Model Serializer

ModelSerializer is a powerful abstract mixin for Django models that centralizes schema generation and serialization configuration directly on the model class.

Overview

Goals:

• Eliminate duplication between Model and separate serializer classes
• Provide clear extension points (sync + async hooks, custom synthetic fields)
• Auto-generate Ninja schemas from model metadata
• Support nested serialization for relationships

Key Features:

• Declarative schema configuration via inner classes
• Automatic CRUD schema generation
• Nested relationship handling
• Sync and async lifecycle hooks
• Custom field support (computed/synthetic fields)

Quick Start

Python

from django.db import models
from ninja_aio.models import ModelSerializer

class User(ModelSerializer):
    username = models.CharField(max_length=150, unique=True)
    email = models.EmailField(unique=True)

    class CreateSerializer:
        fields = ["username", "email"]

    class ReadSerializer:
        fields = ["id", "username", "email"]

    def __str__(self):
        return self.username

Inner Configuration Classes

CreateSerializer

Describes how to build a create (input) schema for a model.

Attributes:

Attribute	Type	Description
fields	list[str]	REQUIRED model field names for creation
optionals	list[tuple[str, type]]	Optional model fields: (field_name, python_type)
customs	list[tuple]	Synthetic inputs. Tuple forms: (name, type) = required (no default); (name, type, default) = optional (literal or callable)
excludes	list[str]	Field names rejected on create

Example:

Python

class User(ModelSerializer):
    username = models.CharField(max_length=150)
    email = models.EmailField()
    password = models.CharField(max_length=128)
    bio = models.TextField(blank=True)

    class CreateSerializer:
        fields = ["username", "email", "password"]
        optionals = [
            ("bio", str),
        ]
        customs = [
            ("password_confirm", str),          # required (no default) equivalent of ("password_confirm", str, ...)
            ("send_welcome_email", bool, True), # optional with default
        ]
        excludes = ["id", "created_at"]

Resolution Order for customs:

1. Payload value (if provided)
2. If default present and callable → invoked
3. Literal default (if provided)
4. If tuple was only (name, type) and no value supplied → validation error (required)

Conceptual Equivalent (django-ninja):

Python

# Without ModelSerializer
from ninja import ModelSchema

class UserIn(ModelSchema):
    class Meta:
        model = User
        fields = ["username", "email"]

ReadSerializer

Describes how to build a read (output) schema for a model.

Attributes

Attribute	Type	Description
fields	list[str]	REQUIRED. Model fields / related names explicitly included in the read (output) schema.
excludes	list[str]	Fields / related names to always omit (takes precedence over fields and optionals). Use for sensitive or noisy data (e.g., passwords, internal flags).
customs	list[tuple]	Computed / synthetic output values. Tuple formats: • (name, type) = required resolvable attribute (object attribute or property). Serialization error if not resolvable. • (name, type, default) = optional; default may be a callable (lambda obj: ...) or a literal value.

Example:

Python

class User(ModelSerializer):
    first_name = models.CharField(max_length=150)
    last_name = models.CharField(max_length=150)
    email = models.EmailField()
    password = models.CharField(max_length=128)

    class ReadSerializer:
        fields = ["id", "first_name", "last_name", "email", "created_at"]
        excludes = ["password"]
        customs = [
            ("full_name", str, lambda obj: f"{obj.first_name} {obj.last_name}".strip()),
            ("is_premium", bool, lambda obj: obj.subscription.is_active if hasattr(obj, 'subscription') else False),
        ]

Resolution Order for customs:

1. Attribute / property on instance
2. Callable default (if provided)
3. Literal default
4. If required (2‑tuple) and still unresolved → error

Generated Output:

JSON

{
  "id": 1,
  "first_name": "John",
  "last_name": "Doe",
  "email": "john@example.com",
  "created_at": "2024-01-15T10:30:00Z",
  "full_name": "John Doe",
  "is_premium": true
}

UpdateSerializer

Describes how to build an update (partial/full) input schema.

Attributes:

Attribute	Type	Description
fields	list[str]	REQUIRED fields for update (rarely used)
optionals	list[tuple[str, type]]	Updatable optional fields (typical for PATCH)
customs	list[tuple]	Instruction fields: (name, type) required; (name, type, default) optional
excludes	list[str]	Immutable fields that cannot be updated

Example:

Python

class User(ModelSerializer):
    username = models.CharField(max_length=150, unique=True)
    email = models.EmailField()
    bio = models.TextField(blank=True)
    is_active = models.BooleanField(default=True)

    class UpdateSerializer:
        optionals = [
            ("email", str),
            ("bio", str),
            ("is_active", bool),
        ]
        customs = [
            ("reset_password", bool, False),  # optional flag
            ("rotate_token", bool),          # required instruction
        ]
        excludes = ["username", "created_at", "id"]

Usage (PATCH request):

JSON

{
  "email": "newemail@example.com",
  "bio": "Updated bio",
  "reset_password": true
}

Schema Generation

Auto-Generated Schemas

ModelSerializer automatically generates four schema types:

Method	Schema Type	Purpose
generate_create_s()	Input ("In")	POST endpoint payload
generate_update_s()	Input ("Patch")	PATCH/PUT endpoint payload
generate_read_s(depth=1)	Output ("Out")	Response with nested relations
generate_related_s()	Output ("Related")	Compact nested representation

Example:

Python

class User(ModelSerializer):
    username = models.CharField(max_length=150)
    email = models.EmailField()

    class CreateSerializer:
        fields = ["username", "email"]

    class ReadSerializer:
        fields = ["id", "username", "email"]

# Auto-generate schemas
UserCreateSchema = User.generate_create_s()
UserReadSchema = User.generate_read_s()
UserUpdateSchema = User.generate_update_s()
UserRelatedSchema = User.generate_related_s()

Nested Relationship Handling

ModelSerializer automatically serializes relationships if the related model is also a ModelSerializer.

ForeignKey (Forward)

Python

class Profile(ModelSerializer):
    bio = models.TextField()

    class ReadSerializer:
        fields = ["id", "bio"]

class User(ModelSerializer):
    username = models.CharField(max_length=150)
    profile = models.ForeignKey(Profile, on_delete=models.CASCADE)

    class ReadSerializer:
        fields = ["id", "username", "profile"]

Output:

JSON

{
  "id": 1,
  "username": "john_doe",
  "profile": {
    "id": 10,
    "bio": "Software developer"
  }
}

ManyToMany

Python

class Tag(ModelSerializer):
    name = models.CharField(max_length=50)

    class ReadSerializer:
        fields = ["id", "name"]

class Article(ModelSerializer):
    title = models.CharField(max_length=200)
    tags = models.ManyToManyField(Tag, related_name="articles")

    class ReadSerializer:
        fields = ["id", "title", "tags"]

Output:

JSON

{
  "id": 1,
  "title": "Getting Started",
  "tags": [
    { "id": 1, "name": "python" },
    { "id": 2, "name": "django" }
  ]
}

Reverse Relationships

Python

class Author(ModelSerializer):
    name = models.CharField(max_length=200)

    class ReadSerializer:
        fields = ["id", "name", "books"]  # Reverse FK

class Book(ModelSerializer):
    title = models.CharField(max_length=200)
    author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name="books")

    class ReadSerializer:
        fields = ["id", "title"]

Output:

JSON

{
  "id": 1,
  "name": "J.K. Rowling",
  "books": [
    { "id": 1, "title": "Harry Potter" },
    { "id": 2, "title": "Fantastic Beasts" }
  ]
}

Async Extension Points

queryset_request(request)

Filter queryset based on request context (user, permissions, tenant, etc.).

Python

class Article(ModelSerializer):
    title = models.CharField(max_length=200)
    author = models.ForeignKey(User, on_delete=models.CASCADE)
    is_published = models.BooleanField(default=False)

    @classmethod
    async def queryset_request(cls, request):
        qs = cls.objects.select_related('author').all()

        # Non-authenticated users see only published
        if not request.auth:
            return qs.filter(is_published=True)

        # Authors see their own + published
        return qs.filter(
            models.Q(author=request.auth) | models.Q(is_published=True)
        )

post_create()

Execute async logic after object creation.

Python

class User(ModelSerializer):
    email = models.EmailField()

    async def post_create(self):
        # Send welcome email
        from myapp.tasks import send_welcome_email
        await send_welcome_email(self.email)

        # Create related objects
        await Profile.objects.acreate(user=self)

        # Log creation
        await AuditLog.objects.acreate(
            action="user_created",
            user_id=self.id
        )

custom_actions(payload)

React to synthetic/custom fields from the payload.

Python

class User(ModelSerializer):
    password = models.CharField(max_length=128)

    class CreateSerializer:
        fields = ["username", "email", "password"]
        customs = [
            ("password_confirm", str, None),
            ("send_welcome_email", bool, True),
        ]

    async def custom_actions(self, payload: dict):
        # Validate password confirmation
        if "password_confirm" in payload:
            if payload["password_confirm"] != self.password:
                raise ValueError("Passwords do not match")

        # Send welcome email if requested
        if payload.get("send_welcome_email", True):
            await send_email(self.email, "Welcome!")

Sync Lifecycle Hooks

Save Hooks

Python

class User(ModelSerializer):
    username = models.CharField(max_length=150)
    slug = models.SlugField(unique=True, blank=True)

    def before_save(self):
        """Executed before every save (create + update)"""
        if not self.slug:
            self.slug = slugify(self.username)

    def on_create_before_save(self):
        """Executed only on creation, before save"""
        self.set_password(self.password)  # Hash password

    def after_save(self):
        """Executed after every save"""
        cache.delete(f"user:{self.id}")

    def on_create_after_save(self):
        """Executed only after creation"""
        send_welcome_email_sync(self.email)

Execution Order:

Text Only

CREATE:
1. on_create_before_save()
2. before_save()
3. super().save()
4. on_create_after_save()
5. after_save()

UPDATE:
1. before_save()
2. super().save()
3. after_save()

Delete Hook

Python

class User(ModelSerializer):

    def on_delete(self):
        """Executed after object deletion"""
        # Clean up related data
        logger.info(f"User {self.username} deleted")

        # Remove from cache
        cache.delete(f"user:{self.id}")

        # Archive data
        ArchivedUser.objects.create(
            username=self.username,
            deleted_at=timezone.now()
        )

Utility Methods

has_changed(field)

Check if a field value has changed compared to database.

Python

class Article(ModelSerializer):
    title = models.CharField(max_length=200)
    status = models.CharField(max_length=20)

    def before_save(self):
        if self.has_changed('status'):
            if self.status == 'published':
                self.published_at = timezone.now()
                notify_subscribers(self)

verbose_name_path_resolver()

Get slugified plural verbose name for URL routing.

Python

class BlogPost(ModelSerializer):
    class Meta:
        verbose_name_plural = "blog posts"

# Returns: "blog-posts"
path = BlogPost.verbose_name_path_resolver()

ModelUtil

Helper class for async CRUD operations with ModelSerializer.

Overview

Python

from ninja_aio.models import ModelUtil

util = ModelUtil(User)

Key Responsibilities:

• Introspect model metadata
• Normalize inbound/outbound payloads
• Handle FK resolution and base64 decoding
• Prefetch reverse relations
• Invoke serializer hooks

Key Methods

get_object(request, pk=None, filters=None, getters=None)

Fetch single object or queryset with optimized queries.

Python

# Get single object
user = await util.get_object(request, pk=1)

# Get with filters
active_users = await util.get_object(
    request,
    filters={"is_active": True}
)

# Get with custom lookup
user = await util.get_object(
    request,
    getters={"email": "john@example.com"}
)

Features:

• Automatic select_related() and prefetch_related()
• Respects queryset_request() filtering
• Raises SerializeError (404) if not found

parse_input_data(request, data)

Convert incoming schema to model-ready dict.

Transformations:

1. Strips custom fields (stored separately)
2. Removes optional fields with None value
3. Decodes BinaryField (base64 → bytes)
4. Resolves FK IDs to model instances

Python

from ninja import Schema

class UserCreateSchema(Schema):
    username: str
    email: str
    profile_id: int
    avatar: str  # base64 for BinaryField
    send_welcome: bool  # custom field

data = UserCreateSchema(
    username="john",
    email="john@example.com",
    profile_id=5,
    avatar="iVBORw0KG...",  # base64
    send_welcome=True
)

payload, customs = await util.parse_input_data(request, data)

# payload = {
#     "username": "john",
#     "email": "john@example.com",
#     "profile": <Profile instance>,
#     "avatar": b'\x89PNG\r\n...'
# }
# customs = {"send_welcome": True}

parse_output_data(request, data)

Post-process serialized output for consistency.

Transformations:

1. Replaces nested FK dicts with actual instances
2. Rewrites nested FK keys to <field>_id format

Python

# Before
{
    "id": 1,
    "author": {"id": 10, "profile": {"id": 5}}
}

# After parse_output_data
{
    "id": 1,
    "author": <Author instance>,
    "author_id": 10,
    "profile_id": 5
}

CRUD Operations

create_s(request, data, obj_schema)

Python

user_data = UserCreateSchema(username="john", email="john@example.com")
result = await util.create_s(request, user_data, UserReadSchema)

# Executes:
# 1. parse_input_data (normalize)
# 2. Model.objects.acreate()
# 3. custom_actions(customs)
# 4. post_create()
# 5. read_s (serialize response)

read_s(request, obj, obj_schema)

Python

user = await User.objects.aget(id=1)
result = await util.read_s(request, user, UserReadSchema)
# Returns parsed dict ready for API response

update_s(request, data, pk, obj_schema)

Python

update_data = UserUpdateSchema(email="newemail@example.com")
result = await util.update_s(request, update_data, 1, UserReadSchema)

# Executes:
# 1. get_object(pk)
# 2. parse_input_data
# 3. Update changed fields
# 4. custom_actions(customs)
# 5. obj.asave()
# 6. read_s (serialize updated object)

delete_s(request, pk)

Python

await util.delete_s(request, 1)
# Returns None

Error Handling

Python

from ninja_aio.exceptions import SerializeError

try:
    user = await util.get_object(request, pk=999)
except SerializeError as e:
    # e.details = {"user": "not found"}
    # e.status_code = 404
    pass

try:
    await util.create_s(request, bad_data, UserReadSchema)
except SerializeError as e:
    # e.details = {"avatar": "Invalid base64"}
    # e.status_code = 400
    pass

Complete Example

Python

from django.db import models
from ninja_aio.models import ModelSerializer
from django.utils.text import slugify

class Category(ModelSerializer):
    name = models.CharField(max_length=100)
    slug = models.SlugField(unique=True, blank=True)

    class CreateSerializer:
        fields = ["name"]

    class ReadSerializer:
        fields = ["id", "name", "slug"]

    class UpdateSerializer:
        optionals = [("name", str)]

    def before_save(self):
        if not self.slug:
            self.slug = slugify(self.name)

class Author(ModelSerializer):
    name = models.CharField(max_length=200)
    email = models.EmailField(unique=True)
    bio = models.TextField(blank=True)

    class CreateSerializer:
        fields = ["name", "email"]
        optionals = [("bio", str)]

    class ReadSerializer:
        fields = ["id", "name", "email", "bio"]
        customs = [
            ("post_count", int, lambda obj: obj.articles.count()),
        ]

    class UpdateSerializer:
        optionals = [
            ("name", str),
            ("email", str),
            ("bio", str),
        ]

class Article(ModelSerializer):
    title = models.CharField(max_length=200)
    slug = models.SlugField(unique=True)
    content = models.TextField()
    author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name="articles")
    category = models.ForeignKey(Category, on_delete=models.SET_NULL, null=True)
    tags = models.ManyToManyField('Tag', related_name="articles")
    is_published = models.BooleanField(default=False)
    views = models.IntegerField(default=0)
    created_at = models.DateTimeField(auto_now_add=True)

    class CreateSerializer:
        fields = ["title", "slug", "content", "author", "category"]
        customs = [
            ("notify_subscribers", bool, True),
        ]

    class ReadSerializer:
        fields = [
            "id", "title", "slug", "content",
            "author", "category", "tags",
            "is_published", "views", "created_at"
        ]

    class UpdateSerializer:
        optionals = [
            ("title", str),
            ("content", str),
            ("category", int),
            ("is_published", bool),
        ]
        excludes = ["slug", "author", "created_at"]

    @classmethod
    async def queryset_request(cls, request):
        qs = cls.objects.select_related('author', 'category').prefetch_related('tags')

        if not request.auth:
            return qs.filter(is_published=True)

        return qs.filter(
            models.Q(author=request.auth) | models.Q(is_published=True)
        )

    async def post_create(self):
        await AuditLog.objects.acreate(
            action="article_created",
            article_id=self.id,
            author_id=self.author_id
        )

    async def custom_actions(self, payload: dict):
        if payload.get("notify_subscribers"):
            await notify_new_article(self)

    def before_save(self):
        if self.has_changed('is_published') and self.is_published:
            self.published_at = timezone.now()

class Tag(ModelSerializer):
    name = models.CharField(max_length=50, unique=True)

    class ReadSerializer:
        fields = ["id", "name"]

Custom Fields Normalization

Custom tuples are normalized by ModelSerializer.get_custom_fields() to (name, python_type, default).

Accepted forms:

• (name, type) -> stored with default = Ellipsis (treated as required)
• (name, type, default) -> default kept (callable or literal)

Invalid lengths raise ValueError.

Example mix:

Python

class CreateSerializer:
    customs = [
        ("password_confirm", str),           # required
        ("send_welcome", bool, True),        # optional
        ("initial_quota", int, lambda: 100)  # optional callable
    ]

At runtime:

Python

# Normalized
[
  ("password_confirm", str, Ellipsis),
  ("send_welcome", bool, True),
  ("initial_quota", int, <function ...>)
]

Required customs (Ellipsis) must be provided in input (create/update) or resolvable (read) or an error is raised.

Error Cases

Situation	Result
1‑item or 4‑item tuple	ValueError
Missing required custom (2‑tuple) in payload	Validation error
Unresolvable required read custom	Serialization error

Best Practices

1. Always exclude sensitive fields:

Python

class ReadSerializer:
    excludes = ["password", "secret_key", "internal_id"]

1. Use optionals for PATCH operations:

Python

class UpdateSerializer:
    optionals = [("email", str), ("bio", str)]  # Partial updates

1. Leverage customs for computed data:

Python

customs = [
    ("full_name", str, lambda obj: f"{obj.first_name} {obj.last_name}"),
]

1. Optimize queries in queryset_request:

Python

@classmethod
async def queryset_request(cls, request):
    return cls.objects.select_related('author').prefetch_related('tags')

1. Keep hooks focused:
   Python

   async def post_create(self):
       # Do ONE thing well
       await send_welcome_email(self.email)
   

See Also

• Model Util - Deep dive into ModelUtil
• API ViewSet - Using ModelSerializer with ViewSets
• Authentication - Securing endpoints