How to Find Out Which Gem Has a Specific Dependency

How do I find out which gem has a specific dependency?

In the bash shell you can do:

gem dependency name_of_the_gem --reverse-dependencies

For instance:

$ gem dependency activesupport --reverse-dependencies                        
Gem activesupport-2.3.14
Used by
actionpack-2.3.14 (activesupport (= 2.3.14))
activerecord-2.3.14 (activesupport (= 2.3.14))
activeresource-2.3.14 (activesupport (= 2.3.14))

Command for displaying a gem's dependencies?

The following information was pulled from the rubygems command reference linked below.

http://guides.rubygems.org/command-reference/#gem-dependency

The first command you're asking for is "gem dependency". Below is the command description.

gem dependency GEMNAME [options]

Options:
-v, --version VERSION            Specify version of gem to uninstall
-r, --[no-]reverse-dependencies  Include reverse dependencies in the output
-p, --pipe                       Pipe Format (name --version ver)

Common Options:
    --source URL                 Use URL as the remote source for gems
-h, --help                       Get help on this command
    --config-file FILE           Use this config file instead of default
    --backtrace                  Show stack backtrace on errors
    --debug                      Turn on Ruby debugging

Arguments:
GEMNAME   name of gems to show

Summary:
Show the dependencies of an installed gem

Defaults:
--version '> 0' --no-reverse

The second command you'll need is "gem install". Dependencies get installed automatically. Read the quote below from the command reference for more detail.

"gem install" will install the named
gem. It will attempt a local
installation (i.e. a .gem file in the
current directory), and if that fails,
it will attempt to download and
install the most recent version of the
gem you want.

If a gem is being installed remotely,
and it depends on other gems that are
not installed, then gem will download
and install those, after you have
confirmed the operation.

How do I figure out which gem(s) caused a particular gem to be bundled?

Normally Gemfile.lock documents which dependencies were generated from other dependencies, it's listed in a rough tree form, but you may have to do some digging to get to the right spot.

How to see the dependency tree just from Gemfile?

gem dependency (with no args) should show you all gems from current system with their dependencies.

bundle exec gem dependency will show you for the current Gemfile

Edit:

You can also do gem dependency -R (or just dep instead of dependency) if you want to find out which gems use specific (or all) gems.

For deeper dependencies I'd parse output (regex maybe?) of first gem dependencies, pick gem's names and call gem dep on each of them, but that's just a loose idea.

Bundler: find which gem depends on this?

You could try looking inside your Gemfile.lock file.
Dependencies are nested underneath what they are required by.

Rails 5: How Do I Find Out Which Gem Depends on a Specific Ruby Version?

Your case seems like not a gem dependencies, but like you have ruby version definition somewhere in your apps code.

Check this files under your projects root folders:

.rvmrc
.ruby-version
.ruby-gemset
.versions.conf

And also check your Gemfiles for string ruby-2.5.2.

Get all dependencies of a specific version of a Gem on RubyGems

I found the solution. The best (and only way) is to use the dependency file of a gem, who list all dependencies of each version of the gem.
To get this file, it's necessary to call the dependency address of the API : http://guides.rubygems.org/rubygems-org-api/#misc-methods (the last one).

After that, this few line do all the stuff to get dependencies of a specific version :

  url          = URI("https://rubygems.org/api/v1/dependencies?gems=#{gem_name}")
  dependencies = Net::HTTP.get(url)
  data         = Marshal.load(dependencies).each do |dependency|
    break dependency if dependency[:number] == gem_version
  end