UpdateBase(Executable, ClauseElement):
             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):