Django Model conditional relationship - django

Question

Newbie question: Django + Postgres + PostGIS
I am trying to setup a Projects model in Django that needs to be conditionally related to a geometry model depending on its geometry type. the geometry types are : Points, Lines or Polygons. The question is how do I define this relationship in the Project Model so that I do not have to save the different geometry types in the same table ( Hence the 3 different geometry models)
=======================Here are my models ============================
PRJ_GEOM = (
(1, "point"),
(2, "line"),
(3, "polygon")
)
class Project(models.Model):
name = models.CharField(max_length=20)
project_geom_type = models.IntegerField(choices=PRJ_GEOM)
project_geometry = models.OneToOneField( ????) # I am stuck here - how do I express this conditional relationship which depends on project_geom_type
# Geometry Model
class Project_Point_Geom(models.Model):
project = models.OneToOne(Project, on_delete=models.CASCADE, related_name='project_point')
point = models.PointField()
class Project_Line_Geom(models.Model):
project = models.OneToOne(Project, on_delete=models.CASCADE, related_name='project_line')
line = models.LineStringField()
class Project_Polygon_Geom(models.Model):
project = models.OneToOne(Project, on_delete=models.CASCADE, related_name='project_polygon')
polygon = models.PolygonField()

Answer 1

Actually you only need one model. All geometry fields are derived from GeometryField so you can use it to store it's sub classes.
PRJ_GEOM = (
(1, "point"),
(2, "line"),
(3, "polygon")
)
class Project(models.Model):
name = models.CharField(max_length=20)
project_geom_type = models.IntegerField(choices=PRJ_GEOM)
project_geometry = models.GeometryField()
That is all! one model, one table instead of four which is much easier to maintain. And the data is slightly smaller because the relation field is eliminated.

Answer 2

Your Project model doesn't need the project_geometry fields. You only need to relate your different geometry models to the project using your OneToOne field. Django automatically creates reverse relationships for ForeignKey, ManyToMany and OneToOne relationships. Your classes would look something like this:
class Project(models.Model):
name = models.CharField(max_length=20)
class Point_Geo(models.Model):
project = models.OneToOne(Project) # truncated for example
point = models.PointField()
class Line_Geo(models.Model):
project = models.OneToOne(Project) # truncated for example
line = models.LineStringField()
class Polygon_Geo(models.Model):
project = models.OneToOne(Project) # truncated for example
polygon = models.PolygonField()
When you've got a Project instance you can walk back along the reverse relationship like this:
project = Project.objects.get(id=1)
point = project.point_geo.point
To query for all projects with point geometry you could query:
projects_with_points = Project.objects.exclude(point_geo=None)