Multiple documents with the same _id field

[aneroid (Anirudh Dutt)]

@Barnabas_Richter

inserts time series data into a collection

I’m going to assume you mean “inserts time series data into a Time Series collection” since the behaviour you describe is not possible in regular collections. Edit: If that did happen for regular collections, it would be a serious bug.

@Barnabas_Richter

Could it be a bug or is it something on my end?

Not a bug and not any issue at your end. By default, Time Series collections don’t create a unique index on the _id field.

@Barnabas_Richter

How is this possible?

When you create a Time Series collection, a compound index will be created on the metaField and timeField. Note that even this is not a unique index.

Using the example from the docs, doing:

db.createCollection(
   "stocks",
   {
      timeseries: {
         timeField: "date",
         metaField: "ticker",
         granularity: "seconds"
      }
   })

will create a time series collection with a compound index on ticker & date, which are the metaField and timeField above. Using db.getCollection("stocks").getIndexes(), gives:

[ { v: 2, key: { ticker: 1, date: 1 }, name: 'ticker_1_date_1' } ]

Again, note that it’s not a unique index and no index on _id is listed above.
Seen here:

However, I would agree that the documentation about this behaviour in Time Series collections could be improved and made clear.