ruby-on-rails - Rails 3 ActiveRecord 查询

Question

给定一个具有以下模型的 Rails 3 应用程序:

# == Schema Information
# Schema version: 20110517144920
#
# Table name: orders
#
#  id         :integer         not null, primary key
#  user_id    :integer         not null
#  name       :string(255)     not null
#  state      :string(255)
#  created_at :datetime
#  updated_at :datetime
#

class Order < ActiveRecord::Base

  belongs_to                          :user
  has_many                            :multipage_pdfs,  :as => :assetable,   :dependent => :destroy
  has_many                            :singlepage_pdfs, :as => :assetable,   :dependent => :destroy
  ...
end


# == Schema Information
# Schema version: 20110511064747
#
# Table name: assets
#
#  id             :integer         not null, primary key
#  assetable_id   :integer         not null
#  assetable_type :string(255)     not null
#  state          :string(255)
#  type           :string(255)     not null
#  name           :string(255)     not null
#  document       :string(255)     not null
#  version        :integer         default(0), not null
#  created_at     :datetime
#  updated_at     :datetime
#

class Asset < ActiveRecord::Base
  ...
end


class SinglepagePdf < Asset
  ...
end

asset#version 每次创建新 Assets 时都会增加，如果存在具有相同名称的 Assets ，这就是我的 Assets 通常的样子(简化):

    | id | assetable_id | assetable_type  |   name  | version | type           |    |  1 |             1 |           Order | my.pdf  |        1 | SinglepagePdf |    |  2 |             1 |           Order | my.pdf  |        2 | SinglepagePdf |    |  3 |             1 |           Order | my.pdf  |        3 | SinglepagePdf |    |  4 |             1 |           Order | bla.pdf |        1 | SinglepagePdf |

Now i need to get all singlepage_pdfs of a single order, but only the last version for each name. How can i achieve this?

o = Order.find(1)
singlepage_pdfs = o.singlepage_pdfs.find(...)

结果应该只包含 id 为 3 和 4 的 Assets ...

Answer 1

试试这个，它按名称将所有 singlepage_pdf 分组并返回每个的最新版本

o.singlepage_pdfs.group("name").order("version DESC")