hash.rb

class Hash
  def rmerge!(other_hash)
    merge!(other_hash) do |key, oldval, newval|
      oldval.class == self.class ? oldval.rmerge!(newval) : newval
    end
  end
 
  def rmerge(other_hash)
    r = {}
    merge(other_hash) do |key, oldval, newval|
      r[key] = oldval.class == self.class ? oldval.rmerge(newval) : newval
    end
  end
end

object.rb

class Object
  def to_mm_criteria(value)
    SymbolOperator.new(self, 'eq').to_mm_criteria(value)
  end
end

query.rb

module MongoMapper
  class Query
    
    def initialize
      @criteria = {}
    end
    
    def where(criteria)
      criteria.each do |key, value|
        @criteria.rmerge!(key.to_mm_criteria(value))
      end
      self
    end
    
    def criteria
      @criteria
    end
  end
end

test_query.rb

require 'test_helper'
require 'models'

class QueryTest < Test::Unit::TestCase
  include MongoMapper
  
  context "where" do
    should "generate criteria" do
      Query.new.where(:foo => 'bar').criteria.should == {:foo => {'$eq' => 'bar'}}
    end
    
    should "merge criteria" do
      Query.new.where(:foo => 'bar').where(:hello => 'world').criteria.should == {
        :foo => {'$eq' => 'bar'}, :hello => {'$eq' => 'world'}
      }
    end
    
    should "merge criteria on same attribute" do
      Query.new.where(:foo => 'bar').where(:foo.ne => 'baz').criteria.should == {
        :foo => {'$eq' => 'bar', '$ne' => 'baz'}
      }
    end
  end
end