Rails 4 query unique by single attribute

问题

So this is more of an arel question than anything but here's what I am trying to do.

I have three objects lets say, called 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.

Something like Item.select('distinct(name), items.*')

This doesn't work though, it still returns all three items.

How can I form this query so that it only returns:

<Item id: 1, name: 'Book'>
<Item id: 2, name: 'Car'>

回答1:

If you want to get the entire model back, but still maintain uniqueness, you can use this:

Item.select('distinct on (name) *')

I've only tested this with a Postgres database. It may or may not work with mysql.

回答2:

Please try this:

Item.select('distinct name')

回答3:

If you only need an array with the names, without the ID, you can do:

Item.pluck(:name).uniq

SQL query result:

#=> SELECT `items`.`name` FROM `items`

** edit **

The uniq is ran on the array of records. Be careful if you expect a lot of records. SQL Distinct is much faster.

If so, use vee's answer above, with a map:

Item.select('distinct name').map(:name)

回答4:

In Rails 4 try Items.all.to_a.uniq { |item| item.name }

In Rails 3 you should be able to just do Items.uniq_by { |item| item.name }

When you call uniq on 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 uniq on that.

回答5:

I can't comment on posts yet, so, putting this as another answer for the question.

In case of someone still searches for this, @BananaNeil's answer is correct. However, putting distinct in select didn't work for me (Rails 5.2.2). Separating these two did fix my problem.

klass.where(
  # Your query or whatever
).distinct.select('on (attribute) *')

回答6:

For Mysql, your can use group with ONLY_FULL_GROUP_BY disabled,

Item.group(:name).to_sql
=> "SELECT `items`.* FROM `items`  GROUP BY name"

See Is there ANY_VALUE capability for mysql 5.6?

来源：https://stackoverflow.com/questions/18050282/rails-4-query-unique-by-single-attribute