This gist by j-mcnally as a rails plugin.
without arguments
If #or is used without arguments, it returns an ActiveRecord::OrChain object that can be used to chain queries with any other relation method, like where:
Post.where("id = 1").or.where("id = 2")
# SELECT `posts`.* FROM `posts` WHERE (('id = 1' OR 'id = 2'))
It can also be chained with a named scope:
Post.where("id = 1").or.containing_the_letter_a
# SELECT `posts`.* FROM `posts` WHERE (('id = 1' OR 'body LIKE \\'%a%\\''))
ActiveRecord::Relation
When #or is used with an ActiveRecord::Relation as an argument, it merges the two relations, with the exception of the WHERE clauses, that are joined using the OR operand.
Post.where("id = 1").or(Post.where("id = 2"))
# SELECT `posts`.* FROM `posts` WHERE (('id = 1' OR 'id = 2'))
anything you would pass to #where
#or also accepts anything that could be passed to the #where method, as a shortcut:
Post.where("id = 1").or("id = ?", 2)
# SELECT `posts`.* FROM `posts` WHERE (('id = 1' OR 'id = 2'))