MongoDB: How do I update a single subelement in an array, referenced by the index within the array?

Question

I'm trying to update a single subelement contained within an array in a mongodb document. I want to reference the field using its array index (elements within the array don't have any fields that I can guarantee will be unique identifiers). Seems like this should be easy to do, but I can't figure out the syntax.

Here's what I want to do in pseudo-json.

Before:

{
  _id : ...,
  other_stuff ... ,
  my_array : [
    { ... old content A ... },
    { ... old content B ... },
    { ... old content C ... }
  ]
}

After:

{
  _id : ...,
  other_stuff ... ,
  my_array : [
    { ... old content A ... },
    { ... NEW content B ... },
    { ... old content C ... }
  ]
}

Seems like the query should be something like this:

//pseudocode
db.my_collection.update(
  {_id: ObjectId(document_id), my_array.1 : 1 },
  {my_array.$.content: NEW content B }
)

But this doesn't work. I've spent way too long searching the mongodb docs, and trying different variations on this syntax (e.g. using $slice, etc.). I can't find any clear explanation of how to accomplish this kind of update in MongoDB.

Answer 1

As expected, the query is easy once you know how. Here's the syntax, in python:

db["my_collection"].update(
    { "_id": ObjectId(document_id) },
    { "$set": { 'documents.'+str(doc_index)+'.content' : new_content_B}}
)