|Version 3 (modified by paulsmith@…, 7 years ago) (diff)|
This section explores the details of the GeoDjango Model API. Throughout this section, we'll be using the following geographic model of a Zip code as our example:
from django.contrib.gis.db import models class Zip(models.Model, models.GeoMixin): code = models.CharField(maxlength=5) poly = models.PolygonField() objects = models.GeoManager()
The following geometry-enabled fields are available:
Each of these fields correspond with OpenGIS Simple Features.
In addition to the regular field options available for Django models, Geographic-enabled models have the following additional options:
- Sets the SRID (Spatial Reference System Identity) of geometry to the given value. Defaults to 4326 (WGS84). See Open GIS Consortium, Inc., OpenGIS Simple Feature Specification For SQL, Document 99-049 (May 5, 1999), at Ch. 2.3.8 (Geometry Values and Spatial Reference Systems, pg. 39).
- Defaults to True. Creates a GiST index for the given geometry. Update the index with the PostgreSQL command VACUUM ANALYZE (may take a while to execute depending on how large your geographic-enabled tables are). Please note that this is different from the db_index field option since geographic indexes are created in a different manner than regular indexes.
The GeoMixin is necessary for all geographic-enabled models. The mixin provides extra instance methods (discussed in the database api docs) for geographic models without subclassing (aka ModelInheritance) -- which is not yet functional in Django.
In order to conduct geographic queries, each geographic model requires GeoManager. This manager allows for the proper SQL construction for geographic queries; thus, without it, all geographic filters will fail. It should also be noted that a GeoManager will be required even if the model does not have a geographic field itself, i.e.,in the case of a ForeignKey to a geographic model. For example, if we had an Address model with a ForeignKey to our Zip geographic model:
from django.contrib.gis.db import models class Address(models.Model): num = models.IntegerField() street = models.CharField(maxlength=100) city = models.CharField(maxlength=100) state = models.USStateField() zip = models.ForeignKey(Zip) objects = models.GeoManager()
The geographic manager is needed to do spatial queries on related Zip objects, for example:
>>> qs = Address.objects.filter(zip__poly__contains='POINT(-104.590948 38.319914)')