Table Of Contents

Previous topic

SQLAlchemy Master Slave Load Balancing

Next topic

SQLAlchemy and Transaction Config Settings

Working With Ming And MongoDB

MongoDB is a high-performance schemaless database that allows you to store and retrieve JSON-like documents. MongoDB stores these documents in collections, which are analogous to SQL tables. Because MongoDB is schemaless, there are no guarantees given to the database client of the format of the data that may be returned from a query; you can put any kind of document into a collection that you want.

While this dynamic behavior is handy in a rapid development environment where you might delete and re-create the database many times a day, it starts to be a problem when you need to make guarantees of the type of data in a collection (because you code depends on it).

Ming allows you to specify the schema for your data in Python code and then develop in confidence, knowing the format of data you get from a query.

TurboGears Integration

TurboGears Ming integration is entirely pushed into the generated quickstart template since version 2.1.3

To generate a Ming based project you just need to pass the --ming option to the quickstart command. For more informations refer to the Quickstarting A TurboGears 2.2.2 Project section.

TurboGears will rely on the unit of work pattern of Ming flushing the session for you at the end of each request. This will happen only if everything went fine. In case of an exception the session won’t be flushed and any change performed throught the ORM layer won’t happen avoiding an incosistent environment due to half made changes.


Note that if you perform any change outside the ming unit of work or if you flush the session yourself you might still end with an inconsistent environment.

Getting Started

If you don’t know how Ming works at all, please take a few minutes to read over these tutorials:

Your quickstarted project will have a subpackage called model, made up of the following files:

  • This is where the database access is set up. Your collections should be imported into this module, and you’re highly encouraged to define them in a separate module - entities, for example.
  • This file defines the session of your database connection. By default TurboGears will use a Session object with multithreading support. You will usually need to import this each time you have to declare a MappedClass to specify the session that has to be used to perform queries.
  • This file will be created if you enabled authentication and authorization in the quickstart. It defines two collections repoze.what.quickstart relies on: User (for the registered members in your website and the groups they belong to) and Permission (a permission granted to one or more groups).

Defining Your Own Collections

By default TurboGears configures Ming in Declarative mode. This is similar to the SQLAlchemy declarative support and needs each model to inherit from the MappedClass class.

The tables defined by the quickstart in model/ are based on the declarative method, so you may want to check it out to see how columns are defined for these tables. For more information, you may read the ORM Tutorial.

Once you have defined your collections in a separate module in the model package, they should be imported from model/ So the end of this file would look like this:

# Import your model modules here.
from auth import User, Permission
# Say you defined these three classes in the 'movies'
# module of your 'model' package.
from movies import Movie, Actor, Director

Indexing Support

TurboGears supports also automatic indexing of MongoDB fields. If you want to guarantee that a field is unique or indexed you just have to specify the unique_indexes or indexes variables for the __mongometa__ attribute of the mapped class.

class Permission(MappedClass):
    class __mongometa__:
        session = DBSession
        name = 'tg_permission'
        unique_indexes = [('permission_name',),]

TurboGears will ensure indexes for your each time the application is started, this is performed inside the init_model function.

Handling Relationships

Ming comes with support to one-to-many and many-to-one Relations they provide an easy to use access to related objects. The fact that this relation is read only isn’t a real issue as the related objects will have a ForeignIdProperty which can be changed to add or remove objects to the relation.

As MongoDB provides too many ways to express a many-to-many relationship, those kind of relations are instead left on their own. TurboGears anyway provides a tool to make easier to access and modify those relationships.

tgming.ProgrammaticRelationProperty provides easy access to those relationships exposing them as a list while leaving to the developer the flexibility to implement the relationship as it best suites the model.

A good example of how the ProgrammaticRelationProperty works is the User to Group relationship:

from tgming import ProgrammaticRelationProperty

class Group(MappedClass):
    class __mongometa__:
        session = DBSession
        name = 'tg_group'

    group_name = FieldProperty(s.String)

class User(MappedClass):
    class __mongometa__:
        session = DBSession
        name = 'tg_user'

    _groups = FieldProperty(s.Array(str))

    def _get_groups(self):
        return Group.query.find(dict(group_name={'$in':self._groups})).all()
    def _set_groups(self, groups):
        self._groups = [group.group_name for group in groups]
    groups = ProgrammaticRelationProperty(Group, _get_groups, _set_groups)

In this case each user will have one or more groups stored with their group_name inside the User._groups array. Accessing User.groups will provide a list of the groups the user is part of. This list is retrieved using User._get_groups and can be set with User._set_groups.

Using Synonyms

There are cases when you will want to adapt a value from the database before loading and storing it. A simple example of this case is the password field, this will probably be encrypted with some kind of algorithm which has to be applied before saving the field itself.

To handle those cases TurboGears provides the tgming.SynonymProperty accessor. This provides a way to hook two functions which have to be called before storing and retrieving the value to adapt it.

from tgming import SynonymProperty

class User(MappedClass):
    class __mongometa__:
        session = DBSession
        name = 'tg_user'

    _password = FieldProperty(s.String)

    def _set_password(self, password):
        self._password = self._hash_password(password)
    def _get_password(self):
        return self._password
    password = SynonymProperty(_get_password, _set_password)

In the previous example the password property is stored encrypted inside the User._password field but it is accessed using the User.password property which encrypts it automatically before setting it.