Atomic update of JSON in Redis

json.lua

# atomic update of a key using LUA.
EVAL "local t1 = cjson.decode(redis.call('get', ARGV[1])); local t2 = cjson.decode(ARGV[2]); for k,v in pairs(t2) do t1[k] = v end; redis.call('set', ARGV[1], cjson.encode(t1))" 0 "foo" '{"bar": 3}'

local id = ARGV[1]
local t1 = cjson.decode(redis.call('get',  id)
local t2 = cjson.decode(ARGV[2])
  
# merge the new hash with the prior hash.
for k,v in pairs(t2) do t1[k] = v end
redis.call('set', id, cjson.encode(t1))