Description

I'm trying to find documents containing an array with equal values at positions 0 and 1. This query produces all documents in the collection:

db.collection.find({
  $expr: {
    $eq: ["$arr.0", "$arr.1"]
  }
})

(see this MongoPlayground)

Is it documented somewhere that Expressions don't support array index dot notation? They do support object dot notation field paths.

More discussion in this SO Q&A, in which user Ashh claimed that `"You cannot use field names inside the expr"`, but I couldn't figure out if they were quoting some official docs, and the ensuing discussion ended up being rather confusing.

Scope of changes

• Investigate behavior of `$expr` with `$eq` and array index dot notation
• Update documentation as appropriate

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)