Performing Intersection with Clipping in Django GIS

To perform an intersection with clipping in Django GIS (where you want to keep only the parts of polygons that are within the car polygon), you need to use the intersection function rather than the within filter.

The within filter, as you've discovered, only returns complete polygons that are entirely contained within the car polygon. The intersects filter returns any polygon that has any overlap with the car polygon, but doesn't clip them.

Here's how you can modify your code to perform the clipping operation:

PYTHONCopied
def get_car_with_fire(cod, inter):
    car = CAR.objects.filter(cod__iexact=cod).first()
    car_geom = car.geom

    if inter == "a":
        # Just check for intersection without clipping
        fire = Fire.objects.filter(geom__intersects=car_geom)
    elif inter == "b":
        # Get all fires that intersect with the car
        intersecting_fires = Fire.objects.filter(geom__intersects=car_geom)
        
        # Create a list to store the clipped geometries
        clipped_fires = []
        
        # For each intersecting fire, perform the intersection operation
        for fire in intersecting_fires:
            # This creates a new geometry that is the intersection of the fire and car
            clipped_geom = fire.geom.intersection(car_geom)
            
            # Create a new Fire object with the clipped geometry
            # (or you could modify the existing one if that's appropriate)
            fire.geom = clipped_geom
            clipped_fires.append(fire)
            
        fire = clipped_fires

This approach:

1. First finds all fire polygons that intersect with the car polygon
2. Then for each intersecting polygon, it performs a geometric intersection operation to get only the part that overlaps with the car
3. Updates the geometry of each fire object with the clipped version
4. Returns the list of modified fire objects

Note that this approach performs the clipping in Python rather than at the database level. If you have a large number of polygons, you might want to consider using a database-level approach for better performance.