db.find({somefield:{$exists:true}}) returns all docs, but no doc contains the somefield!

[creedofcool]

mongoos 6.3.3
mongodb 4.2
express 5.0 beta

why?

[creedofcool]

not happen at mongose@5.13.14 and 4.x
so is it a bug?

[AbdelrahmanHafez]

Is it possible that the field has the value of null in your database? Because setting the value to null/undefined is not the same as $unsetting it.
If not, can you please modify the script below to reproduce the issue you're facing?

11807.js

'use strict';
import mongoose from 'mongoose';
const { Schema } = mongoose;
import assert from 'node:assert';


await mongoose.connect('mongodb://localhost:27017/test');
await mongoose.connection.dropDatabase();

const userSchema = new Schema({
  name: { type: String },
  age: Number
});

const User = mongoose.model('User', userSchema);
await User.insertMany([{ name: 'John' }, { name: 'Doe' }, { name: 'Jane', age: null }]);

const foundUsers = await User.find({ age: { $exists: true } });

// Jane is found, because you can set a field as `null`, and counts as a field
assert.ok(foundUsers.length === 1);

console.log(mongoose.version);
console.log('All assertions passed.');

Output:

$ node 11807.js
6.3.2
All assertions passed.

[creedofcool]

const mongoose = require("mongoose");
const { Schema } = mongoose;

const start = async () => {
  const mongodb_uri = "mongodb://localhost:38517/orm";
  await mongoose.connect(mongodb_uri);
  const userSchema = new Schema({
    name: { type: String, unique:true },
    age: Number,
  });
  const testUser = mongoose.model("testUser", userSchema);
  await testUser.insertMany([
    { name: "John" },
    { name: "Doe" },
    { name: "Jane", age: null },
  ]);

  const foundUsers = await testUser.find({ username: { $exists: true } });
  console.log(foundUsers);

  console.log(\`node version: ${process.version} && mongoose version: ${mongoose.version}\` );

  process.exit(1);
};

start();

output:
[ { _id: new ObjectId("6283441e619ff0529bcaf97d"), name: 'John', __v: 0 }, { _id: new ObjectId("6283441e619ff0529bcaf97e"), name: 'Doe', __v: 0 }, { _id: new ObjectId("6283441e619ff0529bcaf97f"), name: 'Jane', age: null, __v: 0 } ] node version: v16.2.0 && mongoose version: 6.3.2

Thank you @AbdelrahmanHafez
If change the search field to username which is not included in any doc, the result is all docs. I don't understand the logic.

[Uzlopak]

There is no field username in your Schema.

[AbdelrahmanHafez]

@creedofcool
You do not have a username field in your schema, so mongoose removes it from the filter. You can change this behavior by using strictQuery: false

const foundUsers = await testUser.find({ username: { $exists: true } }, null, { strictQuery: false });

You can also set this option globally:

mongoose.set('strictQuery', false); 

// or on a single schema level
schema.set('strictQuery', false);

Does that solve your problem?

[creedofcool]

Yes,
That's right, so awesome.
Thank you so much. @AbdelrahmanHafez