Find Documents Where a Field Compares with Another in an Array

Question

Let\'s say I have a collection of documents that look like this:

{
    \"_id\" : ObjectId(\"5afa6df3a24cdb1652632ef5\"),
    \"createdBy\" : {
        \"_id\         


        

Answer 1

You can still use aggregate() here with MongoDB 3.2, but just using $redact instead:

db.boards.aggregate([
  { "$redact": {
    "$cond": {
      "if": {
        "$and": [
          { "$ne": [ "$createdBy._id", "$owner._id" ] },
          { "$setIsSubset": [["$createdBy._id"], "$acl.profile._id"] }
        ]
      },
      "then": "$$KEEP",
      "else": "$$PRUNE"
    }
  }}
])

Or with $where for the MongoDB 3.2 shell, you just need to keep a scoped copy of this, and your syntax was a bit off:

db.boards.find({
  "$where": function() {
    var self = this;
    return (this.createdBy._id != this.owner._id)
      && this.acl.some(function(e) {
        return e.profile._id === self.createdBy._id
     })
  }
})

Or in an ES6 compatible environment then:

db.boards.find({
  "$where": function() {
    return (this.createdBy._id != this.owner._id)
      && this.acl.some(e => e.profile._id === this.createdBy._id)
  }
})

The aggregate is the most performant option of the two and should always be preferable to using JavaScript evalulation

And for what it's worth, the newer syntax with $expr would be:

db.boards.find({
  "$expr": {
    "$and": [
      { "$ne": [ "$createdBy._id", "$owner._id" ] },
      { "$in": [ "$createdBy._id", "$acl.profile._id"] }
    ]
  }
})

Using $in in preference to $setIsSubset where the syntax is a little shorter.

---

NOTE The only reason the JavaScript comparison here works is because you have mistakenly stored ObjectId values as "strings" in those fields. Where there is a "real" ObjectId just like in the _id field, the comparison needs to take the "string" from valueOf() in order to compare:

    return (this.createdBy._id.valueOf() != this.owner._id.valueOf())
      && this.acl.some(e => e.profile._id.valueOf() === this.createdBy._id.valueOf())

Without that it's actually an "Object Comparison" with JavaScript and { a: 1 } === { a: 1 } is actually false. So avoiding that complexity is another reason there are native operators for this instead.