Dynamic Dropdown Fields in Django Admin

For an ecommerce site where you need admins to manage dress sizes dynamically, the best approach is to create a separate model for sizes and use a foreign key relationship. This is more flexible than hardcoded choices in a CharField.

Solution: Create a Size Model

Here's how to implement this:

1. First, create a model for sizes:

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# models.py
from django.db import models

class Size(models.Model):
    name = models.CharField(max_length=50)
    display_order = models.PositiveIntegerField(default=0)
    
    def __str__(self):
        return self.name
    
    class Meta:
        ordering = ['display_order', 'name']

class Product(models.Model):
    name = models.CharField(max_length=200)
    description = models.TextField()
    price = models.DecimalField(max_digits=10, decimal_places=2)
    # Other product fields
    
    def __str__(self):
        return self.name

class ProductVariant(models.Model):
    product = models.ForeignKey(Product, on_delete=models.CASCADE, related_name='variants')
    size = models.ForeignKey(Size, on_delete=models.PROTECT)
    stock_quantity = models.PositiveIntegerField(default=0)
    # Other variant-specific fields
    
    def __str__(self):
        return f"{self.product.name} - {self.size.name}"

2. Register these models in the admin:

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# admin.py
from django.contrib import admin
from .models import Size, Product, ProductVariant

@admin.register(Size)
class SizeAdmin(admin.ModelAdmin):
    list_display = ['name', 'display_order']
    ordering = ['display_order', 'name']

class ProductVariantInline(admin.TabularInline):
    model = ProductVariant
    extra = 1

@admin.register(Product)
class ProductAdmin(admin.ModelAdmin):
    inlines = [ProductVariantInline]

3. For the frontend form where customers select sizes, you can use a form that queries available sizes for a product:

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# forms.py
from django import forms
from .models import ProductVariant

class AddToCartForm(forms.Form):
    size = forms.ModelChoiceField(queryset=None, empty_label=None)
    quantity = forms.IntegerField(min_value=1, initial=1)
    
    def __init__(self, product, *args, **kwargs):
        super().__init__(*args, **kwargs)
        # Only show sizes that are available for this product
        self.fields['size'].queryset = Size.objects.filter(
            productvariant__product=product,
            productvariant__stock_quantity__gt=0
        ).distinct()

This approach gives you several advantages:

• Admins can add/edit/remove sizes through the admin interface
• You can track inventory per size
• You can control the display order of sizes
• You can reuse sizes across multiple products

If you need to customize the admin interface further, you can use methods like formfield_for_foreignkey to filter or modify how the size selection appears in the admin.