扶手：如何获取这是2'的has_many'级深的记录？ [英] Rails: How to fetch records that are 2 'has_many' levels deep?

问题描述

我有这些模型建立：

Course
  has_and_belongs_to_many Student

Student
  has_and_belongs_to_many Course
  has_many Books

Book
  belongs_to Student

我如何有效地得到所有的书籍为一个疗程用ActiveRecord？

How do I efficiently get all the Books for a Course with ActiveRecord?

推荐答案

试试这个：

Course.includes(:students => { :books })

文件是这里，根据预先加载关联。

Documentation is here, under "Eager loading of associations".

编辑

对不起，我误解了问题。我看到你的重点是书本上的一个给定的课程。在这种情况下，我建议是这样的：

Sorry, I misread the question. I see your focus is on the books for a given course. In that case I would recommend something like this:

Book.includes(:student => :courses).where(["course.id = ?", @course.id]).limit(5)

这可能会更容易添加此作为对课程模式的方法：

class Course < ActiveRecord::Base
  def books(max = 10)
    Book.includes(:student => :courses).where(["course.id = ?", self]).limit(max)
  end
end

这code可能没有完全正确的，但它应该给你正确的想法。

That code might not be exactly correct, but it should give you the right idea.

此外，您可能想看看<一href="http://stackoverflow.com/questions/2150582/has-many-through-a-has-and-belongs-to-many-association">this问题要问一个潜在的替代解决方案来定义这个东西你自己。

Also, you might want to look at this question for a potential alternate solution to defining this stuff yourself.

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