The initial pain point when upgrading a rails app is figuring out which of your dependencies are blocking you upgrading the actual rails gem (& immediate dependencies, actionpack, etc.). One way to start this is to update the rails dependency in your Gemfile and run bundle update rails. Then check the error output (it never works first time…) to see which gems are blocking the upgrade. Repeat, rinse until it works.

I figured I’d cheat a little and eyeball the Gemfile.lock to see which gems had an explicit dependency pinning rails (or actionpack, activejob, etc) to a version lower than I want to upgrade to, so I could get an idea of what needs to be upgraded without having to do them all one-by-one.

Then instead of eyeballing Gemfile.lock, I wrote an awk script to pull out the interesting dependencies (ie, anything that depends on rails gems) so I just have to check which versions they depend on by hand.

# Reads a Gemfile.lock and outputs all dependencies that depend on rails

BEGIN {
  parent = 0
  parent_printed = 0
  rails_gems = "^(rail(s|ties)|action(mailer|pack|view)|active(job|model|record|support))$"
}

# We only want the specs from the GEM section
NR == 1, $1 ~ /GEM/ { next }
$1 == "" { exit }

# Skip parent gems we don't care about (rails itself…)
$0 ~ /^ {4}[^ ]/ &&
$1 ~ rails_gems {
  parent = 0
  parent_printed = 0
  next
}

# Parent gems that aren't part of rails core
# Store the name to be printed if we match below
$0 ~ /^ {4}[^ ]/ {
  parent = $0
  parent_printed = 0
  next
}

# If the nested gem (6 space prefix) matches rails-names and we have a parent value
# set then we print them out - making sure to only print the parent once
$0 ~ /^ {6}[^ ]/ &&
$1 ~ rails_gems &&
parent != 0 {
  if (parent_printed == 0) {
    parent_printed = 1
    print parent
  }

  print $0
}

Run it against your Gemfile.lock for the app you’re upgrading:

awk -f rails5.awk Gemfile.lock

And you’ll get output like this, to run through and see if any of the dependencies are pinning to lower versions than you need.

    coffee-rails (4.0.1)
      railties (>= 4.0.0, < 5.0)
    factory_girl (4.4.0)
      activesupport (>= 3.0.0)
    factory_girl_rails (4.4.1)
      railties (>= 3.0.0)
    globalid (0.3.7)
      activesupport (>= 4.1.0)
    google-api-client (0.8.6)
      activesupport (>= 3.2)
    jquery-rails (3.1.4)
      railties (>= 3.0, < 5.0)
    jquery-ui-rails (5.0.5)
      railties (>= 3.2.16)
    rails-deprecated_sanitizer (1.0.3)
      activesupport (>= 4.2.0.alpha)
    rails-dom-testing (1.0.7)
      activesupport (>= 4.2.0.beta, < 5.0)
    rspec-rails (3.4.2)
      actionpack (>= 3.0, < 4.3)
      activesupport (>= 3.0, < 4.3)
      railties (>= 3.0, < 4.3)
    sass-rails (4.0.5)
      railties (>= 4.0.0, < 5.0)
    sprockets-rails (2.3.3)
      actionpack (>= 3.0)
      activesupport (>= 3.0)

In this case, I’m trying to take this app to rails 5.0, so all the ones specifying < 5 and < 4.3 need upgrading beforehand.