Strongly typed conversion
It's not uncommon for someone to import data into a MongoDB collection and neglect to apply strong typing for the date, number, and boolean fields and store them as strings. This situation is likely to cause friction for subsequent users of the data. This example will show you how to restore these fields to their proper types.
Scenario
A third party has imported a set of retail orders into a MongoDB collection but with all data typing lost (they have stored all field values as strings). You want to reestablish correct typing for all the documents and copy them into a new cleaned collection. You can incorporate such transformation logic in the aggregation pipeline because you know each field's type in the original record structure.
Note
Unlike most examples in this book, in this example, the aggregation pipeline writes its output to a collection rather than streaming the results back to the calling application.
