Reusing code is a determining factor when it comes to the productivity of a programmer.
We refer to other people's code we reuse as a "dependency" since
our program depends on it.
Using a dependency always carries a risk.
Visualisation of modern software development in xkcd
In Ruby the unit of reuse is called a gem. rubygems is the package manager for ruby,
it let's you install gems on your system.
A gem has a name (e.g. rake) and a version number (e.g. 10.4.2).
It can be written in plain ruby or sometimes in ruby and c. Many gems
depend on system libraries that need to be installed before the gem can
be installed. For example the rmagick
gem for image manipulation needs the library ImageMagick.
So most of the time installing a gem is as simple as
>gem install rails_best_practices
Successfully installed rails_best_practices-1.15.7
Parsing documentation for rails_best_practices-1.15.7
Installing ri documentation for rails_best_practices-1.15.7
Done installing documentation for rails_best_practices after 2 seconds
1 gem installed
But sometimes you have to do other installations first.
On your development machine this might look like this:
#install node on a mac
>brew install nodejs
>gem install uglifier
Sometimes you need to set include paths when compiling the c-part of the gem, e.g.:
>gem install eventmachine
... error messages ...
In file included from binder.cpp:20:
./project.h:116:10: fatal error: 'openssl/ssl.h' file not found
#include <openssl/ssl.h>
^
>brew install openssl
>gem install eventmachine ----with-cppflags=-I/usr/local/opt/openssl/include
In production you probably have to deal with Linux, and you
may not have the right permissions to install system libraries.
A typical example would be:
For a rails project you will be using a lot of gems. This will lead
to two problems:
dependency resolution: gem A depends on version 1.1 of gem C, while gem B wants at least version 1.5. You need to find the right version of every gem in your project that actually fits together
different installation: when deploying to a production server, or even just when shareing code with other developers you need to make sure that the same constellation of gems and versions is used on every machine
Bundler is the name of the tool that saves us from dependency hell.
Bundler is itself a gem, so you install it with gem install bundler.
Beware: the command you will be using called bundle, not bundler.
There is how it works: In every ruby project you write
a Gemfile in which you define which gems and (to a certain degree) which versions you want.
When you run bundle install bundler will:
read the Gemfile,
pick out compatible versions of all the gems (if possible),
install all these gems
write Gemfile.lock
The lock-file contains a complete list of all the gems necessary for
your project, and their version numbers. These are now locked down,
and will not change!
When deploying to a new development machine or the production server,
you run bundle install and the exact same versions are now installed.
In the Gemfile you can specify which versions should be used.
But don't overdo it! Bundler does a good job picking versions,
if you specify every version number by hand you are doing too much work.
Some examples of the different ways of specifying version number and source:
The arrow ~> will only allow an increase in the
last (right most) number, so ~> 5.0.0 does allow 5.0.0.7 but not 5.0.1 or 5.1.
This is called a pessimistic version constraint, read more about
it in the rubygem documentation.
The configuration for a gem is found in the initializer directory,
for example for devise the configuration file would be config/initializers/devise.rb.
A gem may install new generators for rails, run rails generate to see a list
of available generators.
A gem may install rake tasks, run rails -T to see a list.
Letting users enter HTML is a dangerous idea - it's really hard to
avoid the security problems. An alternative is to let them enter
a simpler markup language, like Markdown.
You store the markdown in your database, and convert it to HTML when displayed using a gem:
