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Efficiently manage multiple SqlAlchemy connections for Pyramid

Project description

Python package


SQLAssist offers a streamlined integration for handling multiple SQLAlchemy database connections under Pyramid. It ships with first-party support for "reader", "writer" and "logger" concepts - but can be extended to support any design.

SQLAssist also offers some utility mixin/base classes for SQLAlchemy applications that are useful for debugging applications. A debugtoolbar panel is included as well, which can be used to further aid in development.

This package has been working in production environments for many years.

PRs are always appreciated.

Recent changes:

With v0.13.0, SQLAlchemy 1.3.0 and zope.sqlalchemy 1.2.0 are required.

With v0.12.0, there have been some API changes and the introduction of a pyramid_debugtoolbar panel.


This package uses scoped Sessions by default.

v0.9.1 introduced a capability to use non-scoped Sessions. This appears to work, but has not been tested as thoroughly as I'd like.

Non-scoped Sessions are not integrated with the transaction package, as they were incompatible with Zope's transaction extension when support was last attempted.

There is probably a way to get this to work, or things may have changed. Pull Requests are welcome.


The package facilitates managing multiple SQLAlchemy connections under Pyramid through a single API. It has been used in Celery too.

There are 4 steps to using this package:

  1. It is the job of your Pyramid application's model to create SQLAlchemy engines.
  2. Each created engine should be passed into pyramid_sqlassist.initialize_engine
  3. After initializing all the engines, invoke pyramid_sqlassist.register_request_method with the name of the request attribute you wish to use
  4. SQLAlchemy classes in your model must inherit from pyramid_sqlassist.DeclaredTable -- which is just an instance of SQLAlchemy's declarative_base.

Note: If your Pyramid application connects to the database BEFORE a process fork, you must call pyramid_sqlassist.reinit_engine(/engine/). This can be streamlined with the pyramid_forksafe plugin.

What does all this accomplish?

pyramid_sqlassist maintains a private Python dict in it's namespace: _ENGINE_REGISTRY.

Calling initialize_engine will wrap each SQLAlchemy engine into a SQLAssist EngineWrapper and then register it into the _ENGINE_REGISTRY. The wrapper contains a SQLAlchemy sessionmaker created for each engine, along with some convenience functions.

Calling register_request_method will invoke Pyramid's add_request_method to add a DbSessionsContainer onto the Pyramid Request as a specified attribute name.

The DbSessionsContainer automatically register a cleanup function via Pyramid's add_finished_callback if the database is used.

As a convenience, the active Pyramid request is stashed in each SQLAlchmey session's "info" dict.


This is an example for a Pyramid app, which creates a READER and WRITER connection.

import sqlalchemy
import pyramid_sqlassist

from . import model_objects

def initialize_database(config, settings):

	# setup the reader
	engine_reader = sqlalchemy.engine_from_config(settings,

	# setup the writer
	engine_writer = sqlalchemy.engine_from_config(settings,

	# setup the shared interface
	pyramid_sqlassist.register_request_method(config, 'dbSession')

Miscellaneous info

Because Pyramid will lazily create the request database interaction object, it is very lightweight. On initialization, the container will register a cleanup routine via add_finished_callback.

The DbSessionsContainer exposes some methods:

  • reader - property. memoized access to "reader" connection

  • writer - property. memoized access to "writer" connection

  • logger - property. memoized access to "logger" connection

  • any - property. invokes get_any()

  • get_reader - method. lazy access to "reader" connection

  • get_writer - method. lazy access to "writer" connection

  • get_logger - method. lazy access to "logger" connection

  • get_any - method. tries to find memoized connections. otherwise will invoke another method.

On first access of every "Session", the container will re-initialize that Session by invoking it as a callable, issuing a .rollback(), and stashing the current Pyramid request in the Session's info dict.

Within your code, the request can be retrieved via object_session

from sqlalchemy.orm.session import object_session
_session = object_session(ExampleObject)
request =['request']

The cleanup function will call session.remove() for all Sessions that were used within the request.

A postfork hook is available if needed via reinit_engine. For all managed engines, engine.dispose() will be called.

Why it works:

DeclaredTable is simply an instance of sqlalchemy.ext.declarative.declarative_base, bound to our own metadata

# via Pyramid
# Recommended naming convention used by Alembic, as various different database
# providers will autogenerate vastly different names making migrations more
# difficult. See:
	"ix": 'ix_%(column_0_label)s',
	"uq": "uq_%(table_name)s_%(column_0_name)s",
	"ck": "ck_%(table_name)s_%(constraint_name)s",
	"fk": "fk_%(table_name)s_%(column_0_name)s_%(referred_table_name)s",
	"pk": "pk_%(table_name)s"

# store the metadata in the package (GLOBAL)
_metadata = sqlalchemy.MetaData(naming_convention=NAMING_CONVENTION)

# this is used for inheritance only
DeclaredTable = declarative_base(metadata=_metadata)

Subclassing tables from DeclaredTable takes care of all the core ORM setup.

When initialize_engine is called, by default sqlalchemy.orm.configure_mappers is triggered (this can be deferred to first usage of the ORM, but most people will want to take the performance hit on startup and try to push the mapped tables into shared memory before a fork).

Misc Objects


  • core object with utility methods for quick prototyping of applications


  • this namepace is currently unused; it houses some in-progress code for supporting table reflection

debugtoolbar support

Simply add pyramid_sqlassist.debugtoolbar to debugtoolbar.includes for your application.

For example, this is a line from a .ini file

debugtoolbar.includes = pyramid_sqlassist.debugtoolbar

The SQLAssist debugtoolbar panel includes information such as:

  • the request attribute the DbSessionsContainer has been memoized into
  • the connections which were active for the request
  • the engine status for the active request (initialized, started, ended)
  • the engines configured for the application and available to the request

The panel is intended to help debug issues with connections - though there should be none.


  • debugtoolbar: show id information for the underlying connections and connection pool to help troubleshoot potential forking issues


  • PYTHONOPTIMIZE. All logging functions are nested under if __debug__: statements; they can be compiled away during production


Sections of this code were originally taken from or inspired by:

Example Usage

In your env.ini, specify multiple sqlalchemy urls (which might be to different dbs or the same db but with different permissions):

sqlalchemy_reader.url = postgres://myapp_reader:myapp@localhost/myapp
sqlalchemy_writer.url = postgres://myapp_writer:myapp@localhost/myapp


from . import models

    import uwsgi

    def post_fork_hook():

    uwsgi.post_fork_hook = post_fork_hook

except ImportError:

def main(global_config, **settings):


import sqlassist

ENGINES_ENABLED = ['reader', 'writer', ]

def initialize_database(settings):

	engine_reader = sqlalchemy.engine_from_config(settings, prefix="sqlalchemy_reader.")
	sqlassist.initialize_engine('reader',engine_reader,default=True, reflect=myapp.models, use_zope=False)

	engine_writer = sqlalchemy.engine_from_config(settings, prefix="sqlalchemy_writer.")
	sqlassist.initialize_engine('writer',engine_writer,default=False, reflect=myapp.models, use_zope=True)

	# custom property: `request.dbSession`

def database_postfork():

def request_setup_dbSession(request):
	return sqlassist.DbSessionsContainer(request)


import sqlalchemy as sa
from sqlassist import DeclaredTable

class TestTable(DeclaredTable):
	__tablename__ = 'groups'

	id = sa.Column(sa.Integer, primary_key=True)
	name = sa.Column(sa.Unicode(255), nullable=False)
	description = sa.Column(sa.Text, nullable=False)

in your handlers, you have the following. Note, sqlalchemy is only imported to grab an exception:

import sqlalchemy

class BaseHandler(object):
	def __init__(self,request):
		self.request = request

class ViewHandler(BaseHandler):

	def index(self):

		print self.request.dbSession.reader.query(models.actual_models.TestTable).all()

			#this should fail , assuming reader can't write
			dbTestTable = models.actual_models.TestTable() 'Test Case 1'

		except sqlalchemy.exc.ProgrammingError:
			raise ValueError("Commit Failed!")

		#but this should work , assuming writer can write
		dbTestTable = models.actual_models.TestTable() = 'Test Case 2'


If you inherit from this class, your SQLAlchemy objects have some convenience methods:

  • get__by__id( self, dbSession, id , id_column='id' ):
  • get__by__column__lower( self, dbSession, column_name , search , allow_many=False ):
  • get__by__column__similar( self, dbSession , column_name , seed , prefix_only=True):
  • get__by__column__exact_then_ilike( self, dbSession, column_name, seed ):
  • get__range( self, dbSession, start=0, limit=None, sort_direction='asc', order_col=None, order_case_sensitive=True, filters=[], debug_query=False):
  • columns_as_dict(self):

Another important note...


This convenience class ONLY deals with 3 connections right now :

  • reader
  • writer
  • logger

If you have more/different names - subclass (or create a patch to deal with dynamic names!) I didn't have time for that.

The reader and writer classes will start with an automatic rollback; The logger will not.

transaction support

By default, the package will try to load the following libraries:

import transaction
from zope.sqlalchemy import register as zope_register

This can be disabled with an environment variable




if you're using "Zope" & "transaction" modules :

  • you need to call transaction.commit
  • IMPORTANT remember that mark_changed exists!

if you're not using "Zope" & "transaction" modules :

  • you need to call "dbSession_writer.commit()"


you want to call rollback on the specific database Sessions to control what is in each one

catching exceptions if you're trying to support both transaction.commit() and dbsession.commit()

let's say you do this:

except AssertionError , e:
	print "Should fail because Zope wants this"

# add to writer

# commit

in this event, both object1 and object2 will be committed by transaction.commit()

You must explicitly invoke a rollback after the AssertionError

Reflected Tables

this package once supported trying to handle table reflection. It is being removed unless someone wants to do a better job.

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