Should Gemfile.lock be included in .gitignore?

The real problem happens when you are working on an open-source Rails app that needs to have a configurable database adapter. I'm developing the Rails 3 branch of Fat Free CRM. My preference is postgres, but we want the default database to be mysql2.

In this case, Gemfile.lock still needs be checked in with the default set of gems, but I need to ignore changes that I have made to it on my machine. To accomplish this, I run:

git update-index --assume-unchanged Gemfile.lock

and to reverse:

git update-index --no-assume-unchanged Gemfile.lock

It is also useful to include something like the following code in your Gemfile. This loads the appropriate database adapter gem, based on your database.yml.

# Loads the database adapter gem based on config/database.yml (Default: mysql2)
# -----------------------------------------------------------------------------
db_gems = {"mysql2"     => ["mysql2", ">= 0.2.6"],
           "postgresql" => ["pg",     ">= 0.9.0"],
           "sqlite3"    => ["sqlite3"]}
adapter = if File.exists?(db_config = File.join(File.dirname(__FILE__),"config","database.yml"))
  db = YAML.load_file(db_config)
  # Fetch the first configured adapter from config/database.yml
  (db["production"] || db["development"] || db["test"])["adapter"]
else
  "mysql2"
end
gem *db_gems[adapter]
# -----------------------------------------------------------------------------

I can't say if this is an established best practice or not, but it works well for me.

Update for 2022 from TrinitronX

Fast-forward to 2021 and now Bundler docs [web archive] now say to commit the Gemfile.lock inside a gem... ¯_(ツ)_/¯ I guess it makes sense for developers and ease of use when starting on a project. However, now CI jobs need to be sure to remove any stray Gemfile.lock files to test against other versions.

Legacy answer ~2010

Assuming you're not writing a rubygem, Gemfile.lock should be in your repository. It's used as a snapshot of all your required gems and their dependencies. This way bundler doesn't have to recalculate all the gem dependencies each time you deploy, etc.

From cowboycoded's comment below:

If you are working on a gem, then DO NOT check in your Gemfile.lock. If you are working on a Rails app, then DO check in your Gemfile.lock.

Here's a nice article explaining what the lock file is.