Difference between revisions of "MongoDB ObjectIds"
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* pid: process identifier | * pid: process identifier | ||
* inc: auto-incrementing counter | * inc: auto-incrementing counter | ||
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| + | == Non-Default ObjectIds == | ||
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| + | You can assign an objectId yourself to a document, but you do need to ensure it will be globally unique, so treating it as a primary key is not a good idea. | ||
Revision as of 14:15, 21 October 2016
Main Page >> MongoDB >>MongoDB Workbook >> Object IDs
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 are a 12-byte BSON type, with the following structure:
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
| time | machine | pid | inc | ||||||||
Where:
- time: timestamp - the time in seconds
- machine: machine identifier
- pid: process identifier
- inc: auto-incrementing counter
Non-Default ObjectIds
You can assign an objectId yourself to a document, but you do need to ensure it will be globally unique, so treating it as a primary key is not a good idea.
