From: Jim Weirich <jim@...>
Date: 2007-05-24T06:59:14+09:00
Subject: Re: require_gem vs. gem

Dennis Crissman wrote:
> Fran巽ois Beausoleil wrote:
>> The #gem call activates the gem, and the require is the regular
>> require to load a library.
> 
> What is the difference between an "active" gem and a gem that has been 
> "loaded".

In short:
   Gems are activated (i.e. selected and made available).
   Files are loaded.

The long description:

The 'require' command loads files by searching for the file name in a 
list of directories (called the load path).  If a file is in one of the 
directories in the load path, it is loaded into the Ruby program.

By default, the files in a Gem are not in the load path used by Ruby. 
RubyGems extends the standard 'require' command so that if a file is not 
found, the latest gem containing that file will activated (see below).

When a gem is activated, the load path used by Ruby is adjusted so that 
the files in the gem will be in the load path.  Then Ruby can find any 
file in that gem.

The only time you need to use a 'gem' command in your source code is 
when you want to use a particular version of a gem.  Omitting the gem 
command just means that you get the latest version of whatever gem it is 
found in.

Examples:

# Use the latest version of the activerecord gem:
require 'active_record'

# Use the version 1.14.4 of activerecord
gem 'activerecord', '=1.14.4'
require 'active_record'

Question:  Why is require_gem obsolete?

Answer:  Because it did two things that should be separate.

require_gem would activate a gem (i.e. put its files in the load_path) 
and then autorequire the main gem file.

Activating a gem is something that should be done once in a rather 
centrally organized location in the code.  Requiring files is something 
that should be done in every file that uses code from the file being 
requires.  'require_gem' intermingled those two operations and 
encouraged bad coding practices.

Does that help?

-- Jim Weirich

-- 
Posted via http://www.ruby-forum.com/.