Ruby-on-rails Rails：如何从列中获取唯一值
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Rails: how can I get unique values from column
提问by Oleg Pasko
How can I get unique values from column in the table? For example, I have this Products table:
如何从表中的列中获取唯一值？例如，我有这个 Products 表：
ID NAME CATEGORY
1 name1 1st_cat
2 name2 2nd_cat
3 name3 1st_cat
Here I want to get only 2 values - 1st_cat and 2nd_cat:
在这里，我只想获得 2 个值 - 1st_cat 和 2nd_cat：
<%Products.each do |p|%>
<%=p.category%>
<%end%>
回答by user664833
Two more ways:
还有两种方式：
Product.select(:category).map(&:category).uniq # Ruby does the work
Product.uniq.pluck(:category) # DB does the work (superior)
For Rails >= 5.1use:
对于Rails >= 5.1使用：
Product.distinct.pluck(:category) # DB does the work (superior)
...because Relation#uniqwas deprecated.
...因为Relation#uniq已被弃用。
回答by ThatOtherPerson
I think you can do this:
我认为你可以这样做：
<% Products.select("DISTINCT(CATEGORY)").each do |p| %>
<%= p.category %>
<% end %>
Source: http://guides.rubyonrails.org/active_record_querying.html#selecting-specific-fields
来源：http: //guides.rubyonrails.org/active_record_querying.html#selecting-specific-fields
回答by jelder
This does all the work in the database server. The result is a simple array.
这完成了数据库服务器中的所有工作。结果是一个简单的数组。
<% Product.distinct(:category).pluck(:category).each do |category|
<%= category %>
<% end %>
Rails will generate SQL that works on any database (Postgres, MySQL, etc).
Rails 将生成适用于任何数据库（Postgres、MySQL 等）的 SQL。
SELECT DISTINCT "products"."category" FROM "products"
回答by Fabio Ros
I suggest to use Products.all.distinct.pluck(:category)because uniqhas been deprecated since rails 5 and it will be removed on rails 5.1
我建议使用Products.all.distinct.pluck(:category)因为uniq自 rails 5 以来已被弃用，它将在 rails 5.1 上删除
回答by Marek P?íhoda
Try this (in the rails console)
试试这个（在 rails 控制台中）
Product.group(:category)
Product.group(:category).each { |p| p.name }
回答by srghma
For postgres
对于 postgres
<% Product.select("DISTINCT ON (category) *").each do |category|
<%= category %>
<%= name %>
<% end %>
Update
更新
even better
甚至更好
<% Product.select(%(DISTINCT ON (category) "#{Product.table_name}".*)).each do |category|
<%= category %>
<%= name %>
<% end %>
because it can return wrong columns when you do joins(e.g. returns idcolumn from joined table, but not products)
因为它可以在你这样做时返回错误的列joins（例如id从连接表中返回列，但不是products）
回答by Phil_ish
Needed to get unique output and was trying the 'uniq' method unsuccessfully. Tried several solutions posted here unsuccessfully. I'm using devise which gives me access to the current_usermethod and working with two tables, one being a join (an item has_many :things).
需要获得唯一的输出并且尝试使用“uniq”方法失败。尝试了此处发布的几种解决方案均未成功。我正在使用设计，它使我可以访问该current_user方法并使用两个表，一个是连接（一个项目 has_many :things）。
This solution ultimately worked for me :
这个解决方案最终对我有用：
@current_user.things.select(:item_fk).distinct.each do |thing|
<%= thing.item.attribute %>
<% end %>
