2.2. Connecting to MongoDB¶
To connect to a running instance of mongod, use the
connect()
function. The first argument is the name of the
database to connect to:
from mongoengine import connect
connect('project1')
By default, MongoEngine assumes that the mongod instance is running
on localhost on port 27017. If MongoDB is running elsewhere, you should
provide the host
and port
arguments to
connect()
:
connect('project1', host='192.168.1.35', port=12345)
If the database requires authentication, username
and password
arguments should be provided:
connect('project1', username='webapp', password='pwd123')
URI style connections are also supported – just supply the URI as
the host
to
connect()
:
connect('project1', host='mongodb://localhost/database_name')
Note
Database, username and password from URI string overrides
corresponding parameters in connect()
:
connect(
db='test',
username='user',
password='12345',
host='mongodb://admin:qwerty@localhost/production'
)
will establish connection to production
database using
admin
username and qwerty
password.
2.2.1. Replica Sets¶
MongoEngine supports connecting to replica sets:
from mongoengine import connect
# Regular connect
connect('dbname', replicaset='rs-name')
# MongoDB URI-style connect
connect(host='mongodb://localhost/dbname?replicaSet=rs-name')
Read preferences are supported through the connection or via individual queries by passing the read_preference
Bar.objects().read_preference(ReadPreference.PRIMARY)
Bar.objects(read_preference=ReadPreference.PRIMARY)
2.2.2. Multiple Databases¶
To use multiple databases you can use connect()
and provide
an alias name for the connection - if no alias is provided then “default”
is used.
In the background this uses register_connection()
to
store the data and you can register all aliases up front if required.
Individual documents can also support multiple databases by providing a
db_alias in their meta data. This allows DBRef
objects to point across databases and collections. Below is an example schema,
using 3 different databases to store data:
class User(Document):
name = StringField()
meta = {'db_alias': 'user-db'}
class Book(Document):
name = StringField()
meta = {'db_alias': 'book-db'}
class AuthorBooks(Document):
author = ReferenceField(User)
book = ReferenceField(Book)
meta = {'db_alias': 'users-books-db'}
2.2.3. Context Managers¶
Sometimes you may want to switch the database or collection to query against. For example, archiving older data into a separate database for performance reasons or writing functions that dynamically choose collections to write a document to.
2.2.3.1. Switch Database¶
The switch_db
context manager allows
you to change the database alias for a given class allowing quick and easy
access to the same User document across databases:
from mongoengine.context_managers import switch_db
class User(Document):
name = StringField()
meta = {'db_alias': 'user-db'}
with switch_db(User, 'archive-user-db') as User:
User(name='Ross').save() # Saves the 'archive-user-db'
2.2.3.2. Switch Collection¶
The switch_collection
context manager
allows you to change the collection for a given class allowing quick and easy
access to the same Group document across collection:
from mongoengine.context_managers import switch_collection
class Group(Document):
name = StringField()
Group(name='test').save() # Saves in the default db
with switch_collection(Group, 'group2000') as Group:
Group(name='hello Group 2000 collection!').save() # Saves in group2000 collection
Note
Make sure any aliases have been registered with
register_connection()
or connect()
before using the context manager.