Context

We are working with MongoDB queries in a Node.js application, where we need to filter documents based on whether an array field (specifically, a field within a nested array) exists and is not null.

Consider the following sample document:

{
  "name": "Event1",
  "campaigns": [
    {
      "id": "id1",
      "eID": "eID1"
    }
  ]
}

Problem:

We’ve been using the following MongoDB queries:

1. Query with $exists:

{ "campaigns.eID": { $exists: true } }

Result: This works as expected, returning documents where the eID field exists within any element of the campaigns array.

2. Query with $ne: null:

{ "campaigns.eID": { $ne: null } }

Result: This query does not work as expected in our Node.js application. It returns inconsistent results: sometimes it fails to match documents where eID is not null or undefined.

Additional Observations

• When testing these queries in MongoDB Compass:
  • The $exists query consistently returns correct results for all team members.
  • The $ne: null query works correctly in MongoDB Compass for me, but it does not work for my colleague, even though we are using the same query on the same data.

Questions

• Why is the $ne: null query behaving inconsistently across different environments (Node.js vs MongoDB Compass)?
• Is there a known issue or a best practice when using $ne: null with array fields, particularly in nested documents?
• Should we avoid using $ne: null in this context, and instead always use $exists?