Question

I’m trying to learn Ruby as well as Ruby on Rails right now. I’m following along with Learning Rails, 1st edition, but I’m having a hard time understanding some of the code.

I generally do work in C, C++, or Java, so Ruby is a pretty big change for me.

I’m currently stumped with the following block of code for a database migrator:

  def self.up
    create_table :entries do |t|
      t.string :name
      t.timestamps
    end
  end

Where is the t variable coming from? What does it actually represent? Is it sort of like the ‘i’ in a for(i=0;i<5;i++) statement?

Also, where is the :entries being defined at? (entries is the name of my controller, but how does this function know about that?)

Answer 1

:entries is a symbol literal, it’s a literal value like 7 or "a string". There’s nothing to define (incidentally, the function doesn’t know about the name of your controller).

t is the parameter to the block you’ve passed in to the create_tables method. What you’ve written here is roughly analogous to something like:

void anonymous_block(Table *t) {
   t->string("name");
   t->timestamps();
}

...

create_table("entries", &anonymous_block);

in C++. create_table calls your block and passes it a parameter, which you’ve named t. I would suggest you get an introductory book for ruby as opposed to rails. I recommend Ruby For Rails by David A. Black.