[Django]-How to show a many-to-many field with "list_display" in Django Admin?

This way you can do it, kindly checkout the following snippet:

class Categories(models.Model):
    """ Base category model class """

    title       = models.CharField(max_length=100)
    description = models.TextField()
    parent      = models.ManyToManyField('self', default=None, blank=True)
    when        = models.DateTimeField('date created', auto_now_add=True)

    def get_parents(self):
        return ",".join([str(p) for p in self.parent.all()])

    def __unicode__(self):
        return "{0}".format(self.title)

And in your admin.py module call method as follows:

class categories(admin.ModelAdmin):
    list_display    = ('title', 'get_parents', 'when')

If you want to save extra queries, you can use prefetch_related in the get_queryset method like below:

class PurchaseOrderAdmin(admin.ModelAdmin):
    fields = ['product', 'dollar_amount']
    list_display = ('get_products', 'vendor')

    def get_queryset(self, request):
        qs = super().get_queryset(request)
        return qs.prefetch_related('product')

    def get_products(self, obj):
        return ",".join([p.products for p in obj.product.all()])

According to the Docs, In this way, there would be just one extra query needed to fetch related Product items of all PurchaseOrder instances instead of needing one query per each PurchaseOrder instance.

For example, there are Category and Product models which have many-to-many relationship as shown below. *I use Django 4.2.1:

# "models.py"

from django.db import models

class Category(models.Model):
    name = models.CharField(max_length=20)

    def __str__(self):
        return self.name

class Product(models.Model):
    categories = models.ManyToManyField(Category)
    name = models.CharField(max_length=50)
    price = models.DecimalField(decimal_places=2, max_digits=5)

Then, there are Category and Product admins as shown below:

# "admin.py"

from django.contrib import admin
from .models import Category, Product

@admin.register(Category)
class CategoryAdmin(admin.ModelAdmin):
    list_display = ('id', 'name')
    ordering = ('id',)

@admin.register(Product)
class ProductAdmin(admin.ModelAdmin):
    list_display = ('id', 'name', 'price')
    ordering = ('id',)

Then, Category admin has 5 objects as shown below:

And, Product admin has 6 objects as shown below:

Now, define get_products and get_categories() with @admin.display(), then set them to list_display in Category and Product admins respectively as shown below. *description argument in @admin.display() can rename the columns in Django Admin from GET PRODUCTS and GET CATEGORIES to PRODUCTS and CATEGORIES respectively:

# "admin.py"

@admin.register(Category)
class CategoryAdmin(admin.ModelAdmin):
    list_display = ('id', 'name', 'get_products')
    ordering = ('id',)            # Here

    # Here
    @admin.display(description='products')
    def get_products(self, obj):
        return [product.name for product in obj.product_set.all()]

@admin.register(Product)
class ProductAdmin(admin.ModelAdmin):      # Here  
    list_display = ('id', 'name', 'price', 'get_categories')
    ordering = ('id',)
    
    # Here
    @admin.display(description='categories')
    def get_categories(self, obj):
        return [category.name for category in obj.categories.all()]

Then, PRODUCTS column is displayed in Category admin as shown below:

Then, CATEGORIES column is displayed in Product admin as shown below:

Also, you can display PRODUCTS and CATEGORIES columns by defining get_products() and get_categories() with @admin.display() in Category and Product models respectively as shown below:

# "models.py"

from django.db import models
from django.contrib import admin

class Category(models.Model):
    name = models.CharField(max_length=20)

    def __str__(self):
        return self.name

    # Here
    @admin.display(description='products')
    def get_products(self):
        return [product.name for product in self.product_set.all()]

class Product(models.Model):
    categories = models.ManyToManyField(Category)
    name = models.CharField(max_length=50)
    price = models.DecimalField(decimal_places=2, max_digits=5)

    # Here
    @admin.display(description='categories')
    def get_categories(self):
        return [category.name for category in self.categories.all()]

Then, set them to list_display in Category and Product admins respectively as shown below:

# "admin.py"

...

@admin.register(Category)
class CategoryAdmin(admin.ModelAdmin):
    list_display = ('id', 'name', 'get_products')
    ordering = ('id',)            # Here

@admin.register(Product)
class ProductAdmin(admin.ModelAdmin):      # Here
    list_display = ('id', 'name', 'price', 'get_categories')
    ordering = ('id',)