Redis - Lua tables as return values - why is this not working

Question

When I run this code through redis EVAL it return no results. Any idea why this is not working?

redis-cli EVAL "$(cat bug.lua)" 0

bug.lua

local retv = {}
retv["test"] = 1000

return retv

If I initialize the table that value alone gets printed.

$ cat bug.lua 
--!/usr/bin/env lua


local retv = {"This", "is", "a", "bug" }
retv["test"] = 1000

return retv

$ redis-cli EVAL "$(cat bug.lua)" 2 a b
1) "This"
2) "is"
3) "a"
4) "bug"

Answer 1

If you refer to the Redis EVAL documentation you can see what are the rules Redis uses to convert a Lua table into a Redis reply:

1. Lua table (array) -> Redis multi bulk reply (truncated to the first nil inside the Lua array if any)
2. Lua table with a single ok field -> Redis status reply
3. Lua table with a single err field -> Redis error reply

So except with special cases 2 and 3, Redis assumes your table is a sequence (i.e list) which means it reads retv[1], retv[2], ... until it encounters a nil element (here is the corresponding source code section).

This explains why retv["test"] is ignored in your case.

If you change your code with:

local retv = {"This", "is", "a", "bug" }
retv[5] = 1000
return retv

Then this additional element gets returned:

1) "This"
2) "is"
3) "a"
4) "bug"
5) (integer) 1000

Answer 2

Answer from @deltheil is valid.

Remember though that you can use cjson library to pack tables and pass them to consumers.

Your lua file:

local retv = {"This", "is", "a", "bug" }
retv["test"] = 1000

return cjson.encode(retv)

Command:

redis-cli EVAL "$(cat bug.lua)" 0

Result:

"{\"1\":\"This\",\"2\":\"is\",\"3\":\"a\",\"4\":\"bug\",\"test\":1000}"