postgresql Rails 4 查询唯一的单个属性
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Rails 4 query unique by single attribute
提问by goddamnyouryan
So this is more of an arel question than anything but here's what I am trying to do.
所以这更像是一个问题而不是任何问题，但这就是我想要做的。
I have three objects lets say, called Items
我有三个对象可以说，称为 Items
<Item id: 1, name: 'Book'>
<Item id: 2, name: 'Car'>
<Item id: 3, name: 'Book'>
I want to do a query that will just return only one of each unique "name" attributes.
我想做一个查询，它只会返回每个唯一的“名称”属性中的一个。
Something like Item.select('distinct(name), items.*')
就像是 Item.select('distinct(name), items.*')
This doesn't work though, it still returns all three items.
但这不起作用，它仍然返回所有三个项目。
How can I form this query so that it only returns:
我怎样才能形成这个查询，以便它只返回：
<Item id: 1, name: 'Book'>
<Item id: 2, name: 'Car'>
回答by BananaNeil
If you want to get the entire model back, but still maintain uniqueness, you can use this:
如果你想恢复整个模型，但仍然保持唯一性，你可以使用这个：
Item.select('distinct on (name) *')
I've only tested this with a Postgres database. It may or may not work with mysql.
我只用 Postgres 数据库对此进行了测试。它可能与 mysql 一起工作，也可能不工作。
回答by vee
Please try this:
请试试这个：
Item.select('distinct name')
回答by Frexuz
If you only need an array with the names, without the ID, you can do:
如果你只需要一个有名字的数组，没有 ID，你可以这样做：
Item.pluck(:name).uniq
SQL query result:
SQL查询结果：
#=> SELECT `items`.`name` FROM `items`
** edit **
** 编辑 **
The uniqis ran on the array of records. Be careful if you expect a lot of records.
SQL Distinct is much faster.
该uniq是记录阵列上跑。如果您期望有很多记录，请小心。SQL Distinct 快得多。
If so, use vee's answer above, with a map:
如果是这样，请使用上面 vee 的答案，并带有map：
Item.select('distinct name').map(:name)
Item.select('distinct name').map(:name)
回答by Oguzhan Ozdemir
I can't comment on posts yet, so, putting this as another answer for the question.
我还不能对帖子发表评论，所以，把它作为这个问题的另一个答案。
In case of someone still searches for this, @BananaNeil's answeris correct. However, putting distinctin selectdidn't work for me (Rails 5.2.2). Separating these two did fix my problem.
如果有人仍在搜索此内容，@BananaNeil 的回答是正确的。然而，将distinct在select没有对我来说有效（Rails的5.2.2）。将这两个分开确实解决了我的问题。
klass.where(
# Your query or whatever
).distinct.select('on (attribute) *')
回答by TalkativeTree
In Rails 4 try Items.all.to_a.uniq { |item| item.name }
在 Rails 4 中尝试 Items.all.to_a.uniq { |item| item.name }
In Rails 3 you should be able to just do Items.uniq_by { |item| item.name }
在 Rails 3 中，你应该能够做到 Items.uniq_by { |item| item.name }
When you call uniqon an array, you can pass it a block to dictate how to determine uniqueness. In Rails 3 you used to be able to use uniq_by, but it became deprecated in Rails 4. So one method I found is to just convert the ActiveRecord Relation to an array and call uniqon that.
当您调用uniq一个数组时，您可以向它传递一个块来指示如何确定唯一性。在 Rails 3 中，您曾经可以使用uniq_by，但它在 Rails 4 中已被弃用。所以我发现的一种方法是将 ActiveRecord Relation 转换为数组并调用uniq它。
回答by fangxing
For Mysql, your can use groupwith ONLY_FULL_GROUP_BYdisabled,
对于 Mysql，您可以group在 禁用ONLY_FULL_GROUP_BY的情况下使用，
Item.group(:name).to_sql
=> "SELECT `items`.* FROM `items` GROUP BY name"
