In ActiveRecord::Associations::CollectionProxy, a call to respond_to? triggers load_target before checking the proxy_association class, which in turn calls .all on the association, forcing immediate load of all records. In my application this is causing the early load of millions of records, since scopes have not yet been applied.
Check for :accessible_by on proxy_association.klass on active_record
Attached is a suggestion for fixing this on AR, for master and 2.0 branches. No specs included, not sure how best to go about testing it...
See also #398 and linked issues