Adding Information: the _id field

• Should you include or exclude the _id field?

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  • _id is like an RDBMS "auto-increment" field
  • If _id is not specified, MongoDB creates this for you
  • If you specify _id it must be unique within the collection!
  • ObjectID is generated by the client
  • 4 byte Timestamp . 3 byte machine ID . 2 byte PID . 3 byte counter
  • More info: docs.mongodb.org/manual/reference/object-id/

• What are the advantages of overwriting the _id field?

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  • Easier to locate documents

• What are the disadvantages of overwriting the _id field?

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  • Duplicate key error can occur when inserting a document while overwriting the _id field
  • Fatal error: Uncaught exception MongoCursorException with message localhost:27017: E11000 duplicate key error index: mydb.collection.$_id_ dup key: { : ObjectId(528908a9ef5c881f68000000) }

• How can you uniquely ID a document without overwriting _id?

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  • Add an additional field with a your own unique identifying system