Spatial Relationships in Django GIS

To create a relationship between your Plant and Location models based on spatial intersection without using explicit foreign keys, you can implement a custom solution using Django's GeoDjango capabilities.

Option 1: Create a Property Method with Annotation

You can add a property method to your Plant model that uses spatial lookups to find intersecting locations:

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from django.contrib.gis.db import models
from django.db.models import Prefetch

class Location(models.Model):
    area = models.PolygonField()
    name = models.CharField(max_length=100)  # Optional field for demonstration

    def __str__(self):
        return self.name

class Plant(models.Model):
    position = models.PointField()
    name = models.CharField(max_length=100)  # Optional field for demonstration

    def __str__(self):
        return self.name
    
    @property
    def locations(self):
        return Location.objects.filter(area__contains=self.position)

However, this approach still suffers from the N+1 query problem.

Option 2: Custom Manager with Prefetch Support

A better approach is to create a custom manager for your Plant model:

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from django.contrib.gis.db import models
from django.db.models import Prefetch, OuterRef, Subquery

class PlantManager(models.Manager):
    def with_locations(self):
        """Prefetch locations that contain each plant's position"""
        from django.contrib.gis.db.models.functions import Intersection
        
        # Create a subquery to find locations that contain each plant's position
        locations_subquery = Location.objects.filter(
            area__contains=OuterRef('position')
        )
        
        # Annotate plants with their locations
        return self.annotate(
            location_ids=Subquery(
                locations_subquery.values('id')
            )
        ).prefetch_related(
            Prefetch(
                'prefetched_locations',
                queryset=Location.objects.all(),
                to_attr='locations'
            )
        )

class Plant(models.Model):
    position = models.PointField()
    name = models.CharField(max_length=100)
    
    objects = PlantManager()
    
    @property
    def prefetched_locations(self):
        """This is used by the prefetch_related in the manager"""
        if hasattr(self, 'locations'):
            return self.locations
        return Location.objects.filter(area__contains=self.position)

Option 3: Create a Through Model

Another approach is to create an explicit through model that maintains the spatial relationship:

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from django.contrib.gis.db import models
from django.db.models.signals import post_save
from django.dispatch import receiver

class Location(models.Model):
    area = models.PolygonField()
    name = models.CharField(max_length=100)

class Plant(models.Model):
    position = models.PointField()
    name = models.CharField(max_length=100)
    locations = models.ManyToManyField(Location, through='PlantLocation')

class PlantLocation(models.Model):
    plant = models.ForeignKey(Plant, on_delete=models.CASCADE)
    location = models.ForeignKey(Location, on_delete=models.CASCADE)
    
    class Meta:
        unique_together = ('plant', 'location')

@receiver(post_save, sender=Plant)
def update_plant_locations(sender, instance, **kwargs):
    """Update the plant's locations whenever it's saved"""
    # Clear existing relationships
    PlantLocation.objects.filter(plant=instance).delete()
    
    # Create new relationships for locations that contain this plant
    locations = Location.objects.filter(area__contains=instance.position)
    for location in locations:
        PlantLocation.objects.create(plant=instance, location=location)

With this approach, you can use standard prefetch_related:

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plants = Plant.objects.prefetch_related('locations')

Recommendation

Option 3 is the most Django-friendly approach as it uses standard Django relationships, but it requires maintaining the relationships when plants or locations change. You'll need to add similar signals for Location changes.

If you prefer a more dynamic approach without maintaining relationships in the database, Option 2 provides a good balance of performance and flexibility.