django-hstore

5 years after

============= 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 as well as nose, and a PostgreSQL 9.0+ server up and running::

DB_USER=<username> HSTORE_SQL=<path-to-contrib/hstore.sql> ./runtests

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(db_index=True)
    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().

.. _hstore: http://www.postgresql.org/docs/9.0/interactive/hstore.html

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