Can you explain what's going on in this Ruby code?

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.

Answer 2

I'll take the t thing. The create_table method is like a C function that takes a function pointer that takes one argument, the table definition object (forgive my non-existent C skills):

void entries_table_constructor(TableDef table_definition) {
  table_def_add_string_column(table_definition, "name");
  table_def_add_timestamps_column(table_definition);
}    

create_table("entries", entries_table_constructor);

But in Ruby, the definition of the passed function can be done at the moment the create_table method is called. So the bit between the do and end is like the entries_table_constructor function, and the t variable is like the table_definition argument.

There is a big difference between function pointers in C and blocks in Ruby however. In Ruby, all the local variables outside the block are available inside the block:

a = 10
create_table :entries do |t|
  puts a
  ...
end

Check out the yield keyword in Ruby to see how to write your own methods like create_table.