Union fails with mysql2 driver

[maletor]

I'm working with trying to make a union of two queries with ActiveRecord and Arel. I have been able to produce a Gist that proves the sqlite3 driver creates the correct query, while the mysql2 raises an error.

https://gist.github.com/maletor/af1aae5086ba10ed7cad

The failing line is https://github.com/rails/rails/blob/v4.0.8/activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb#L14 where there is an undefined method reverse for nil.

I have also tested this against Rails 4.1.5 and Arel 5.0.

[sodabrew]

Is this a mysql2 gem bug?

[maletor]

@sodabrew, I've also opened a bug on Rails. To be honest, I'm not completely sure, but you can see that only with the mysql2 driver does this fail.

[sodabrew]

Closing here. The issue is well discussed in rails/rails#16978.