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ActiveRecord UNION left out

Imported from Lighthouse. Original ticket at: http://rails.lighthouseapp.com/projects/8994/tickets/6591 Created by clyfe - 2011-03-17 19:22:47 UTC

ActiveRecord returns Arel::Nodes::Union when I call model.union(active_relation), but it should return an ActiveRelation

  1. What steps will reproduce the problem:

    class User << AR::Base; end
    User.where(:a => 1).union(User.where(:a => 2))

  2. What is the wrong result:

    The result is an instance of Arel::Nodes::Union

  3. What is the result that should happen instead:

    The computation should return an instance of ActiveRelation

THE GOOD

The resulting Arel::Nodes::Union object can be transformed to_sql and then we can use User.find_by_sql(sql)

a = User.where(:a => 1).union(User.where(:a => 2))
sql = a.to_sql # ( SELECT "users".* FROM "users" WHERE "users"."a" = 1 UNION SELECT "users".* FROM "users" WHERE "users"."a" = 2 )"
User.find_by_sql(sql)

THE ISSUE

The issue is obtaining an ActiveRelation object that we can further chain. Caling a method (select, where, includes) on this (ActiveRelation unioned) object would have the behavior of further calling that method on each of the ActiveRelation objects involved in the UNION

All tough in general a UNION query can be avoided, there are some cases where the corect active relation #union functionality is needed.

For example this wold allow fixing issue #213 of CanCan https://github.com/ryanb/cancan/issues/213

I would be glad to work in this issue, with a little help.