Question

class User < ActiveRecord::Base
  has_many :events
  has_many :trainings, -> { distinct }, through: :events
end

class Event < ActiveRecord::Base
  belongs_to :user
  belongs_to :training
end

class Training < ActiveRecord::Base
  has_many :events
  has_many :user, -> { distinct },  through: :events
end

events table contains:

t.integer  "user_id"
t.integer  "training_id"
t.due      "date"

想象一下，events表包含以下user_id，training_id和截止日期

[ 1, 1, 25/06/16]
[ 1, 1, 25/06/15]
[ 1, 1, 25/06/14]

[ 1, 2, 25/06/16]
[ 1, 2, 25/06/15]
[ 1, 2, 25/06/14]

[ 2, 1, 25/06/16]
[ 2, 1, 25/06/15]
[ 2, 1, 25/06/14]

[ 2, 2, 25/06/16]
[ 2, 2, 25/06/15]
[ 2, 2, 25/06/14]

我想根据每个“群组”（用户和培训密钥对）的截止日期选择最新值。

即

[ 1, 1, 25/06/16]
[ 1, 2, 25/06/16]
[ 2, 1, 25/06/16]
[ 2, 2, 25/06/16]

知道如何实现这个目标吗？

Answer 1

使用自定义查询进行管理。只有轻微的问题是它会选择行，即使它们不是过期的，只是最新的行。如果我输入WHERE due＆lt; ＆＃39;＃{Time.now}＆＃39;它基本上选择了一个不是我想要的东西，但我可以在视图中放置一些东西来过滤日期＆lt; Time.now as compramise。

  @overdue = Event.find_by_sql("SELECT DISTINCT ON (user_id, training_id)
         id, user_id, training_id, date, due FROM events 
  ORDER BY user_id, training_id, due DESC")

Answer 2

您可以执行此操作以对其进行清理

Model
.select('DISTINCT ON (col_1, col_2) *')
.reorder(:col_1, :col_2, date_col: :desc)

在rails中选择每组唯一密钥对的日期

2 个答案: