schemas.py

#  Request - Response validation

from typing import Optional
from pydantic import BaseModel, EmailStr, Field, model_validator

# it is to validate incoming JSON request body
class UserCreate(BaseModel):
    name: str = Field(min_length = 3, max_length = 30)
    email: EmailStr  # EmailStr is already highly specialized pre-validated type
    
    age: int = Field(gt = 0, lt = 100)  # gt -> greater than , lt -> less than

class UserResponse(UserCreate):
    id: int

    class Config:    
        from_attributes = True
        # After API route finishes running, it returns an ORM object, if return is specified. 

        # from_attributes = True, tells pydantic not to crash and process it if it gets ORM object 
        # to validate instead of python dictionary

# for PUT - full update, all fields are required (same as UserCreate)
class UserUpdate(UserCreate):
    pass

# for PATCH - partial update, all fields are optional
class UserPatch(BaseModel):
    name: Optional[str] = Field(default = None, min_length = 3, max_length = 30)
    email: Optional[EmailStr] = None
    age: Optional[int] = Field(default = None, gt = 0, lt = 100)

    @model_validator(mode = 'before')
    @classmethod
    def check_at_least_one_field(cls, values):
        # Reject completely empty payloads — at least one field must be provided
        if not any(values.values()):
            raise ValueError("At least one field must be provided for update")
        return values