gistfile1.rb

require 'rubygems'
require 'dm-core'
require 'dm-migrations'

DataMapper::Logger.new($stdout, :debug)
DataMapper.setup(:default, "sqlite::memory")

class User
  include DataMapper::Resource

  has n, :answers
  property :id, Serial
  property :name, String
end

class Answer
  include DataMapper::Resource

  belongs_to :user
  has n, :topics, :through => Resource
  property :id, Serial
  property :text, Text
end

class Topic
  include DataMapper::Resource

  has n, :answers, :through => Resource
  property :name, String, :key => true
end

DataMapper.finalize
DataMapper.auto_migrate!

user = User.create(:name => 'tom')

a = Answer.create(:user => user, :text => 'a1', :topics => [
                     Topic.first_or_create(:name => 'aboutme'),
                     Topic.first_or_create(:name => '@onetom')
])

answers      = Answer.all(:"user.name" => 'tom', :"topics.name" => 'aboutme')
user_answers = user.answers.all(:"topics.name" => 'aboutme')

puts answers == user_answers # => true

gistfile2.sql

SELECT "answers"."id", "answers"."user_id" FROM "answers" INNER JOIN "users" ON "answers"."user_id" = "users"."id" INNER JOIN "answer_topics" ON "answers"."id" = "answer_topics"."answer_id" INNER JOIN "topics" ON "answer_topics"."topic_name" = "topics"."name" WHERE ("users"."name" = 'tom' AND "topics"."name" = 'aboutme') GROUP BY "answers"."id", "answers"."user_id" ORDER BY "answers"."id"


SELECT "answers"."id", "answers"."user_id" FROM "answers" INNER JOIN "answer_topics" ON "answers"."id" = "answer_topics"."answer_id" INNER JOIN "topics" ON "answer_topics"."topic_name" = "topics"."name" WHERE ("answers"."user_id" = 1 AND "topics"."name" = 'aboutme') GROUP BY "answers"."id", "answers"."user_id" ORDER BY "answers"."id"