mc-manager/src/lib/mongodb.ts

import mongoose from 'mongoose'

interface MongooseCache {
  conn: typeof mongoose | null
  promise: Promise<typeof mongoose> | null
}

declare global {
  var mongoose: MongooseCache | undefined
}

const cached: MongooseCache = global.mongoose || { conn: null, promise: null }

if (!global.mongoose) {
  global.mongoose = cached
}

export default async function connectToDatabase(): Promise<typeof mongoose> {
  if (cached.conn) {
    return cached.conn
  }

  if (!cached.promise) {
    const MONGODB_URI = process.env.MONGODB_URI

    if (!MONGODB_URI) {
      throw new Error('Please define the MONGODB_URI environment variable')
    }

    cached.promise = mongoose.connect(MONGODB_URI, {
      bufferCommands: false,
    })
  }

  try {
    cached.conn = await cached.promise
  } catch (e) {
    cached.promise = null
    throw e
  }

  return cached.conn
}