导轨 - 包括动态条件协会 [英] Rails - Including associations with dynamic conditions

问题描述

由于学校的模式，并具有的has_many关系到学生的学生模型与学校：

Given a school model and a student model with the school having a has_many relation to student:

has_many :students, :conditions => proc  { "year_id=#{send(:active_year_id)}" }

在这里active_year_id是在学校的模型中定义的方法，我遇到一个错误，active_year_id是不确定的打电话时：

where active_year_id is a method defined in the school model, I'm encountering an error that "active_year_id is undefined" when calling:

School.where(:active => true).includes(:students)

条件正常工作时，我做的，比方说，

The condition works fine when I do, say,

School.where(:id => 10).students

只有当我尝试使用包括做我得到这个错误。那是正确的行为。如果不是，有什么我做错了，我如何修复？

Only when I try to use includes do I get that error. Is that the right behavior. If not, what am I doing wrong and how do I fix ?

使用Rails 3.0.9，REE 1.8.7。

Using Rails 3.0.9, REE 1.8.7.

推荐答案

这是一个有点老了，但我看到的问题很多，也没有看到任何满意的解决方案。添加这样的条件基本上等同于创建一个协会有两个公钥/私钥。

This is a bit old, but I see the question a lot and have not seen any satisfactory solutions. Adding a condition like this is essentially equivalent to creating an association with two public/private keys.

@Fabio说得很的在执行PROC取决于它如何被称为是不同的上下文。的不过我认为你可以克服active_year_id是不确定的的问题。

@Fabio is right in saying "the context in where the proc is executed is different depending of how it's called." However I think you can overcome the "active_year_id is undefined" problem.

在这个例子：

class School < ActiveRecord::Base
  has_many :students, :conditions => proc  { "year_id=#{send(:active_year_id)}" }
  ...

的问题是，在某些情况下，在proc是在一个特定的校对象的上下文中执行，有时作为一个ActiveRecord ::协会:: JoinDependency :: JoinAssociation。我使用的是稍微复杂一点的进程内，这样解决了这个：

the issue is that in some situations, the proc is executed in the context of a particular School object and sometimes as an ActiveRecord::Associations::JoinDependency::JoinAssociation. I solved this using a slightly more complicated proc, like this:

class School < ActiveRecord::Base
  has_many :students, :conditions => proc  { 
      self.respond_to?(:active_year_id) ?
        {year_id: self.active_year_id} : 
        'students.year_id = schools.active_year_id'
    }

所以，当条件计算出实际的学校对象，自响应active_year_id属性访问器，你可以提供一个哈希值作为条件（其作品更漂亮不仅仅是一个插字符串创建关联的对象，等等。）

So, when the condition is calculated for an actual school object, self responds to the active_year_id attribute accessor and you can provide a hash as the condition (which works more nicely than just an interpolated string for creating associated objects, etc.)

在上下文中没有present 自作为一个实际的校对象，（正如你提到的，发生时使用的包含条款，例如被称为，）上下文是使用的字段名称，而不是值JoinAssociation和条件的字符串形式的作品就好了。

When the context does not present self as an actual school object, (which, as you noted, occurs when called using an include clause, for example,) the context is a JoinAssociation and the string form of the condition works just fine using field names rather than values.

我们发现这个解决方案让我们成功地使用动态关联。

We found this solution let us use the dynamic association successfully.

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