接下来，previous记录使用命名范围 [英] Next, Previous Records Using Named Scope

问题描述

我有，我要检索下一个记录（S）和previous结果的模型。我想通过模型上的named_scope要做到这一点，同时也传递作为参数的下一个/ previous记录在X号返回。

I have a model for which I want to retrieve the next record(s) and previous record(s). I want to do this via a named_scope on the model, and also pass in as an argument the X number of next/previous records to return.

例如，假设我有5个记录：

For example, let's say I have 5 records:

• 记录1
• RECORD2
• RECORD3
• RECORD4
• Record5

我希望能够调用模型。$ ​​P $ pvious或模型。$ ​​P $ pvious（1）返回RECORD2。同样，我希望能够调用Model.next或Model.next（1）返回RECORD4。再举一个例子我希望能够调用模型。$ ​​P $ pvious（2）返回RECORD3。我觉得你的想法。

I want to be able to call Model.previous or Model.previous(1) to return Record2. Similarly, I want to be able to call Model.next or Model.next(1) to return Record4. As another example I want to be able to call Model.previous(2) to return Record3. I think you get the idea.

我怎样才能做到这一点？

How can I accomplish this?

推荐答案

要实现类似'模式。previous'，类本身就必须有一个当前的状态。这将使意义，如果当前的记录（或许在发布调度系统？）是RECORD3在你的榜样，但你的例子并不表明这一点。

To implement something like 'Model.previous', the class itself would have to have a 'current' state. That would make sense if the 'current' record (perhaps in a publishing scheduling system?) was Record3 in your example, but your example doesn't suggest this.

如果你想利用一个模型的实例，并获得下一个或previous记录，下面是一个简单的例子：

If you want to take an instance of a model and get the next or previous record, the following is a simple example:

class Page < ActiveRecord::Base
  def previous(offset = 0)    
    self.class.first(:conditions => ['id < ?', self.id], :limit => 1, :offset => offset, :order => "id DESC")
  end

  def next(offset = 0)
    self.class.first(:conditions => ['id > ?', self.id], :limit => 1, :offset => offset, :order => "id ASC")
  end
end

如果这样你可以这样做：

If so you could do something like:

@page = Page.find(4)
@page.previous

另外的工作将是：

Also working would be:

@page.previous(1)
@page.next
@page.next(1)

显然，这种假设的'下一个'和'previous'的理念是由'身份证'字段，这可能是没有在应用程序的寿命延长很好。

Obviously, this assumes that the idea of 'next' and 'previous' is by the 'id' field, which probably wouldn't extend very well over the life of an application.

如果你确实想使用这个类上，你也许可以扩展该到一个名为范围，是以当前记录作为参数。事情是这样的：

If you did want to use this on the class, you could perhaps extend this into a named scope that takes the 'current' record as an argument. Something like this:

named_scope :previous, lambda { |current, offset| { :conditions => ['id < ?', current], :limit => 1, :offset => offset, :order => "id DESC" }}

这意味着你可以调用：

Which means you could call:

Page.previous(4,1)

在哪里'4'是要启动对记录的ID，1是要导航回的数量。

Where '4' is the id of the record you want to start on, and 1 is the number you want to navigate back.

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