@router.post("/", response_model=UserBook)
async def add_book(book: Book, user: User = Depends(active_user)) -> UserBook:
    async def validate(con, user_id: int, book: Book) -> None:
        count = await con.fetchval(
            "SELECT count(*) FROM user_book WHERE user_id=$1", user_id
        )
        if count > 1000:
            raise HTTPException(
                400,
                "You already have too many books added, "
                "please contact our support",
            )
        if book.google_id and (
            await con.fetchrow(
                "SELECT 1 FROM user_book WHERE user_id=$1 AND google_id=$2",
                user_id,
                book.google_id,
            )
        ):
            raise HTTPException(400, "You have already added this book")

    async def insert_book(con, user_id: int, book: Book) -> UserBook:
        data = book.dict()
        data["user_id"] = user_id
        sql, params = insert_sql("user_book", data)
        data["id"] = await con.fetchval(sql, *params)
        return UserBook(**data)

    async def update_head(con, user_id: int, book_id: int) -> None:
        sql = """
            UPDATE user_book
            SET prev_id = $1
            WHERE id != $1
              AND user_id = $2
              AND prev_id is NULL
        """
        await con.execute(sql, book_id, user_id)

    user_id = user["id"]
    async with db.acquire() as con:
        await validate(con, user_id, book)
        async with con.transaction():
            user_book = await insert_book(con, user_id, book)
            await update_head(con, user_id, user_book.id)
    return user_book