Summery of "The $out stage" confusing: "if specified"

MongoDB University M121: The MongoDB Aggregation Framework
#1

In de summary of “The $out stage” the first bullet says "Will create a new collection of overwrite an existing collection if specified.

The documentation says an existing collection is always overwritten, so what do you mean with “if specified”?

What I understand when I read this sentence is that it is possible to choose to overwrite an existing collection or not, by providing an additional document like {allowDiskUse: true}.

#2

@Harry_39420

No. If you specify an $out collection that matches an existing collection, the existing collection will be overwritten., On the other hand, if you specify an $out collection that does not yet exist, it will be created. So an existing collection is overwritten if the collection specified already exists.

#3

Hi. Thanks, but that part is already couvert: “Will create a new collection or overwrite an existing collection”. Maybe it is my onderstanding of the language, but for me it is confusing.

As I said before. There could be three possibilities:

  1. the collection is created when non existant
  2. the documents are appended is the collection exists
  3. the collection is replaced “if specified”, for example with {“append”:false"} or something like that

When I see the sentence “if specified” I ask myself “what if not specified”. And how do I do that?

#4

@Harry_39420

I think this is a language problem, but in any case you should always take the official MongoDB documentation as the canonical source.