djorm-ext-hstore 0.6

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.

• Hstore extension is not automatically installed on use this package. You must install it manually. (For execute tests, you must install hstore extension on template1 database.

• For run tests, hstore extension must be installed on template1 database. (Short version)

Classes

The library provides three principal classes:

djorm_hstore.fields.DictionaryField

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

djorm_hstore.fields.ReferencesField

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

djorm_hstore.models.HStoreManager

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

NOTE: the predefined hstore manager inherits all functionality of djorm-ext-expressions module (which is part of django orm extensions package)

Usage

Initially define some sample model:

from django.db import models
from djorm_hstore.fields import DictionaryField
from djorm_hstore.models import HStoreManager

class Something(models.Model):
    name = models.CharField(max_length=32)
    data = DictionaryField(db_index=True)
    objects = 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 djorm_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 djorm_hstore.functions import HstoreSlice, HstorePeek, HstoreKeys

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

Psycopg2 hstore registration

If for some reason you have to use djorm_hstore along databases that don’t have hstore extension installed, you can skip hstore registration by setting HAS_HSTORE to False in your database config:

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql_psycopg2',
        'NAME': 'name',
        'USER': 'user',
        'PASSWORD': 'pass',
        'HOST': 'localhost',
        'PORT': '',
    },
    'other': {
        'ENGINE': 'django.db.backends.postgresql_psycopg2',
        'NAME': 'other',
        'USER': 'user',
        'PASSWORD': 'pass',
        'HOST': 'localhost',
        'PORT': '',
        'HAS_HSTORE': False,
    }
}

If you do that, then don’t try to create DictionaryField in this database. Be sure to check out allow_syncdb documentation.