MongoDB ObjectIds
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Object IDs
You may have noticed that the database creates an unique object for each document:
db.deptCollection.find().pretty()
Examine the output carefully and you will notice for each department in the collection something like:
"_id" : ObjectId("5808e3d2ec0ff55100af2649")
An ObjectId is like a primary key found in relational databases, except in this case it is globally unique across the whole database, not just one table. It is similar to objectIds found in object-oriented programming languages.
ObjectIds created by MongoDB consist of 12 bytes:
ObjectID layout
| 0 | 1 | 3|4|5|6|7|8|9|10| 11| | |
| time | machine | pid | inc| |
Where:
- time: system time
- machine: machine id
- pid: process id
- inc: incremental number
