README.markdown

Experimental Mongo support for ActiveRecord.

Here is an instructional video:

http://www.youtube.com/watch?v=kPfHxygdOjc

identity.rb

module Arel
  module Visitors
    ###
    # The Identity visitor returns anything passed to the `accept` method.
    class Identity < Arel::Visitors::Visitor
      def initialize engine
      end

      def accept ast
        ast
      end
    end
  end
end

mongo.rb

module Arel
  module Visitors
    class Mongo < Arel::Visitors::Visitor
      attr_reader :db

      def initialize db
        @db     = db
      end

      Query = Struct.new(:collection_name, :fields, :conditions)

      private

      def visit_Arel_Nodes_SelectStatement o
        o.cores.map { |c| visit_Arel_Nodes_SelectCore c }.map { |query|
          collection = db.collection query.collection_name

          fields   = query.fields
          selector = Hash[query.conditions]

          opts = {}
          opts[:fields] = fields unless fields.empty?
          opts[:limit]  = o.limit.expr.to_i if o.limit

          collection.find(selector, opts).to_a
        }.flatten
      end

      def visit_Arel_Nodes_SelectCore o
        fields     = o.projections.map { |proj| visit(proj) }.compact
        conditions = o.wheres.map { |condition| visit(condition) }.flatten(1)

        Query.new(visit(o.source), fields, conditions)
      end

      def visit_Arel_Nodes_And o
        o.children.map { |child| visit child }
      end

      def visit_Arel_Nodes_Equality o
        [visit(o.left), visit(o.right)]
      end

      def visit_Arel_Attributes_Attribute o
        return if o.name == '*'
        o.name
      end

      def visit_Arel_Nodes_JoinSource o
        visit o.left
      end

      def visit_Arel_Table o
        o.name
      end

      def literal o; o; end
      alias :visit_String :literal
      alias :visit_Fixnum :literal

    end
  end
end

mongo_adapter.rb

require 'active_record/connection_adapters/abstract_adapter'
require 'arel/visitors/identity'
require 'arel/visitors/mongo'

# Ensure that to_sql returns the AST.
Arel::Visitors::VISITORS['mongo'] = Arel::Visitors::Identity

require 'mongo'

module ActiveRecord
  class Base
    def self.mongo_connection(config)
      connection = Mongo::Connection.new
      ConnectionAdapters::MongoAdapter.new(connection, logger, config)
    end
  end

  module ConnectionAdapters
    class MongoAdapter < AbstractAdapter
      attr_reader :db

      def initialize connection, logger, config
        super(connection, logger)
        @config = config
        @db     = connection.db config[:database]
      end

      ###
      # There is no concept of columns, so return an empty set.
      def columns(table_name, name = nil)
        []
      end

      def table_exists? name
        true
      end

      def primary_key table
        "_id"
      end

      def select(ast, name = nil, binds = [])
        visitor = Arel::Visitors::Mongo.new db
        visitor.accept ast
      end
    end
  end
end