ruby-on-rails - Rails SELECT 与 postgres 不同

Question

所以我试图在我的桌子上使用 SELECT Distinct 选项

Hashlog.select("DISTINCT tag").select("created_at").order("created_at DESC").limit(10)

 1.9.3-p286 :017 > Hashlog.select("DISTINCT tag").select("created_at").order("created_at DESC").limit(10)
      Hashlog Load (0.7ms)  SELECT DISTINCT tag, created_at FROM "hashlogs" ORDER BY created_at DESC LIMIT 10
     => [#<Hashlog tag: "new", created_at: "2012-12-11 04:06:37">, 
     #<Hashlog tag: "now", created_at: "2012-12-11 04:06:33">, 
     #<Hashlog tag: "googleold", created_at: "2012-12-11 04:06:28">, 
     #<Hashlog tag: "google", created_at: "2012-12-11 04:06:26">, 
     #<Hashlog tag: "facebook", created_at: "2012-12-11 04:06:21">, 
     #<Hashlog tag: "facebook", created_at: "2012-12-11 04:06:18">, 
     #<Hashlog tag: "faceboot", created_at: "2012-12-11 04:06:15">]

所以我希望结果只在标签列上是唯一的，但它不会让我按 created_at 排序，除非它通过选择。

Answer 1

我认为您要做的是为每个标签选择按最早（即最小）排序的不同标签值created_at。这将是解决由具有多个关联created_at值的标签引起的歧义的一种方法。

如果是这种情况，请尝试以下操作：

Hashlog.select("tag, min(created_at) as earliest").group("tag").order("earliest DESC").limit(10)

.

Answer 2

我在我的项目中使用了以下代码段

Product.select("DISTINCT(products.material_id), products.material_id, products.class_id").where('products.class_id' => current_user.class_id)