Rails: MySQL and database url

[PavelPenkov]

Steps to reproduce

Configure database using an URL with mysql protocol, with something like this in database.yml

development:
  url: mysql://user:password@localhost/app_development
  adapter: mysql2

Expected behavior

The application should load

Actual behavior

Instead it crashes with

/home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-5.0.0.rc2/lib/active_support/dependencies.rb:293:in `require': Could not load 'active_record/connection_adapters/mysql_adapter'. Make sure that the adapter in config/database.yml is valid. If you use an adapter other than 'mysql2', 'postgresql' or 'sqlite3' add the necessary adapter gem to the Gemfile. (LoadError)
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-5.0.0.rc2/lib/active_support/dependencies.rb:293:in `block in require'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-5.0.0.rc2/lib/active_support/dependencies.rb:259:in `load_dependency'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-5.0.0.rc2/lib/active_support/dependencies.rb:293:in `require'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activerecord-5.0.0.rc2/lib/active_record/connection_adapters/connection_specification.rb:174:in `spec'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activerecord-5.0.0.rc2/lib/active_record/connection_handling.rb:53:in `establish_connection'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activerecord-5.0.0.rc2/lib/active_record/railtie.rb:125:in `block (2 levels) in <class:Railtie>'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-5.0.0.rc2/lib/active_support/lazy_load_hooks.rb:38:in `instance_eval'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-5.0.0.rc2/lib/active_support/lazy_load_hooks.rb:38:in `execute_hook'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-5.0.0.rc2/lib/active_support/lazy_load_hooks.rb:45:in `block in run_load_hooks'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-5.0.0.rc2/lib/active_support/lazy_load_hooks.rb:44:in `each'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-5.0.0.rc2/lib/active_support/lazy_load_hooks.rb:44:in `run_load_hooks'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activerecord-5.0.0.rc2/lib/active_record/base.rb:324:in `<module:ActiveRecord>'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activerecord-5.0.0.rc2/lib/active_record/base.rb:24:in `<top (required)>'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-5.0.0.rc2/lib/active_support/dependencies.rb:293:in `require'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-5.0.0.rc2/lib/active_support/dependencies.rb:293:in `block in require'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-5.0.0.rc2/lib/active_support/dependencies.rb:259:in `load_dependency'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-5.0.0.rc2/lib/active_support/dependencies.rb:293:in `require'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/spring-1.7.1/lib/spring/application.rb:338:in `active_record_configured?'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/spring-1.7.1/lib/spring/application.rb:259:in `disconnect_database'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/spring-1.7.1/lib/spring/application.rb:97:in `preload'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/spring-1.7.1/lib/spring/application.rb:143:in `serve'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/spring-1.7.1/lib/spring/application.rb:131:in `block in run'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/spring-1.7.1/lib/spring/application.rb:125:in `loop'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/spring-1.7.1/lib/spring/application.rb:125:in `run'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/spring-1.7.1/lib/spring/application/boot.rb:19:in `<top (required)>'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /home/synapse/.rbenv/versions/2.3.0/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from -e:1:in `<main>'

As far as I understand, Rails tries to guess adapter name from protocol, and guesses wrong. Everything works fine when protocol is changed to mysql2

System configuration

Rails 5.0.0.rc2

Ruby 2.3.0

[matthewd]

This is expected behaviour. We've removed the mysql adapter from core, but it would be dangerous to simultaneously alias it to mysql2, which has some observable differences.

Existing applications that are using URLs and intend to use mysql2 should already be using mysql2: in the protocol; they will continue to work. Applications that are currently using mysql: should be forced to choose between using a third-party copy of the old mysql adapter, or consciously switch to mysql2.