Rails 5 has been released for a while, recently I was asked to upgrade from rails 4.2 to rails 5.1, it is quite straightforward, but there are some issues worth recording.

First thing try first

After some googling, many people suggest remove all the version lock-in Gemfile and run bundle update, this might help for some projects, but in my case, I got into the issue below for 1 hours, cannot be resolved, but it’s still good to try first.

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Patch update rails first

Doing patch update first to the latest version of 4.2.x, vim Gemfile to ~>
```
gem rails, '~> 4.2.0'
```
then run bundle update
In my case, this will update my rails to rails 4.2.10, without any issue.
```
bundle exec rails s(bundle exec thin start)
```
I have seen a lot of deprecation warnings in the console, I fixed them one by one.

Upgrade to Rails 5.0.0

Edit Gemfile then run bundle update rails
```
gem 'rails', '5.0.0'
```
If your project is anything like mine, you would have a lot of bundler’s cannot find compatible version errors for many gems. I have found most of them are just for development or test, my solution is:
1. checkout the github issues of the gem you are using for supporting rails 5;
2. comment that line out in the Gemfile for now if it’s must have for production
Luckily I commented out a few gems and run bundle update rails, toda installing all the dependencies.
After rails 5 is installed, I uncomment these gems and do bundle install, it auto fetches them again.
Now is actually the good time to upgrade all the Gems as many as possible, but be carefully some will having breaking changes, luckily I have Rspec tests as the baseline, you should have tests before upgrading rails.

Post-install Rails 5

It’s very likely your application will be borken at this point,
```
bundle exec rake rails:update
```
For Rails 5.1, the command is bin/rake app:update. Use git diff to carefully merge your old configurations with the new rails configurations.
Then run bin/rake app:update:bin to update the bin binary, which i think should be included in the git repo.

A list of breaking changes:

Gem quiet_assets, test_after_commit are included in Rails 5, should delete in Gemfile.
Gem factory_girl_rails is renamed to factory_bot_rails.
Enum type, where(field: string) no longer work, changed to where(field: fields[field]) or where(field: :symbol)
Enum finally support _prefix and _suffix
render nothing: true, status: 401 is deprecated, change to head :unauthorized.
belongs_to default to optional: false, so association is required, it’s said you can configure the setting
```
Rails.application.config.active_record.belongs_to_required_by_default = false
```
however, according to the known issue, it cannot be turned off when activerecord-session_store is used.
config.active_job.queue_adapter = :async by default which uses concurrent-ruby. But this is not recommended for production environments.
Finally can use hash#dig, {}.dig(:firstkey, :secondkey)
ActiveRelation#uniq is removed in 5.1, will have to use ActiveRelation#distinct