Updating a Nested Array with MongoDB

General Scope and Explanation

There are a few things wrong with what you are doing here. Firstly your query conditions. You are referring to several _id values where you should not need to, and at least one of which is not on the top level.

In order to get into a “nested” value and also presuming that _id value is unique and would not appear in any other document, you query form should be like this:

Model.update(
    { "array1.array2._id": "123" },
    { "$push": { "array1.0.array2.$.answeredBy": "success" } },
    function(err,numAffected) {
       // something with the result in here
    }
);

Now that would actually work, but really it is only a fluke that it does as there are very good reasons why it should not work for you.

The important reading is in the official documentation for the positional $ operator under the subject of “Nested Arrays”. What this says is:

The positional $ operator cannot be used for queries which traverse more than one array, such as queries that traverse arrays nested within other arrays, because the replacement for the $ placeholder is a single value

Specifically what that means is the element that will be matched and returned in the positional placeholder is the value of the index from the first matching array. This means in your case the matching index on the “top” level array.

So if you look at the query notation as shown, we have “hardcoded” the first ( or 0 index ) position in the top level array, and it just so happens that the matching element within “array2” is also the zero index entry.

To demonstrate this you can change the matching _id value to “124” and the result will $push an new entry onto the element with _id “123” as they are both in the zero index entry of “array1” and that is the value returned to the placeholder.

So that is the general problem with nesting arrays. You could remove one of the levels and you would still be able to $push to the correct element in your “top” array, but there would still be multiple levels.

Try to avoid nesting arrays as you will run into update problems as is shown.

The general case is to “flatten” the things you “think” are “levels” and actually make theses “attributes” on the final detail items. For example, the “flattened” form of the structure in the question should be something like:

 {
   "answers": [
     { "by": "success", "type2": "123", "type1": "12" }
   ]
 }

Or even when accepting the inner array is $push only, and never updated:

 {
   "array": [
     { "type1": "12", "type2": "123", "answeredBy": ["success"] },
     { "type1": "12", "type2": "124", "answeredBy": [] }
   ]
 }

Which both lend themselves to atomic updates within the scope of the positional $ operator

---

MongoDB 3.6 and Above

From MongoDB 3.6 there are new features available to work with nested arrays. This uses the positional filtered $[<identifier>] syntax in order to match the specific elements and apply different conditions through arrayFilters in the update statement:

Model.update(
  {
    "_id": 1,
    "array1": {
      "$elemMatch": {
        "_id": "12","array2._id": "123"
      }
    }
  },
  {
    "$push": { "array1.$[outer].array2.$[inner].answeredBy": "success" }
  },
  {
    "arrayFilters": [{ "outer._id": "12" },{ "inner._id": "123" }] 
  }
)

The "arrayFilters" as passed to the options for .update() or even
.updateOne(), .updateMany(), .findOneAndUpdate() or .bulkWrite() method specifies the conditions to match on the identifier given in the update statement. Any elements that match the condition given will be updated.

Because the structure is “nested”, we actually use “multiple filters” as is specified with an “array” of filter definitions as shown. The marked “identifier” is used in matching against the positional filtered $[<identifier>] syntax actually used in the update block of the statement. In this case inner and outer are the identifiers used for each condition as specified with the nested chain.

This new expansion makes the update of nested array content possible, but it does not really help with the practicality of “querying” such data, so the same caveats apply as explained earlier.

You typically really “mean” to express as “attributes”, even if your brain initially thinks “nesting”, it’s just usually a reaction to how you believe the “previous relational parts” come together. In reality you really need more denormalization.

Also see How to Update Multiple Array Elements in mongodb, since these new update operators actually match and update “multiple array elements” rather than just the first, which has been the previous action of positional updates.

NOTE Somewhat ironically, since this is specified in the “options” argument for .update() and like methods, the syntax is generally compatible with all recent release driver versions.

However this is not true of the mongo shell, since the way the method is implemented there ( “ironically for backward compatibility” ) the arrayFilters argument is not recognized and removed by an internal method that parses the options in order to deliver “backward compatibility” with prior MongoDB server versions and a “legacy” .update() API call syntax.

So if you want to use the command in the mongo shell or other “shell based” products ( notably Robo 3T ) you need a latest version from either the development branch or production release as of 3.6 or greater.

See also positional all $[] which also updates “multiple array elements” but without applying to specified conditions and applies to all elements in the array where that is the desired action.