When creating associations to classes that use Single Table Inheritance (STI), rails is smarter about the hierarchy than I would have expected. A call to find on a base class yields a query that does not filter on type. But what about a call to find on a subclass? Instead of making a single filter on type, rails finds all the subclasses of that subclass and creates an “OR” expression in the “WHERE” clause.

Generated SQL respects class heirarchy

# ActiveRecord::Base < User < Admin < SuperAdmin
something.admin    # has_one :admin

Generates the following SQL:

# SELECT * FROM users WHERE (type = 'Admin' OR type = 'SuperAdmin')

When it doesn’t work

Since it relies on the class hierarchy, the query is only accurate if every subclass has been loaded. If class preloading is off, for example, very weird things can happen. The query will depend non-deterministically on which subclasses have been loaded.