Update the key data type from string to int64

[steevej (Steeve Juneau)]

If you look at the $map documentation you will read the following about $$this:

A name for the variable that represents each individual element of the input array. If no name is specified, the variable name defaults to this.

In you sample document, the field fulfill is an array of objects. You $map using input:$fulfill and $toLong:$$this.tagSerialNumbers, but tagSerialNumbers is an array and this is why you get:

@Amarendra_Krishna

MongoServerError: Unsupported conversion from array to long in $convert with no onError value

But I already mentioned:

@steevej

The field tagSerialNumbers in the document you shared is an array.

An hint to the solution was also mentioned:

@steevej

You have to $map tagSerialNumbers and convert with { $toLong : $$this }.