ruby - 数据关联 through，到底用什么用？

Question

有3个模型，document, section, paragraph。

d = Document.new

直接执行 d.sections.to_sql 或者 d.paragraphs.to_sql或者是 d.sections.to_sql会报错，因为没有关联。

问题1：

现在把他们关联了，（我测试后发现添加了has_many后显示的d.sections.to_sql或者是其他模型.to_sql在添加through关系前和后打印都是一样的），那么我就不知道这个through到底有什么用？不是多余的吗？

问题2:

添加了如下关系后，d.section.to_sql报错，为什么会出现这样的错误？

irb(main):008:0> d.sections
SystemStackError: stack level too deep
class Document < ActiveRecord::Base
    has_many :paragraphs, through: :sections
    has_many :sections 
end

class Paragraph < ActiveRecord::Base
     belongs_to :section
end

class Section < ActiveRecord::Base
     belongs_to :document
  has_many :paragraphs
end

Answer 1

The relationship between your partner is like this:

Document 1 n Section
Section 1 n Paragraph

Document and Section have a one-to-many relationship, and Section and Paragraph have a one-to-many relationship. It can be concluded that Document has multiple Paragraphs, but they are not directly related, so they are connected through (through) Section association. This is what trough does