PostgreSQL HStore

Hstore is a niche library which integrates the `hstore`_ extension of PostgreSQL into Django, assuming one is using Django 1.3+, PostgreSQL 9.0+, and Psycopg 2.4+.

Limitations and notes

• 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.

Note to postgresql 9.0 users:

If using postgresql9.0 must manually install the extension hstore to create the database or make hstore already installed in the corresponding template. For an example, you can see the file “runtests-pg90”.

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'}

Classes

The library provides three principal classes:

django_orm.postgresql.hstore.DictionaryField

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

django_orm.postgresql.hstore.ReferencesField

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

django_orm.postgresql.hstore.HStoreManager

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

Example of model declaration:

from django.db import models
from django_orm.postgresql 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:

from django_orm.postgresql.hstore.expressions import HstoreExpression as HE

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

# subset by key/value mapping
Something.objects.where(HE("data").contains({'a':'1'}))

# subset by list of keys
Something.objects.where(HE("data").contains(['a', 'b']))

# subset by single key
Something.objects.where(HE("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.filter(id=1).hkeys(attr='data')
['a', 'b']

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

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

In addition to filters and specific methods to retrieve keys or hstore field values, we can also use annotations, and then we can filter for them.

from django_orm.postgresql.hstore.functions import HstoreSlice, HstorePeek, HstoreKeysç

queryset = SomeModel.objects.annotate_functions(
    sliced = HstoreSlice("hstorefield", ['v']),
    peeked = HstorePeek("hstorefield", "v"),
    keys = HstoreKeys("hstorefield"),
)