In the current stable, ActiveRecord::Associations::AssociationProxy#method_missing calls yield() if a block is given, causing the block to always be evaluated in its calling context. However, in the case of instance_eval, correct behavior requires that the block be passed directly to the @target, rather than being evaluated inside a different block. Incidentally, this also simplifies the code slightly.

[#3412 state:committed]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>