Question

Any idea why this query

{
  "_id": ObjectId("4e2a4ca7f21a81331f0006c3"),
  "categories": {
    "$nin": [
      "Arts"
    ]
  }
}

returns the following record when we explicitly excluded “Arts” with $nin above?

{
   "_id": ObjectId("4e2a4ca7f21a81331f0006c3"),
   "categories": {
     "0": "Arts",
     "1": "Arts - Performance" 
  }
}

as a sanity check, I changed $nin to $in

{
  "_id": ObjectId("4e2a4ca7f21a81331f0006c3"),
  "categories": {
    "$in": [
      "Arts"
    ]
  }
}

and it didn’t return anything as expected.. but I’m still puzzled by the $nin result above!

Answer 1

Your first query is requesting a document with a specific ID and where the value for categories is not in a list of strings (in this case, ["Arts"]). The value for categories for this document is an embedded document, { "0": "Arts", "1": "Arts - Performance" }.

The second query returns nothing because that embedded document is not in the provided list of values for the categories field.

Edit: If you want to be able to search for categories using $in and $nin, you should consider converting the categories subdocument into an array:

> db.foo.save({categories: ["Arts", "Arts - Performance"]})                                                     
> db.foo.save({categories: ["Sports", "Sports - Baseball"]})
> db.foo.find({categories: {$nin: ["Arts"]}})                 
{ "_id" : ObjectId("4e2e3b49aed2871354b374eb"), "categories" : [ "Sports", "Sports - Baseball" ] }
> db.foo.find({categories: {$in: ["Arts"]}})                  
{ "_id" : ObjectId("4e2e3b1eaed2871354b374ea"), "categories" : [ "Arts", "Arts - Performance" ] }