Writing programs using MongoDB - MongoDB in Action - page 44

One of the incidental benefits of using MongoDB object ID s is that they include a
timestamp. Most of the drivers allow you to extract the timestamp, thus providing the
document creation time, with resolution to the nearest second, for free. Using the
Ruby driver, you can call an object ID 's generation_time method to get that ID 's cre-
ation time as a Ruby Time object:

irb(main):002:0> id = BSON::ObjectId.new
=> BSON::ObjectId('4c41e78f238d3b9090000001')
irb(main):003:0> id.generation_time
=> Sat Jul 17 17:25:35 UTC 2010
Naturally, you can also use object ID s to issue range queries on object creation time.
For instance, if you wanted to query for all documents created between October 2010
and November 2010, you could create two object ID s whose timestamps encode those
dates and then issue a range query on _id . Since Ruby provides methods for generat-
ing object ID s from any Time object, the code for doing this is trivial:

oct_id = BSON::ObjectId.from_time(Time.utc(2010, 10, 1))
nov_id = BSON::ObjectId.from_time(Time.utc(2010, 11, 1))

@users.find({'_id' => {'$gte' => oct_id, '$lt' => nov_id}})
I've explained the rationale for MongoDB object ID s and the meaning behind the
bytes. All that remains is to see how they're encoded. That's the subject of the next
section, where we discuss BSON .

3.2.2 BSON

BSON is the binary format used to represent documents in MongoDB. BSON acts as
both a storage and command format: all documents are stored on disk as BSON , and
all queries and commands are specified using BSON documents. Consequently, all
MongoDB drivers must be able to translate between some language-specific document
representation and BSON .
BSON defines the data types you can use with MongoDB. Knowing which types
BSON comprises, as well as a bit about their encoding, will take you a long way in using
MongoDB effectively and diagnosing certain performance issues when they occur.
At the time of this writing, the BSON specification includes 19 data types. What this
means is that each value within a document must be convertible into one of these
types in order to be stored in MongoDB. The BSON types include many that you'd
expect: UTF -8 string, 32- and 64-bit integer, double, Boolean, timestamp, and UTC
datetime. But a number of types are specific to MongoDB. For instance, the object ID
format described in the previous section gets its own type; there's a binary type for
opaque blobs; and there's even a symbol type for languages that support it.
Figure 3.2 illustrates how you serialize a Ruby hash into a bona fide BSON docu-
ment. The Ruby document contains an object ID and a string. When translated to a
BSON document, what comes first is a 4-byte header indicating the document's size
(you can see that this one is 38 bytes). Next are the two key-value pairs. Each pair
begins with a byte denoting its type, followed by a null-terminated string for the key