Rails:rails 模型的默认排序顺序? [英] Rails: Default sort order for a rails model?

问题描述

我想在我的模型中指定一个默认的排序顺序.

I would like to specify a default sort order in my model.

这样，当我在不指定 .order() 的情况下执行 .where() 时，它使用默认排序.但是如果我指定一个 .order()，它会覆盖默认值.

So that when I do a .where() without specifying an .order() it uses the default sort. But if I specify an .order(), it overrides the default.

推荐答案

default_scope

这适用于 Rails 4+:

default_scope

This works for Rails 4+:

class Book < ActiveRecord::Base
  default_scope { order(created_at: :desc) }
end

对于 Rails 2.3、3，你需要这个:

For Rails 2.3, 3, you need this instead:

default_scope order('created_at DESC')

对于 Rails 2.x:

For Rails 2.x:

default_scope :order => 'created_at DESC'

其中 created_at 是您希望对其进行默认排序的字段.

Where created_at is the field you want the default sorting to be done on.

注意:^ASC 是用于升序的代码，_DESC 用于降序(desc, NOT dsc ！).

一旦你习惯了，你也可以使用scope:

Once you're used to that you can also use scope:

class Book < ActiveRecord::Base
  scope :confirmed, :conditions => { :confirmed => true }
  scope :published, :conditions => { :published => true }
end

对于 Rails 2，您需要 named_scope.

For Rails 2 you need named_scope.

:published 范围为您提​​供 Book.published 而不是Book.find(:published => true).

:published scope gives you Book.published instead of Book.find(:published => true).

从 Rails 3 开始，您可以通过在它们之间用句点连接它们来链接"这些方法，因此对于上述范围，您现在可以使用 Book.published.confirmed.

Since Rails 3 you can 'chain' those methods together by concatenating them with periods between them, so with the above scopes you can now use Book.published.confirmed.

使用此方法，直到需要实际结果(延迟评估)才会实际执行查询，因此可以将 7 个范围链接在一起，但只会产生 1 个实际数据库查询，以避免执行 7 个单独查询时出现性能问题.

With this method, the query is not actually executed until actual results are needed (lazy evaluation), so 7 scopes could be chained together but only resulting in 1 actual database query, to avoid performance problems from executing 7 separate queries.

您可以使用传入的参数，例如 date 或 user_id(会在运行时更改的参数，因此需要使用 lambda 进行惰性求值"，如下所示:

You can use a passed in parameter such as a date or a user_id (something that will change at run-time and so will need that 'lazy evaluation', with a lambda, like this:

scope :recent_books, lambda 
  { |since_when| where("created_at >= ?", since_when) }
  # Note the `where` is making use of AREL syntax added in Rails 3.

最后，您可以通过以下方式禁用默认范围:

Finally you can disable default scope with:

Book.with_exclusive_scope { find(:all) } 

甚至更好:

Book.unscoped.all

这将禁用任何过滤器(条件)或排序(排序依据).

which will disable any filter (conditions) or sort (order by).

请注意，第一个版本适用于 Rails2+，而第二个(无作用域)仅适用于 Rails3+

所以......如果你在想，嗯，那么这些就像方法一样......，是的，这正是这些范围！
它们就像拥有 def self.method_name ...code... end 但与 ruby​​ 一样，它们是很好的小语法快捷方式(或糖")，让您更轻松！

So ... if you're thinking, hmm, so these are just like methods then..., yup, that's exactly what these scopes are!
They are like having def self.method_name ...code... end but as always with ruby they are nice little syntactical shortcuts (or 'sugar') to make things easier for you!

实际上，它们是类级别的方法，因为它们对 1 组所有"记录进行操作.

In fact they are Class level methods as they operate on the 1 set of 'all' records.

然而，它们的格式正在发生变化，在 Rails 4 中，在使用 #scope 而不传递可调用对象时会出现弃用警告. 例如范围 :red， where(color: 'red') 应该更改到 scope :red, ->{ where(color: 'red') }.

Their format is changing however, with rails 4 there are deprecation warning when using #scope without passing a callable object. For example scope :red, where(color: 'red') should be changed to scope :red, -> { where(color: 'red') }.

附带说明，如果使用不当，默认_scope 可能会被误用/滥用.
这主要是关于它何时被用于where 限制(过滤)默认 选择(默认的坏主意)等操作而不仅仅是用于排序结果.
对于 where 选择，只需使用常规命名范围.并在查询中添加该范围，例如Book.all.published 其中 published 是命名范围.

As a side note, when used incorrectly, default_scope can be misused/abused.
This is mainly about when it gets used for actions like where's limiting (filtering) the default selection (a bad idea for a default) rather than just being used for ordering results.
For where selections, just use the regular named scopes. and add that scope on in the query, e.g. Book.all.published where published is a named scope.

总而言之，范围真的很棒，可以帮助您将事情推到胖模型瘦控制器"DRYer 方法的模型中.

In conclusion, scopes are really great and help you to push things up into the model for a 'fat model thin controller' DRYer approach.

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