Why I am getting Query was already executed error?

July 14, 2022

I have the following Schema and Model:

const userSchema = new mongoose.Schema({
  name: String,
})

const UserModel = mongoose.model('User', userSchema, 'users')

and I have written the following express middleware, which simply takes one argument, awaits that argument, set the returned value from that awaiting job to the req object on a property called gottenDocs, (i.e.: req.gottenDocs)

function getDocumentsMw(query) {
  return async (req, res, next) => {
    const dbRes = await query
    req.gottenDocs = dbRes
    
    next()
  }
}

and I have the following route:

app.get(
  '/users',
  getDocumentsMw(UserModel.find({})), 
  (req, res, next) => {
    const gottenDoc = req.gottenDocs
    res.status(200).json({
      status: 'success',
      data: gottenDoc,
    })
})

That’s all I have, now, when I request [ GET " /users " ] I recieve the following response which is great, and nothing is wrong:

{
  "status": "success",
  "data": []
}

but, the weird thing is when I request this route again, it throws this error:

MongooseError: Query was already executed: User.find({})

What could be the problem? is it a bug in Nodejs? which could be hmmm, something like, that it is not removing the function call from the call stack after the response has been sent?

any help appreciated.

>Solution :

The problem is on this line

getDocumentsMw(UserModel.find({})),

Here you create a query once the application start, because of that the query is created once but executed multiple times.

You may need to refactor your code to something like that

getDocumentsMw(() => UserModel.find({})),

Now you are passing a function and not a query. The function creates a query, aka factory. Next step is to refactor getDocumetnsMw to call the function to create a query when it needs to do something with it.

function getDocumentsMw(queryFactory) {
  return async (req, res, next) => {
    const dbRes = await queryFactory()
    req.gottenDocs = dbRes
    
    next()
  }
}