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I'm making an app similar to instagram using Rails/Mongoid. I want a unique ID that I can use in a url like http://instagr.am/p/DJmU8/
What's the easiest way to do that? Can I derive such an ID from the default BSON ObjectID Mongo creates?
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MongoDB uses ObjectIds as the default value of _id field of each document, which is generated during the creation of any document. Object ID is treated as the primary key within any MongoDB collection. It is a unique identifier for each document or record. Syntax: ObjectId(<hexadecimal>).
According to MongoDB, ObjectID can be considered globally unique. The first nine bytes in a MongoDB _ID guarantee its uniqueness across machines and processes, in relation to a single second; the last three bytes provide uniqueness within a single second in a single process.
To create a unique index, use the db. collection. createIndex() method with the unique option set to true .
ObjectId values are 12 bytes in length, consisting of: A 4-byte timestamp, representing the ObjectId's creation, measured in seconds since the Unix epoch. A 5-byte random value generated once per process. This random value is unique to the machine and process.
You may try to use first 4 bytes of ObjectID (they will represent timestamp).
But, to be 100% safe, it's better to produce really unique short id, by implementing a counter. You can use separate collection to maintain current value of your counter.
More details on mongo's ObjectID structure can be found here: http://www.mongodb.org/display/DOCS/Object+IDs
As an alternative you can convert convert hex string id representation to a representation based on 36 symbols (26 latin letters + 10 digits). It will obviously be shorter.
It seems, that there is a ruby library, that can do such conversions http://rubyworks.github.com/radix/
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