什么是def to_s功能？

Question

I'm going through the "Tags" section of the Blogger tutorial and am a bit confused on one part: the def to_s function (in tag.rb); why it's required and how it's included.

I've included some relevant parts of relevant files for context.

MODELS

article.rb

class Article < ActiveRecord::Base
  attr_accessible :tag_list
  has_many :taggings
  has_many :tags, through: :taggings

  def tag_list
    return self.tags.collect do |tag|
      tag.name
    end.join(", ")
  end

  def tag_list=(tags_string)
    self.taggings.destroy_all

      tag_names = tags_string.split(",").collect{|s| s.strip.downcase}.uniq

      tag_names.each do |tag_name|
        tag = Tag.find_or_create_by_name(tag_name)
        tagging = self.taggings.new
        tagging.tag_id = tag.id
      end
    end
  end

tag.rb

class Tag < ActiveRecord::Base
  has_many :taggings
  has_many :articles, through: :taggings

  def to_s
    name
  end
end

class Tagging < ActiveRecord::Base
  belongs_to :tag
  belongs_to :article
end

CONTROLLERS

class TagsController < ApplicationController

  def index
    @tags = Tag.all
  end

  def show
    @tag = Tag.find(params[:id])
  end

  def destroy
    @tag = Tag.find(params[:id]).destroy
    redirect_to :back
  end
end

HELPERS

module ArticlesHelper

  def tag_links(tags)
    links = tags.collect{|tag| link_to tag.name, tag_path(tag)}
    return links.join(", ").html_safe
  end
end

VIEWS

<%= form_for(@article, html: {multipart: true}) do |f| %>
  <p>
    <%= f.label :tag_list %>
    <%= f.text_field :tag_list %>
  </p>
  <p>
    <%= f.submit %>
  </p>
<% end %>

Tags: <%= tag_links(@article.tags) %>

Answer 1

I got your point. When you concatenate value in string you have to write eg

"hello #{@user.name}" 

So instead of calling @user.name u can specify whatever u have to display user as a string you can directly specify in to_s method so that you dont need to call .to_s again just write

"hello #{@user}"

above line of code search for .to_s method for @user's class and print returning value.

same is for routing like

user_path(@user)

will give you >> users/123 # where 123 is id of @user

if you write

def to_params
 self.name
end

Then it will give >> users/john # where john is the @user's name

Answer 2

to_s is the standard Ruby method for converting an Object into a string. You define to_s when you want a custom string representation for your class. Example:

1.to_s #=> "1"
StandardError.new("ERROR!").to_s #=> "ERROR!"

And so on.

Answer 3

to_s is a function that returns the string equivalent of the object.

In this case, you have Tag.to_s defined, which allows you to do something like this

tag = Tag.new(:name => "SomeTag")
tag.to_s #=> "SomeTag"

You can add more logic to to_s to have a more friendly string, rather than an object hashcode when you want string from a tag (ex. while printing to console).