Ruby on Rails- :symbols、@iVars 和“字符串”-天哪！

Question

New to Rails and trying to get my head around when/why to use :symbols , @ivars , "strings" within the framework.

I think I understand the differences between them conceptually

• only one :symbol instance per project
• one @ivar per instance
• multiple "strings" - as they are created whenever referenced (?)

Feel free to correct me!

The main confusion comes from understanding the rules & conventions of what Rails expects - where and WHY?

I'm sure there's an "Ah ha!" moment coming but I haven't had it yet...as it seems pretty arbitrary to me (coming from C/Obj-C).

-thx

Answer 1

The @instance_variable is an instance variable. It is usually defined in the controller and accessible in the views.

The "string" is a string, like as in any other language.

The :symbol , is as you mentioned it's an efficient way of representing names and strings; they are literal values. It is initialized and exists only once during the ruby session. It's not a string, since you don't have access to String methods; it's a Symbol. On top of that, it's immutable. For those reasons, it becomes very handy in representing keys in hashs. Rails methods uses hashes, thus, you find symbols a bit everywhere in Rails.

Answer 2

Instance variables are pretty straightforward: they track properties/values of a particular instance, so you use them when you the values will vary across instances.

Symbols vs. strings are a bit more arbitrary. Symbols are generally used for constant values, in much the same way that a language such as C would use enums; Ruby doesn't have enums, so symbols are often used to fill that gap. Strings are used for more varied pieces of text that won't be used as a flag or similar constant.

Answer 3

Symbols are kind of like pointers (not in the C-ish way, but in C-ish thinking, they point). Well, you use symbols when you are manipulating properties. They are one of the great benefits of dynamic typing if you'd ask me. (For potential voters I do not mean any harm, I do know that they are not pointers, but it felt 'ah-ha!' for me).

:action => "index"

Instance variables are needed when you fetch data from your model and you want to use them across your views (inside your controller method).

def my_controller_method
@myposts = Post.find(:all)
end

# inside view
<% for @myposts do |m| %>
<i><%= m.title %></i>
<% end %>

Just a heads up, the rules and conventions kinda change rapidly (as I discovered on my Rails journey) quite a lot per version. Having the right guide with the right Rails helps. Good luck with coding!

Answer 4

Instance variables don't really belong in the same list as strings and symbols. Strings and Symbols are types of classes whereas instance variables are a type of variable . So instance variables ( @var ) are just a way to store a value between methods of one instance of one class:

class Calculator
  @counter = 0

  def inc
    @counter += 1
  end

  def dec
    @counter -= 1
  end
end

Here is a good article on the distinction between symbols and strings.

Answer 5

Rails 控制器通过 ORM（对象关系映射）通过 Models 访问 rails 数据库，即 Model 类将映射到其对应的表，Objects 直接映射到表中的行。为了获取给定用户查询的结果，实例变量 (@instance_variable) 是处理它的完美选择。