local mt1 = {
    __index = { pie = "yummy" }
}

local mt2 = {
    __index = function(i) return i == "pie" and "yummy" end
}

local t1 = setmetatable({}, mt1)
local t2 = setmetatable({}, mt2)

print(next(t1))
print(next(t2))

nil
nil