With TanStack Start

import { createServerFn } from "@tanstack/react-start";
import { M, model, mongster } from "mongster";

const todoSchema = M.schema({
  title: M.string().min(1),
  completed: M.boolean().default(false),
}).withTimestamps();

const Todo = model("todos", todoSchema);

let connected = false;

async function ensureDb() {
  if (connected) return;
  await mongster.connect("mongodb://127.0.0.1:27017/start-app");
  connected = true;
}

export const createTodo = createServerFn({ method: "POST" })
  .inputValidator((input: { title: string }) => input)
  .handler(async ({ data }) => {
    await ensureDb();

    const todo = await Todo.createOne({
      title: data.title,
    });

    return todo
      ? {
          id: todo._id.toHexString(),
          title: todo.title,
          completed: todo.completed,
        }
      : null;
  });