12 Validation Deep

title: Deep Validation in FastAPI REST APIs weight: 22 date: 2026-06-28 lastmod: 2026-06-28 description: Master advanced FastAPI validation with custom validators, nested model validation, conditional validation, cross-field validation, and custom error messages. tags: [api-development, fastapi]

Deep validation in FastAPI uses Pydantic's field_validator and model_validator for complex rules including cross-field validation, conditional validation, custom error messages, and nested model validation.

```python
from pydantic import BaseModel, Field, field_validator, model_validator
from typing import Optional
from datetime import date, datetime

class UserRegistration(BaseModel):
    username: str = Field(..., min_length=3, max_length=20, pattern="^[a-z0-9_]+$")
    email: str
    password: str = Field(..., min_length=8)
    confirm_password: str
    birth_date: Optional[date] = None
    accept_terms: bool = False

    @field_validator("email")
    @classmethod
    def validate_email(cls, v):
        if "@" not in v or "." not in v.split("@")[-1]:
            raise ValueError("Invalid email format")
        return v.lower()

    @field_validator("username")
    @classmethod
    def validate_username(cls, v):
        if v in ["admin", "root", "system"]:
            raise ValueError("This username is reserved")
        return v

    @model_validator(mode="after")
    def check_passwords_match(self):
        if self.password != self.confirm_password:
            raise ValueError("Passwords do not match")
        return self

    @model_validator(mode="after")
    def check_age_requirement(self):
        if self.birth_date:
            age = (date.today() - self.birth_date).days // 365
            if age < 13:
                raise ValueError("You must be at least 13 years old")
        return self

    @model_validator(mode="after")
    def check_terms(self):
        if not self.accept_terms:
            raise ValueError("You must accept the terms of service")
        return self

What's Next

Now learn about SQLAlchemy integration in Building REST APIs with FastAPI.