willhardy / django-hstore

django-hstore

Django-hstore is a niche library which integrates the hstore extension of PostgreSQL into Django, assuming one is using Django 1.2+, PostgreSQL 9.0+, and Psycopg 2.3+.

Limitations

• Due to how Django implements its ORM, you will need to use the custom postgresql_psycopg2 backend defined in this package, which naturally will prevent you from dropping in other django extensions which require a custom backend (unless you fork and combine).
• PostgreSQL's implementation of hstore has no concept of type; it stores a mapping of string keys to string values. This library makes no attempt to coerce keys or values to strings.

Running the tests

Assuming one has the dependencies installed, and a PostgreSQL 9.0+ server up and running:

python setup.py test

Usage

First, update your settings module to specify the custom database backend:

DATABASES = {
    'default': {
        'ENGINE': 'django_hstore.postgresql_psycopg2',
        ...
    }
}

Note to South users: If you keep getting errors like There is no South database module 'south.db.None' for your database., add the following to settings.py:

SOUTH_DATABASE_ADAPTERS = {'default': 'south.db.postgresql_psycopg2'}

The library provides three principal classes:

django_hstore.hstore.DictionaryField

An ORM field which stores a mapping of string key/value pairs in an hstore column.

django_hstore.hstore.ReferencesField

An ORM field which builds on DictionaryField to store a mapping of string keys to django object references, much like ForeignKey.

django_hstore.hstore.HStoreManager

An ORM manager which provides much of the query functionality of the library.

Model definition is straightforward:

from django.db import models
from django_hstore import hstore

class Something(models.Model):
    name = models.CharField(max_length=32)
    data = hstore.DictionaryField()
    objects = hstore.HStoreManager()

    def __unicode__(self):
        return self.name

You then treat the data field as simply a dictionary of string pairs:

instance = Something.objects.create(name='something', data={'a': '1', 'b': '2'})
assert instance.data['a'] == '1'

empty = Something.objects.create(name='empty')
assert empty.data == {}

empty.data['a'] = '1'
empty.save()
assert Something.objects.get(name='something').data['a'] == '1'

You can issue indexed queries against hstore fields:

# equivalence
Something.objects.filter(data={'a': '1', 'b': '2'})

# subset by key/value mapping
Something.objects.filter(data__contains={'a': '1'})

# subset by list of keys
Something.objects.filter(data__contains=['a', 'b'])

# subset by single key
Something.objects.filter(data__contains='a')

You can also take advantage of some db-side functionality by using the manager:

# identify the keys present in an hstore field
>>> Something.objects.hkeys(id=instance.id, attr='data')
['a', 'b']

# peek at a a named value within an hstore field
>>> Something.objects.hpeek(id=instance.id, attr='data', key='a')
'1'

# do the same, after filter
>>> Something.objects.filter(id=instance.id).hpeek(attr='data', key='a')
'1'

# remove a key/value pair from an hstore field
>>> Something.objects.filter(name='something').hremove('data', 'b')

The hstore methods on manager pass all keyword arguments aside from attr and key to .filter().