使用关联模型上的属性来订购Rails模型

Question

我有一个名为Retreat的Rails模型 Retreat与模型实例具有has_many关联，因此Retreat具有多个实例，实例属于Retreat。

这里有两个模式：

create_table "instances", force: :cascade do |t| 
t.date "date" 
t.string "venue" 
t.string "schedule" 
t.decimal "price", precision: 8, scale: 2 
t.datetime "created_at", null: false 
t.datetime "updated_at", null: false 
t.integer "retreat_id" 
t.index ["retreat_id"], name: "index_instances_on_retreat_id" 
end 

create_table "retreats", force: :cascade do |t| 
t.string "title" 
t.string "tagline" 
t.string "type_of" 
t.datetime "created_at", null: false 
t.datetime "updated_at", null: false 
t.string "description" 
t.string "schedule" 
t.boolean "available", default: false 
end 

我想订购的最直接的实例的日期方面退却模型（即是最接近今天实例的日期）。如果撤退没有实例，则按照顺序排列。

我该怎么做？

编辑＃1：

这是怎么了，我终于做到了：

def Retreat.order_by_upcoming(order='asc') 
    if order == 'asc' 
    Retreat.find_by_sql("select retreats.* from retreats left outer join \ 
    (select retreats.title, min(instances.date) as upcoming, 
     count(instances.date) from retreats left outer join instances on 
     retreats.id = instances.retreat_id where 
     instances.date >= '#{Date.today}' group by retreats.title) as s on \ 
     retreats.title = s.title order by case when s.upcoming is null then 1 
     else 0 end, s.upcoming, retreats.title") 
    elsif order == 'desc' 
    Retreat.find_by_sql("select retreats.* from retreats left outer join \ 
     (select retreats.title, min(instances.date) as upcoming, 
     count(instances.date) from retreats left outer join instances on 
     retreats.id = instance s.retreat_id where 
     instances.date >= '#{Date.today}' group by retreats.title) as s on \ 
     retreats.title = s.title order by s.upcoming desc, retreats.title") 
    end 
end 

可能有更好的方法来做到这一点。

Answer 1

这应该有助于

Retreat.joins(:instances).group('retreats.id').order('MAX(in‌​stances.date) desc') 

假设Retreathas_manyInstance小号