mongodb - How to reference each _id in a collection -

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i have 2 collections on db.

at first have "tvs" collection have list of tvs, @ second have tvcomments holds comments made in each tv.

i update "tvs" collection new field shows count of comments each tv.

the "tv" field "_id" field "tvs" collection references commented tv.

i prove code bellow isn´s works

db.tvs.update( { $set: { numcomments: db.tvcomments.find( { tv: this._id } ).count() } }, { multi: true } )

default _id keys in mongo mongo object ids. have convert this._id objectid: objectid(this._id).

your command becomes:

db.tvs.update( { $set: { numcomments: db.tvcomments.find( { tv: objectid(this._id) } ).count() } }, { multi: true } )

objectid 12-byte bson type, constructed using:

  • a 4-byte value representing seconds since unix epoch,
  • a 3-byte machine identifier,
  • a 2-byte process id, and
  • a 3-byte counter, starting random value.

see more information in documentation.


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