It's a MongoDB ORM for Python.

$ pip install mongoengine

To start using MongoEngine we will import like this:

import mongoengine

To connect a DB we will use the connect function, its first argument is the DB:

connect('project1')
connect('project1', host='192.168.1.35', port=12345)
connect('project1', username='webapp', password='pwd123')
connect('project1', host='mongodb://localhost/database_name')

Next schema implements a Blog where a User can create different type Posts. Those types are inherited classes from Post.

class User(Document):
    email = StringField(required=True)
    first_name = StringField(max_length=50)
    last_name = StringField(max_length=50)
 
class Post(Document):
    title = StringField(max_length=120, required=True)
    author = ReferenceField(User)
    meta = {'allow_inheritance': True}
 
class TextPost(Post):
    content = StringField()
 
class ImagePost(Post):
    image_path = StringField()
 
class LinkPost(Post):
    link_url = StringField()

• tags = ListField(StringField(max_length=30)), a ListField is a field object that allows to have a list of other field objects (including other lists).

class Comment(EmbeddedDocument):
    content = StringField()
    name = StringField(max_length=120)
 
class Post(Document):
    title = StringField(max_length=120, required=True)
    author = ReferenceField(User)
    tags = ListField(StringField(max_length=30))
    comments = ListField(EmbeddedDocumentField(Comment))

When we define a ReferenceField we can indicate reverse_delete_rule=CASCADE to delete all the objects when a referenced object is deleted (it does not work with MapFields neither DictFields).

class Post(Document):
    title = StringField(max_length=120, required=True)
    author = ReferenceField(User, reverse_delete_rule=CASCADE)
    tags = ListField(StringField(max_length=30))
    comments = ListField(EmbeddedDocumentField(Comment))

ross = User(email='ross@example.com', first_name='Ross', last_name='Lawley').save()
 
# this is other way to save ross 
ross1 = User(email='ross1@example.com')
ross1.first_name = 'Ross1'
ross1.last_name = 'Lawley'
ross1.save()
 
post1 = TextPost(title='Fun with MongoEngine', author=john)
post1.content = 'Took a look at MongoEngine today, looks pretty cool.'
post1.tags = ['mongodb', 'mongoengine']
post1.save()
 
post2 = LinkPost(title='MongoEngine Documentation', author=ross)
post2.link_url = 'http://docs.mongoengine.com/'
post2.tags = ['mongoengine']
post2.save()

A Document object is actually a QuerySet object. It lazily queries the database when you need the data. QuerySet objects perform searches on the data base using functions like…

• first() to retrieve the first object that matches with the provided query.
• count() to know the number of objects that matche with the provided query.

for post in Post.objects:
    print post.title

for post in TextPost.objects:
    print post.content

for post in Post.objects(tags='mongodb'):
    print post.title