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How _id field is constructed in mongoDB ?

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How _id field is constructed in mongoDB ?
posted Jun 13, 2015 by Vikram Singh

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1 Answer

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In RDMS, a primary key is used to identify a record uniquely. Similar to RDMS, mongoDB uses objectId to identify a document uniquely. In RDMS, a user takes care of primary key value while in case mongoDB, mongoDB server may take care to maintain uniqueness of a document.

" _id " field is used to identify a document uniquely in a database. _id is a 12-bytes long BSON type string.
These 12 bytes are constructed by following components.
a) First 4 bytes represents seconds since Unix epoch.
b) The next 3 bytes are machine identifiers.
c) The next 2 bytes consist of process Id.
d) Remaining 3 bytes are a random counter value.

answer Jun 13, 2015 by Alok
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Unfortunately, this doesnt work when searching for sub field "a1.0" (field "a1" is an array):

db.table_0.aggregate({"$group" : { _id: "$a1.0", "count": { "$sum": 1 } } }
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Any workaround ?

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