Question

我试图找到一种方法来查询如何通过多个唯一字段查找项目。所以例如我想找到这个项目，其中＆＃34;国家＆＃34;和＆＃34;城市＆＃34;是独一无二的。

对于我一直在使用的单个字段：

@foo = Foo.where(created_at: from..to).to_a.uniq{ |foo| foo.bar }

其中bar是我Foo表中的唯一字段。

如何Foo bar和baz字段是唯一的？

Answer 1

您可以将unique方法与数组一起用作return：

@foo = Foo.where(created_at: from..to).to_a.uniq{ |foo| [foo.bar, foo.baz] }

Answer 2

请使用DISTINCT ON

 Foo.where(created_at: from..to)
    .select("DISTINCT ON (foo) foo, bar")
    .order("foo, bar")

这是一个示例，显示此查询如何在多列上提供唯一记录。让我们创建一个表：

app_development=# select * from clients;
 id | age | name |         created_at         |         updated_at
----+-----+------+----------------------------+----------------------------
  1 |  26 | Arup | 2015-03-23 08:56:45.968966 | 2015-03-23 08:56:45.968966
  2 |  26 | Arup | 2015-03-23 08:56:57.269637 | 2015-03-23 08:56:57.269637
  3 |  25 | Ayan | 2015-03-23 08:57:06.956179 | 2015-03-23 08:57:06.956179
(3 rows)

appdevelopment=# SELECT DISTINCT ON (name) name, age FROM "clients"   ORDER BY name, age;
 name | age
------+-----
 Arup |  26
 Ayan |  25
(2 rows)

你也可以在Rails中完全相同：

2.1.2 :005 > Client.select("DISTINCT ON (name) name, age").order("name, age")
  Client Load (0.9ms)  SELECT DISTINCT ON (name) name, age FROM "clients"   ORDER BY name, age
 => #<ActiveRecord::Relation [#<Client id: nil, age: 26, name: "Arup">, #<Client id: nil, age: 25, name: "Ayan">]>
2.1.2 :006 > Client.select("DISTINCT ON (name) name, age").order("name, age").map { |r| [r.name, r.age] }
  Client Load (0.5ms)  SELECT DISTINCT ON (name) name, age FROM "clients"   ORDER BY name, age
 => [["Arup", 26], ["Ayan", 25]]

通过多个唯一字段Rails查找

2 个答案: