How do I know that a gem is compatible with a version of rails?

Question

I'm just getting started with ruby on rails and as I follow the tutorial, there isn't an explanation of how certain gems were gathered and placed in the Gemfile. I've just been copying and pasting them and putting them in my Gemfile and running bundle install.

How does one go about downloading specific versions of gems, and their dependencies as well as making sure that they are compatible with the version of rails I'm using?

Answer 1

Bundler figures out which versions to install depending on your current Rails version if you don't specify the Gem version. Usually Bundler will warn you also when it can't install a version you specified.

gem 'gemname'

This installs whatever version is compatible with your Rails version.

gem 'gemname', '1.5'

This installs version 1.5 only if it supports your current Rails version.

gem 'gemname',  '>=1.0'

This installs version 1.0 or greater if available and compatible.

If you want to install a specific version (2.2) but you know that version 3.0 will break your code (some gems do that like Mailchimp gem) you can specify a minimum version and maximum version:

gem 'gemname', '>= 2.2.0', '< 3.0'

Since it is more or less common there is a shortcut for this:

gem 'gemname', '~> 2.2'

The "~>" will match any version smaller than 3.0. It tells bundler to install only 2.X never reaching 3.0.

Every gem you want to install will tell you which version is compatible with your Rails version. Usually it will say the minimum version number. For example the boostrap gem:

https://rubygems.org/gems/bootstrap/versions/4.0.0.alpha3.1

If you look at the site, it tells you the dependencies. It doesn't mention a minimum Rails version so you can install always the latest version:

RUNTIME DEPENDENCIES (2):
autoprefixer-rails >= 6.0.3
sass >= 3.4.19
DEVELOPMENT DEPENDENCIES (13):
actionpack >= 4.1.5
activesupport >= 4.1.5
capybara >= 2.6.0
compass ~> 1.0.3
jquery-rails >= 3.1.0
json >= 1.8.1
minitest ~> 5.8.0
minitest-reporters ~> 1.0.5
poltergeist >= 0
slim-rails >= 0
sprockets-rails >= 2.3.2
term-ansicolor >= 0
uglifier >= 0

If it specifies a Rails version under dependencies like this:

rails >= 4

It means that you need at least Rails 4.0.

Answer 2

For rails 4 and 5 you can check here. If gem ready or not.

Rails automatically install best suited newest version of gem when you run bundle install if you write gem 'gemname'. Specifying a gem version is mention in other answers.

If a specific version of ruby or rails or any dependency is required in a gem. Then it is specified in gemspec file of gem and Gemfile for that gem.

You can cross-check in gemspec or Gemfile of gem also if something break while bundle install.