ActiveRecord Arel OR condition

Question

How can you combine 2 different conditions using logical OR instead of AND?

NOTE: 2 conditions are generated as rails scopes and can't be easily changed into something like where("x or y") directly.

Simple example:

admins = User.where(:kind => :admin)
authors = User.where(:kind => :author)

It's easy to apply AND condition (which for this particular case is meaningless):

(admins.merge authors).to_sql
#=> select ... from ... where kind = 'admin' AND kind = 'author'

But how can you produce the following query having 2 different Arel relations already available?

#=> select ... from ... where kind = 'admin' OR kind = 'author'

It seems (according to Arel readme):

The OR operator is not yet supported

But I hope it doesn't apply here and expect to write something like:

(admins.or authors).to_sql

Answer 1

ActiveRecord queries are ActiveRecord::Relation objects (which maddeningly do not support or), not Arel objects (which do).

[ UPDATE: as of Rails 5, "or" is supported in ActiveRecord::Relation; see https://stackoverflow.com/a/33248299/190135 ]

But luckily, their where method accepts ARel query objects. So if User < ActiveRecord::Base...

users = User.arel_table
query = User.where(users[:kind].eq('admin').or(users[:kind].eq('author')))

query.to_sql now shows the reassuring:

SELECT "users".* FROM "users"  WHERE (("users"."kind" = 'admin' OR "users"."kind" = 'author'))

For clarity, you could extract some temporary partial-query variables:

users = User.arel_table
admin = users[:kind].eq('admin')
author = users[:kind].eq('author')
query = User.where(admin.or(author))

And naturally, once you have the query you can use query.all to execute the actual database call.