从Django PolygonField获取随机点 [英] Get random point from django PolygonField

问题描述

TL，DR ； 我想使用 ST_GeneratePoints (可能)从多边形中获取随机点. /em>

TL,DR; I want to get a random point from a polygon (potentially) using ST_GeneratePoints.

背景

我正在制作GeoDjango网络服务，并收集了具有各自边界的英国邮政编码，例如:

I'm making a GeoDjango web service and have a collection of UK Postcodes with respective boundaries like so:

from django.db import models as dj_models
from django.contrib.gis.db import models as gis_models

class Postcode(gis_models.Model):
      pretty_postcode = dj_models.CharField( max_length=8 )
      coords = gis_models.PolygonField( default='POLYGON EMPTY' )

我找到了一个令人愉悦的PostGIS小函数 ST_GeneratePoints ，该函数可以为我找到随机点在我的coords地区.

I've found a delightful little PostGIS function ST_GeneratePoints, which can find me random points in my coords region.

问题

如何在我的python django应用程序中使用此功能(或者您可以提出更好的方法?).理想的情况是以这样的函数结束:

How can I use this function from within my python django app (or can you suggest a better way?). Ideally ending up with a function like so:

from django.contrib.gis import geos
# ... other imports ...

class Postcode(gis_models.Model):
     # ... fields ...

     def get_random_point(self):
         rand_point = # code that executes ST_GeneratePoints
                      # and returns a geos.Point instance
         return rand_point

推荐答案

我在这里回答了类似的问题:

I have answered a similar question here: Equivalent of PostGIS ST_MakeValid in Django GEOS

由于您本质上是想调用数据库函数，因此您无法像您想象的那样完全做到这一点.
相反，您可以做的是将ST_GeneratePoints包裹为GeoFunc:

Since you essentially want to call a database function, you cannot quite do it as you imagine.
What you can do instead is to wrap the ST_GeneratePoints as a GeoFunc:

from django.contrib.gis.db.models.functions import GeoFunc

class GeneratePoints(GeoFunc):
    function='ST_GeneratePoints'

并在 aggregation/annotation 中使用它:

and use it in an aggregation/annotation:

from django.db.models import Value

Postcode.objects.annotate(
    rand_point=GeneratePoints(
        'coords',
        Value(1) # to get only one point
    )
)

做同一件事的另一种方法是:

Another way of doing the same thing is:

from django.contrib.gis.db.models.functions import GeoFunc
from django.db.models import F, Value

Postcode.objects.annotate(
    rand_point=GeoFunc(
        F('coords'),
        Value(1),
        function='ST_GeneratePoints',
    )
)

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