Working with Multi-Polygons in Django Forms: A Guide to gis.forms.MultiPolygonField

Purpose

• Useful for representing areas with complex shapes or consisting of disconnected regions, like islands in a lake or the layout of a building with courtyards.
• Represents a field in a Django model that can store collections of multiple polygons.

Inheritance

• Inherits from django.contrib.gis.db.models.MultiPolygonField, which in turn inherits from django.contrib.gis.db.models.GeometryField.

Key Points

• Works seamlessly with GeoDjango's model API and form fields.
• Allows for efficient storage and retrieval of complex geometric shapes.
• Stores data in the database using a specific spatial database backend, typically PostGIS or SpatiaLite, which provide functions for manipulating geographic data.

Form Integration

• Leverages GeoDjango's form widgets, like OpenLayersWidget, to provide visual map interfaces for editing and creating multi-polygon geometries.
• Use gis.forms.MultiPolygonField in your Django forms to create input fields for user interaction with multi-polygon data.

Example

from django.contrib.gis.db import models
from django.contrib.gis.forms import MultiPolygonField

class MyModel(models.Model):
    multi_polygon_field = MultiPolygonField()  # Field to store the multi-polygon data

In a form

from django import forms
from .models import MyModel

class MyModelForm(forms.ModelForm):
    class Meta:
        model = MyModel
        fields = ['multi_polygon_field']
        widgets = {
            'multi_polygon_field': forms.MultiPolygonField(widget=OpenLayersWidget),
        }

1. Import necessary modules.
2. Define a model MyModel with a multi_polygon_field of type MultiPolygonField.
3. Create a form MyModelForm that inherits from forms.ModelForm.
4. Specify the model (MyModel) and fields (multi_polygon_field) in the Meta class.
5. Use MultiPolygonField with the OpenLayersWidget in the widgets dictionary to enable visual editing on a map.

• Consider using appropriate validation techniques (e.g., custom form validation methods) to ensure the integrity of the multi-polygon data entered by users.
• Ensure your database backend (e.g., PostGIS) is correctly configured for geospatial data storage.

---

from django.contrib.gis.db import models
from django.contrib.gis.geos import GEOSGeometry
from django.contrib.gis.validators import validate_multipolygon

class MyModel(models.Model):
    multi_polygon_field = MultiPolygonField(validators=[validate_multipolygon])  # Add validator

def validate_multipolygon_area(value):
    """Custom validation to ensure minimum area."""
    if value.area < 10000:  # Replace with your minimum area threshold
        raise ValidationError('Multi-polygon must have a minimum area of 10000 square meters.')

class MyModelForm(forms.ModelForm):
    class Meta:
        model = MyModel
        fields = ['multi_polygon_field']
        widgets = {
            'multi_polygon_field': forms.MultiPolygonField(widget=OpenLayersWidget),
        }

    def clean_multi_polygon_field(self):
        """Clean method to call custom validation."""
        multi_polygon = self.cleaned_data['multi_polygon_field']
        validate_multipolygon_area(multi_polygon)
        return multi_polygon

1. Model
   
   • The multi_polygon_field now includes the validate_multipolygon validator to ensure basic validity.
2. Custom Validation Function
   
   • validate_multipolygon_area checks if the multi-polygon area is greater than a minimum threshold (replace 10000 with your desired value).
   • Raises a ValidationError if the area is insufficient.
3. Form
   
   • The clean_multi_polygon_field method is added to the form.
   • It retrieves the cleaned multi-polygon data from self.cleaned_data.
   • Calls the validate_multipolygon_area function for custom validation.
   • Returns the validated multi-polygon data.

---

1. • Break down the multi-polygon into individual polygons and store them in separate PolygonField instances.
   • This approach is useful when dealing with a limited number of distinct polygons.
   • However, it can become unwieldy for large or complex multi-polygons.

2. GeoJSONField
   

   • Represent the multi-polygon data as a GeoJSON string, a standard format for encoding geographic data.
   • Store the GeoJSON string in a CharField or TextField.
   • Use GeoDjango's from_ewkt and to_ewkt functions to convert between GeoJSON and GeoDjango geometry objects.
   • This approach provides flexibility and interoperability with other GIS systems.
   • But it requires additional processing and may not be as user-friendly for form input.

3. Custom Form Field
   

   • Develop a custom form field that encapsulates the logic for handling and validating multi-polygon data.
   • This approach offers complete control over the field's behavior and presentation.
   • However, it requires more development effort and may not be as well-integrated with GeoDjango's form framework.

The choice between these alternatives depends on the specific requirements of your application:

• Control
  For complex scenarios or when you need precise control over the field's behavior, a custom form field provides the most flexibility.
• Flexibility
  If you need to exchange data with other GIS systems, GeoJSONField offers a versatile solution.
• Simplicity
  For simple cases with a few distinct polygons, multiple PolygonField instances may suffice.

Consider the following factors when making your decision:

• Development effort
  The time and resources available for custom development.
• Integration needs
  Whether you need to interact with other GIS systems or tools.
• Data complexity
  The number and complexity of the polygons involved.