Conditional ruby gem dependencies within a gemspec

Question

I'm building a rails engine which uses foreign keys in migrations.

    add_foreign_key "theblog_content_nodes",
                    "theblog_content_statuses", column: :content_status_id

From the version 4.2 rails supports foreign keys by itself but before we used foreigner gem for this. If we try to use foreigner with rails 4.2 and newer we get an error.

So since I'm going to support rails starting from 4.0.1 I have to use conditional dependency in my gemspec.

I found possible solution here but I have no idea how to check rails version in the gemspec.

# sidekiq-spy.gemspec

if RUBY_VERSION >= '2.1'
  spec.add_development_dependency "curses", "~> 1.0"
end

NOTE:

I have another temporary solution: I just check Foreigner availability in my migrations. If it is unavailable I just don't create foreign keys:

if defined?(Foreigner)
  add_foreign_key "theblog_content_nodes",
                  "theblog_content_statuses", column: :content_status_id
end

But I'd like to add foreigner dependency for old rails versions.

Chapen

Did you think of providing two different versions of your gem for that scenarios? – Krumm

@iltempo, I can even use the one version for both scenarios. Just add note that we have to add gem "foreigner" to Gemfile if Rails version is less than 4.2 and we want to use foreign keys. – Chapen

Answer 1

To access rails version, we can use something like below (based on this answer):

require 'rubygems'

rails_gem = Gem::Specification.select {|z| z.name == "rails"}.max_by {|a| a.version}
p rails_gem.version.version
#=> "4.2.5"

Newness

Unfortunately it seems we have no access to Rails inside the gemspec. I've just got the following error: uninitialized constant Rails from /Users/kont/projects/theblog/theblog.gemspec:21:in 'block in <main>' – Chapen

Updated the answer to use rubygems API. Please check whether this will work for you – Newness

I tried this in both engine itself and in the app using this engine. It seems to work properly. Thanks! – Chapen

Recently The CI failed with the following error: There was a NoMethodError while loading theblog.gemspec: undefined method version' for nil:NilClass from /Users/kont/projects/drevo/theblog/theblog.gemspec:26:in block in <main>' I found that Gem::Specification does not contain Rails yet – Chapen