from pydantic import BaseModel, Field
from typing import Optional

# Base Schema
class BookBase(BaseModel):
    title: str = Field(..., description="Book title")
    description: Optional[str] = Field(None, description="Book description")
    author: str = Field(..., description="Book author")
    pages: int = Field(..., gt=0, description="Number of pages")

    class Config:
        schema_extra = {
            "example": {
                "title": "The Great Gatsby",
                "description": "A novel about the decadence of the Jazz Age.",
                "author": "F. Scott Fitzgerald",
                "pages": 180
            }
        }

# Schema for creating a new Book
class BookCreate(BookBase):
    user_id: str = Field(..., description="ID of the user creating the book")

    class Config:
        schema_extra = {
            "example": {
                "title": "The Great Gatsby",
                "description": "A novel about the decadence of the Jazz Age.",
                "author": "F. Scott Fitzgerald",
                "pages": 180,
                "user_id": "user123"
            }
        }

# Schema for Book responses
class Book(BookBase):
    id: int
    user_id: str

    class Config:
        orm_mode = True
```

This code defines three Pydantic models for the Book entity:

1. `BookBase`: A base model that defines the common fields for a book (title, description, author, and pages).
2. `BookCreate`: A model that inherits from `BookBase` and adds the `user_id` field, which is required for creating a new book.
3. `Book`: A model that inherits from `BookBase` and adds the `id` and `user_id` fields, which are typically required for API responses.

The `Field` class is used to define validation rules and descriptions for each field. For example, the `title` and `author` fields are required, the `pages` field must be a positive integer, and the `description` field is optional.

The `Config` class is used to provide additional configuration options for each model, such as example values and ORM mode for API responses.

Note that this code assumes that the `user_id` field is a string representing the ID of the user creating the book. If the `user_id` field is a foreign key to the `users` table, you may need to modify the field type accordingly.