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django-earthdistance

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Using PostgreSQL's EarthDistance extension with django

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django-earthdistance
====================

Using PostgreSQL\'s EarthDistance extension for django 1.11, 2.2 and 3.2
(for older versions see *with\_djorm\_expressions* branch)

Earthdistance allows to do fast geolocalized queries without using
PostGIS

Usage
-----

Cube and EarthDistance extensions must be enabled in postgreSQL BD, so
log in database using pgsql and install extensions:

``` {.sql}
=> create extension cube;
=> create extension earthdistance;
```

Filter by rows inside a circunference of radius r
-------------------------------------------------

``` {.python}
from django.db import models

from django_earthdistance.models import EarthDistanceQuerySet

class MyModel(models.Model):
    latitude = models.FloatField()
    longitude = models.FloatField()

objects = EarthDistanceQuerySet.as_manager()

# Define fields to query in DistanceExpression initialization
# search with lat=0.2546 and lon=-38.25 and distance 1500 meters
# use param `annotate` to set a custom field for the distance, `_ed_distance` as default

MyModel.objects.in_distance(1500, fields=['latitude', 'longitude'], points=[0.2546, -38.25])
```

Annotate each row returned by a query with distance between two points
----------------------------------------------------------------------

``` {.python}
from django_earthdistance.models import EarthDistance, LlToEarth

MyModel.objects.filter(....).annotate(
    distance=EarthDistance([
        LlToEarth([0.2546, -38.25]),
        LlToEarth(['latitude', 'longitude'])
    ]))
```

Optimizing perfomance with indexes
----------------------------------

PostgreSQL allow to use GiST indexes with functions results, a good
perfomance improvement is to store [ll\_to\_earth]{.title-ref} results
in an index, [ll\_to\_earth]{.title-ref} is a function that calculates
the position of a point on the surface of the earth (assuming earth is
perfectly spherical)

``` {.sql}
-- Example MyModel table is app_mymodel and points columns are latitude and longitude
CREATE INDEX mymodel_location ON app_mymodel USING gist (ll_to_earth(latitude, longitude));
```

### For django \< 1.7

Also, using south is preferred, just add this migration to migrations/
folder and edit it to your needs, index will be created

``` {.python}
class Migration(SchemaMigration):

def forwards(self, orm):
        cursor = connection.cursor()
        cursor.execute("CREATE INDEX mymodel_location ON app_mymodel USING gist (ll_to_earth(latitude, longitude));")

def backwards(self, orm):
        # Deleting field 'Venue.coords'
        cursor = connection.cursor()
        cursor.execute("DROP INDEX mymodel_location ON app_mymodel;")
```

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