why bundle exec rake db:migrate

Some Ruby Idioms

• Methods such as empty! and empty?

Ruby method names can end with an exclamation mark (a bang method) or a question mark (a predicate method). Bang methods normally do something destructive to the receiver. Predicate methods return true or false depending on some condition.

• a

  b

The expression a || b evaluates a. If it isn ’t false or nil, then evaluation stops, and the expression returns a. Otherwise, the statement returns b. This is a common way of returning a default value if the first value hasn ’t been set.

• a

  = b

The assignment statement supports a set of shortcuts: a op= b is the same as a = a op b. This works for most operators.

count += 1 # same as count = count + 1
price *= discount # price = price * discount
count ||= 0 # count = count || 0

• obj = self.new

Sometimes a class method needs to create an instance of that class.

class Person < ActiveRecord::Base
	def self.for_dave
		Person.new(name: 'Dave' )
	end
end

This works fine, returning a new Person object. But later, someone might subclass our class.

class Employee < Person
	# ..
end
dave = Employee.for_dave # returns a Person

The for_dave() method was hardwired to return a Person object, so that’s what is returned by Employee.for_dave. Using self.new instead returns a new object of the receiver’s class, Employee.

• lambda

The lambda operator converts a block into an object of type Proc. An alternate syntax, introduced in Ruby 1.9, is ->.

• require File.expand_path(‘../../config/environment’, FILE)

Ruby’s require method loads an external source file into our application. This is used to include library code and classes that our application relies on. In normal use, Ruby finds these files by searching in a list of directories, the LOAD_PATH.

Sometimes we need to be specific about what file to include. We can do that by giving require a full filesystem path. The problem is, we don ’t know what that path will be —our users could install our code anywhere.

Wherever our application ends up getting installed, the relative path between the file doing the requiring and the target file will be the same. Knowing this, we can construct the absolute path to the target by using the File.expand_path() method, passing in the relative path to the target file, as well as the absolute path to the file doing the requiring (available in the special variable FILE).

In addition, there are many good resources on the Web showing Ruby idioms and Ruby gotchas. Here are just a few:

1. http://www.ruby-lang.org/en/documentation/ruby-from-other-languages/
2. http://en.wikipedia.org/wiki/Ruby_programming_language
3. http://www.zenspider.com/Languages/Ruby/QuickRef.html