CODEWITHSUNDEEP
ruby-on-railsrubysymbols
Peter.Ward
Peter.Ward

Reputation: 7

Symbols versus variables

I'm trying to migrate my database in a ruby on rails app, and I'm a bit confused about the proper syntax shown below:

class CreateCategories < ActiveRecord::Migration
    def change 
        create_table :categories do |t|
            t.string :name
            t.string :thumburl
            t.timestamps
        end
    end
end

(1) Why is a symbol :Migration being used at the class naming area? The way I'm reading it is that a class, CreateCategories, is being created, and it inherits all attributes of the ActiveRecord class. I was taught that the double colon signifies a class method call, yet the syntax coloring makes me think the :Migration component is actually a symbol. What's going on here?

(2) I get that create_table is a method, yet why use :categories, i.e. a symbol, instead of a class or instance variable, i.e. @@categories or @categories? I suppose you have to name the table that's being created, but why a symbol?

(3) Why are :name and :thumburl symbols and not variables?

Upvotes: 0

Views: 91

Answers (2)

Lesleh
Lesleh

Reputation: 1675

ActiveRecord::Migration isn't a symbol, it's the name of the superclass being inherited from. :: is the scope resolution operator, so Migration is a member of the ActiveRecord module.

As for why symbols are being used in various places, they're used in place of where you might otherwise use a String, such as giving the table a name. You could use 'categories' but as symbols are essentially just a pointer to an object, there's a certain amount of extra efficiency in terms of memory storage, as you don't have a new string being created everywhere that symbol is used.

To illustrate this idea, take the following script for example:

puts 'categories'.object_id
puts 'categories'.object_id

puts '---------------------'

puts :categories.object_id
puts :categories.object_id

Running of the script might produce something similar to this:

19608760
19608660
---------------------
839388
839388

The string versions have two different IDs, which means two separate String objects were created. The symbols version, however, prints the same ID, which means only a single object was created to represent the symbol, and the same object is used wherever the symbol is used.

Upvotes: 2

sawa
sawa

Reputation: 168071

(1) Your tokenization :Migration is wrong. It should be understood as ::Migration. The :: is a scope resolution operator. It means Migration as understood within the namespace of ActiveRecord. You are right that :: is used as a class method call. Actually, :: has two uses: the scope resolution operator, and a class method call; it is ambiguous. Here, it is the former.

(2, 3) @@categories or @categories or any other variable will be evaluated to a certain value it holds. It would not mean anything like "category". In order to tell something like "category", expressing it as "category" in a string, or :category as in a symbol is the way to go.

Upvotes: 0

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