STI slow, result of ActiveRecord#find_sti_class should be cached!

Hi,

after getting perftools.rb running with 1.9.2 and rails3rc i found that a lot of cycles are burned inside find_sti_class (actually compute_type). this is caused by looking up a constant fails to load:

base.rb:1141

        # Build a list of candidates to search for
        candidates = []
        name.scan(/::|$/) { candidates.unshift "#{$`}::#{type_name}" }
        candidates << type_name

class A < ActiveRecord::Base
end

class B < A
end

this will set candidates to ["A::B", "B"], now "A::B" will go though the long codepath of constantize - and it will everytime a B is instantiated from the database as the const A::B will never come into existance and we don't do negative caching.

my suggestion would be to add a class level cache to the find_sti_class method. that will improve performace of largish queries with sti.

i would write a patch myself, but failed to get the AR testsuite pass on my box.

re,
thies