gistfile1.rb

def association_names_to_joins_argument(names)
  joins = names.pop
  joins = { names.pop => joins } until names.empty?
  joins
end

association_names_to_joins_argument([:memberships])
# => :memberships

association_names_to_joins_argument([:memberships, :members])
# => { memberships: :members }

association_names_to_joins_argument([:users, :posts, :comments])
# => { users: { posts: :comments } }

BTW, even though it won't really apply without modifications in this case, it seems like Tail Call Optimization is still not turned on by default on 2.0.0:

RUBY_VERSION
# => "2.0.0"

RubyVM::InstructionSequence.compile_option
# => {:inline_const_cache=>true,
#     :peephole_optimization=>true,
#     :tailcall_optimization=>false,
#     :specialized_instruction=>true,
#     :operands_unification=>true,
#     :instructions_unification=>false,
#     :stack_caching=>false,
#     :trace_instruction=>true,
#     :debug_level=>0}

More on this on stackoverflow and this "turn by default" ticket.