ruby on rails, colon at back or front of variables

What i have understand so far is that :variable in ruby, is to say that this variable will not be able to change, which is similar to constant in other language.

I'm not sure if I understand that statement. In Ruby, constants start with an uppercase letter:

Foo = 1

Reassignment generates a warning:

Foo = 1Foo = 2 #=> warning: already initialized constant Foo

Variables start with a lowercase letter and reassignment doesn't cause a warning (they are supposed to change):

foo = 1foo = 2 # no warning

Symbols start with a colon:

:a_symbol:Uppercase_symbol:"i'm a symbol, too"

They often represent static values, e.g. :get and :post. Symbols are memory efficient, because they are created only once - the same symbol literal always returns the same object. Checking if two symbols are equal is a cheap operation.

Both key: and method: (...) What does that this represent?

This is an alternate syntax for hashes. You can type it in IRB to see the result:

{ foo: 1, bar: 2 }#=> {:foo=>1, :bar=>2}

There are double colons inbetween variables? now I am guessing that Blog: is one variable, and :Application is constant.

No, Blog and Application are both constants and :: is the scope resolution operator. It can be used to access nested constants, e.g.:

module Foo  class Bar    BAZ = 123  endendFoo::Bar::BAZ #=> 123

Rails.application.config.session_store :cookie_store, key: '_blog_session'

session_store is a method that takes two "Arguments":

• :cookie_store is a Symbol
• key: '_blog_session' is actually a short way of writing a Hash.

(could also be session_store :cookie_store, { key: '_blog_session' })

Similarly for link_to "Delete", article, confirm: "Are you sure?", method: :delete

• "Delete" is a string
• article a variable
• { confirm: '...', method: :delete } hash where confirm:, method: and :delete are Symbols again.

While Blog::Application :: is basically a namespace resolution operator. A way for you to address the Application class in the Blog module.

Hope this helps. Have a look at the documentation I referenced, it is explained rather nicely.

:presence => true

presence: true

In the bottom example, the colon is saying, “Hey, I am pointing from presence to true.