This snippet checks whether current riak node is a primary for the given key. (Works only for the N=1 case; use itr_pop for N > 1)

is_primary.erl

is_primary_for_doc(BKey) ->    
    {ok, CHBin} = riak_core_ring_manager:get_chash_bin(),
    DocIdx = riak_core_util:chash_key(BKey),
    Itr = chashbin:iterator(DocIdx, CHBin),
    MyNode = node(),
    case chashbin:itr_value(Itr) of
        {Idx, Node} when Node == MyNode -> true;
        {_Idx, _Node} -> false
    end.