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|
SQL Expressions
===============
.. contents::
:local:
:class: faq
:backlinks: none
.. _faq_sql_expression_string:
How do I render SQL expressions as strings, possibly with bound parameters inlined?
------------------------------------------------------------------------------------
The "stringification" of a SQLAlchemy statement or Query in the vast majority
of cases is as simple as::
print(str(statement))
this applies both to an ORM :class:`~.orm.query.Query` as well as any :func:`.select` or other
statement. Additionally, to get the statement as compiled to a
specific dialect or engine, if the statement itself is not already
bound to one you can pass this in to :meth:`.ClauseElement.compile`::
print(statement.compile(someengine))
or without an :class:`.Engine`::
from sqlalchemy.dialects import postgresql
print(statement.compile(dialect=postgresql.dialect()))
When given an ORM :class:`~.orm.query.Query` object, in order to get at the
:meth:`.ClauseElement.compile`
method we only need access the :attr:`~.orm.query.Query.statement`
accessor first::
statement = query.statement
print(statement.compile(someengine))
The above forms will render the SQL statement as it is passed to the Python
:term:`DBAPI`, which includes that bound parameters are not rendered inline.
SQLAlchemy normally does not stringify bound parameters, as this is handled
appropriately by the Python DBAPI, not to mention bypassing bound
parameters is probably the most widely exploited security hole in
modern web applications. SQLAlchemy has limited ability to do this
stringification in certain circumstances such as that of emitting DDL.
In order to access this functionality one can use the ``literal_binds``
flag, passed to ``compile_kwargs``::
from sqlalchemy.sql import table, column, select
t = table('t', column('x'))
s = select([t]).where(t.c.x == 5)
print(s.compile(compile_kwargs={"literal_binds": True}))
the above approach has the caveats that it is only supported for basic
types, such as ints and strings, and furthermore if a :func:`.bindparam`
without a pre-set value is used directly, it won't be able to
stringify that either.
To support inline literal rendering for types not supported, implement
a :class:`.TypeDecorator` for the target type which includes a
:meth:`.TypeDecorator.process_literal_param` method::
from sqlalchemy import TypeDecorator, Integer
class MyFancyType(TypeDecorator):
impl = Integer
def process_literal_param(self, value, dialect):
return "my_fancy_formatting(%s)" % value
from sqlalchemy import Table, Column, MetaData
tab = Table('mytable', MetaData(), Column('x', MyFancyType()))
print(
tab.select().where(tab.c.x > 5).compile(
compile_kwargs={"literal_binds": True})
)
producing output like::
SELECT mytable.x
FROM mytable
WHERE mytable.x > my_fancy_formatting(5)
|
