Ruby-on-rails 如何使用activerecord获取最后N条记录？
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How to get last N records with activerecord?
提问by JtR
With :limitin query, I will get first N records. What is the easiest way to get last N records?
随着:limit查询，我会得到前N个记录。获取最后 N 条记录的最简单方法是什么？
采纳答案by Dan McNevin
An active record query like this I think would get you what you want ('Something' is the model name):
我认为这样的活动记录查询可以满足您的需求（'Something' 是模型名称）：
Something.find(:all, :order => "id desc", :limit => 5).reverse
edit: As noted in the comments, another way:
编辑：如评论中所述，另一种方式：
result = Something.find(:all, :order => "id desc", :limit => 5)
while !result.empty?
puts result.pop
end
回答by Bongs
This is the Rails 3 way
这是 Rails 3 的方式
SomeModel.last(5) # last 5 records in ascending order
SomeModel.last(5).reverse # last 5 records in descending order
回答by Arthur Neves
new way to do it in rails 3.1 is SomeModel.limit(5).order('id desc')
在 rails 3.1 中做到这一点的新方法是 SomeModel.limit(5).order('id desc')
回答by Gagan Gami
For Rails 4 and above version:
对于Rails 4 及以上版本：
You can try something like this If you want first oldest entry
你可以尝试这样的事情如果你想要第一个最旧的条目
YourModel.order(id: :asc).limit(5).each do |d|
You can try something like this if you want last latest entries..
如果你想要最新的条目，你可以尝试这样的事情..
YourModel.order(id: :desc).limit(5).each do |d|
回答by Amin Ariana
For Rails 5 (and likely Rails 4)
对于 Rails 5（可能还有 Rails 4）
Bad:
坏的：
Something.last(5)
because:
因为：
Something.last(5).class
=> Array
so:
所以：
Something.last(50000).count
will likely blow up your memory or take forever.
可能会破坏你的记忆或永远。
Good approach:
好办法：
Something.limit(5).order('id desc')
because:
因为：
Something.limit(5).order('id desc').class
=> Image::ActiveRecord_Relation
Something.limit(5).order('id desc').to_sql
=> "SELECT \"somethings\".* FROM \"somethings\" ORDER BY id desc LIMIT 5"
The latter is an unevaluated scope. You can chain it, or convert it to an array via .to_a. So:
后者是一个未评估的范围。您可以链接它，或通过 将其转换为数组.to_a。所以：
Something.limit(50000).order('id desc').count
... takes a second.
...需要一秒钟。
回答by Developer
Solution is here:
解决方案在这里：
SomeModel.last(5).reverse
Since rails is lazy, it will eventually hit the database with SQL like: "SELECT table.* FROM tableORDER BY table.idDESC LIMIT 5".
由于 rails 是惰性的，它最终会使用 SQL 命中数据库，例如：“SELECT table.* FROM tableORDER BY table. idDESC LIMIT 5”。
回答by Thorin
If you need to set some ordering on results then use:
如果您需要对结果进行排序，请使用：
Model.order('name desc').limit(n) # n= number
if you do not need any ordering, and just need records saved in the table then use:
如果您不需要任何排序，只需要保存在表中的记录，则使用：
Model.last(n) # n= any number
回答by timlentse
In my rails (rails 4.2)project, I use
在我的 rails(rails 4.2)项目中，我使用
Model.last(10) # get the last 10 record order by id
and it works.
它有效。
回答by Joydeep Banerjee
we can use Model.last(5)or Model.limit(5).order(id: :desc)in rails 5.2
我们可以在 Rails 5.2 中使用Model.last(5)或Model.limit(5).order(id: :desc)
回答by Thorin
Just try:
你试一试：
Model.order("field_for_sort desc").limit(5)
