DataMapper: promoting columns into classes

mongodb | ruby | databases

I recently had to deal with reading a schema in a MySQL database in Ruby and migrating data to MongoDB. It has been made real easy with DataMapper, an awesome library that doesn’t force me into creating a complete and rigid model for the MySQL database.

Today’s problem looks as follows. I have a table called fruits. Each fruit _has a field called _color _amongst two dozen other fields. Colors are referred by name, for example _yellow or green. What I would like to do is to promote the colors into a first class object, therefore creating a collection of colors based on the data in the fruits table. Effectively I’d like to SELECT DISTINCT color from fruits, but I’d like the results to be of type Color.

Turns out to be very straightforward with DataMapper.

class Color
  include DataMapper::Resource
  storage_names[:default] = "fruits"
  property :name, String, :field => "color"

  class << self
    alias_method :_all, :all

    def all
      coll = []
      repository(:default).adapter.select("SELECT DISTINCT color FROM fruits").each do |c|
        coll << Color.new({name: c})
      end
      coll
    end
  end
end

This creates a class called Color with a name field that comes from the color column of the fruits table. We replace the Color.all method with our own implementation that makes a DISTINCT query at the adapter level. As a bonus you can see how you can rename the all method (we don’t really call the old _all implementation here, but it’s often useful). This lets you do Color.all.