Incorrect SQL from find with :limit and function call in :order

Given two associated Models one can do:
ModelA.find(:all, :include => :model_b, :order => 'some_field, another_field', :limit => 10)

This works fine. But if we use a function call with comma separated parameter list in the order-by clause,
as in

ModelA.find(:all, :include => :model_b, :order => 'substr(some_field, 3, 5)', :limit => 10)

we get a PGError (Syntaxerror). The generated SQL is:

SELECT * FROM (SELECT DISTINCT ON ("model_a".id) "model_a".id, substr(model_b.some_field AS alias_0, 3 AS alias_1, 5) AS alias_2 FROM "model_a" LEFT OUTER JOIN "model_b" ON "model_b"."model_a_id" = "model_a"."id") AS id_list ORDER BY id_list.alias_0 , id_list.alias_1 , id_list.alias_2 LIMIT 2

Looks like rails generated a field list from the order by clause by separating the clause on commas, regardless
for what purpose they are there.