Clever Custom Datatypes with MongoMapper

Rich Objects. This is a pattern we’re using more and more with MongoDB^[1] and it is so easy to get started.

Example: Version Numbers

We^[2] had an app today that stored a simple version number (e.g. 1.0, 2.1). We could have done this with a string, a pair of integers, or a float. We decided on something better:

class Article
  include MongoMapper::Document

  key :title,    String
  key :body,     String
  key :version,  Version, :default => '1.0'
  …
end

Version? That’s not a Rails or Ruby thing. That’s our own. We can easily cast data into our own objects. Here’s our basic Version class:

class Version
  attr_accessor :major, :minor

  def self.from_mongo(value)
    new(value.to_s)
  end

  def self.to_mongo(value)
    value.to_s
  end

  def initialize(string)
    self.major, self.minor = string.split('.').map(&:to_i)
  end

  def to_s
    "#{major}.#{minor}"
  end
end

We’re storing the actual version as a string in MongoDB (you could do an array too, as that’s a native type) but the key info is the from_mongo and to_mongo to define how they get [un]serialized to MongoDB.

from_mongo takes a value and turns it into a Version object (we call value.to_s in case value is already a Version or a Fixnum). to_mongo simply turns it into a string for saving. In our instances we decided to keep major and minor versions separate.

Bumping Versions

We added some simple methods to bump the versions:

def bump_minor
  self.minor += 1
end

def bump_major
  self.major += 1
end

Now we can bump our article versions easily:

>> @article.version
=> "1.2"
>> @article.version.bump_minor
>> @article.version
=> "1.3"

Comparing Versions

Since this is just a Ruby object, the ability to compare and sort versions is simple:

class Creator::Version
  include Comparable

  def <=>(value)
    other = self.class.new(value.to_s)
    [major, minor] <=> [other.major, other.minor]
  end

  …
  end

Next Steps

There are lots of other objects you can do this with. Once you start to think in terms of rich objects, you find all sorts of great applications.

There are a couple other examples in the MongoMapper documentation^[3], too. Head over there to see more!