xxxxxxxxxx
Save ()
/ \
/ \
Having _id Not Having _id
->In this case save will do -> It will do normal insertion
upsert to insert.Now in this case as insert() do.
what that means, it means
take the document and replace
the complete document having same
_id.
Insert()
/ \
/ \
Doc Having _id in collection Doc Not Having _id
-> E11000 duplicate key ->Insert a new doc inside the collection.
error index: