Plugin Details
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This is an extremely naive 'railsification' of a SQL union.

The first parameter, parts, is an array of hashes. Each hash is what you would normally send into a single find and represents each SELECT that will eventually be unioned together.

The second paramter, options, is a hash of remaining options to be applied to the UNION of the parts (ie: order, limit, offset).

A simple (and useless) example would be:
User.union([{:conditions => ['name = ?', 'tom']}, {:conditions => ['name = ?', 'gary']}], {:order => 'created_at'})

This example produces the following SQL:
(SELECT * FROM `users` WHERE (name = 'tom')) UNION (SELECT * FROM `users` WHERE (namae = 'gary')) ORDER BY created_at;

Essentially you can do any union, but it's up to you to make sure you don't pass the wrong stuff in because it's a pretty dumb implementation.

License
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This plugin is available under the MIT license.

Author
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Thomas Mango
http://slicedsoftware.com