To be generated id is marked as duplicate

[dmdum]

I’m trying to insert multiple data using a for-loop (there will be individual processing after each insertion). The _id for each should be auto-generated so I’m providing null in that field.

I’m getting a E11000 duplicate key error collection: . . . . . . . . dup key: { id: null }

Nothing is inserted.
I’m assuming that the id will be generated before the insertion.
Why is null being flagged as duplicate?

Edit: The same thing happens with insertMany()

[steevej (Steeve Juneau)]

If the

@dmdum

_id for each should be auto-generated

you should not supply any values. Null is a value so you should not

@dmdum

providing null in that field

The error message does not match part of you post. You write

@dmdum

The _id for each

with an leading underscore but the error complains about

@dmdum

dup key: { id: null }

without the leading underscore. Have you redacted the error message? Is it _id or id? The only auto-generated field in native mongo is the top level _id. If you have an error with id rather than _id then you must have a unique index on that field. If you do, then you cannot have 2 documents with the same value and null is a value.

To conclude

1. Only top level _id is auto-generated
2. If you want top level _id to be auto-generated, you must not supply a value (and null is a value)

[dmdum]

Yeah, I realized that after looking at the indexes. There was an unwanted index probably created from the earlier stages of the code. I should’ve created a reply instead of an edit. Let me mark your reply as the solution though. Thanks for the detailed explanation!