Lists & Dicts

Django MongoDB Engine provides two fields for storing arbitrary (BSON-compatible) Python list and dict objects in Django model objects, ListField and DictField, which can be used to store information that is not worth a separate model or that should be queryable in efficient manner (using an index).

Both fields may optionally be provided with type information. That restricts their usage to one single type but has the advantage of automatic type checks and conversions.

ListField

Stores Python lists (or any other iterable), represented in BSON as arrays.

from djangotoolbox.fields import ListField

class Post(models.Model):
    ...
    tags = ListField()

>>> Post(tags=['django', 'mongodb'], ...).save()
>>> Post.objects.get(...).tags
['django', 'mongodb']

The typed variant automatically does type conversions according to the given type:

class Post(models.Model):
    ...
    edited_on = ListField(models.DateTimeField())

>>> post = Post(edited_on=['1010-10-10 10:10:10'])
>>> post.save()
>>> Post.objects.get(...).edited_on
[datetime.datetime([1010, 10, 10, 10, 10, 10])]

As described in the tutorial, ListFields are very useful when used together with Embedded Models to store lists of sub-entities to model 1-to-n relationships:

from djangotoolbox.fields import EmbeddedModelField, ListField

class Post(models.Model):
    ...
    comments = ListField(EmbeddedModelField('Comment'))

class Comment(models.Model):
    ...
    text = models.TextField()

Please head over to the Embedded Models topic for more about embedded models.

SetField

Much like a ListField except that it’s represented as a set on Python side (but stored as a list on MongoDB due to the lack of a separate set type in BSON).

DictField

Stores Python dicts (or any dict-like iterable), represented in BSON as subobjects.

from djangotoolbox.fields import DictField

class Image(models.Model):
    ...
    exif = DictField()

>>> Image(exif=get_exif_data(...), ...).save()
>>> Image.objects.get(...).exif
{u'camera_model' : 'Spamcams 4242', 'exposure_time' : 0.3, ...}

The typed variant automatically does type conversion on values. (Not on keys as the are required to be strings on MongoDB.)

class Poll(models.Model):
    ...
    votes = DictField(models.IntegerField())

>>> Poll(votes={'bob' : 3.14, 'alice' : '42'}, ...).save()
>>> Poll.objects.get(...).votes
{u'bob' : 3, u'alice' : 42}

DictFields are useful mainly for storing objects of varying shape, i.e. objects whose structure is unknow at coding time. If all your objects have the same structure, you should consider using Embedded Models.