Hash fields in Redis not ordered the same way as input - redis

Question

I've got an associative array of the type date => data, f.e.:
[
'2015-11-18' => 'some_data',
'2015-11-17' => 'some_data',
'2015-11-16' => 'some_data'
]
and I push them into a hash, where the array key (the date) is the field of the hash and the value is the value... But in Redis they are not sorted in the same order as they were input (and I need them to be). Furthermore, when I get all keys (hkeys) they are ordered in a completely different way of the order they are stored in Redis.
Is there a way to sort them by the same way I input them, both when storing and getting the keys?

Answer 1

You'll need to use two structures to implement an associative array in redis. One way to do it would be to store the keys in-order in a list, and also store the key => value mapping in a hash.
keys list:
[
'2015-11-18',
'2015-11-17',
'2015-11-16'
]
hash:
{
'2015-11-18' => 'some data',
'2015-11-16' => 'some data',
'2015-11-17' => 'some data'
}
You can use scripts to atomically update the two structures. An add operation script could look like:
eval "
redis.call('rpush', KEYS[1], ARGV[1]);
local i = redis.call('llen', KEYS[1]);
return redis.call('hset', KEYS[2], ARGV[1], ARGV[2]);
" 2 'keys' 'values' '2015-11-15' 'some data'
And a remove operation script could look like:
eval "
redis.call('lrem', KEYS[1], ARGV[1]);
return redis.call('hdel', KEYS[2], ARGV[1]);
" 2 'keys' 'values' '2015-11-15'
A get-by-key operation script could look like a normal hash get:
hget 'values' '2015-11-15'
A get-by-index operation script could look like:
eval "
local k = redis.call('lindex', KEYS[1], ARGV[1]);
return redis.call('hget', KEYS[2], k);
" 2 'keys' 'values' 1
To get the keys in-order would be a simple lrange:
lrange 'keys' 0 -1
To get the values in-order, you could use:
eval "
local k = redis.call('lrange', KEYS[1], 0, -1);
return redis.call('hmget', KEYS[2], unpack(k));
" 2 'keys' 'values'

Answer 2

Redis Hashes do not maintain order, nor do they make any assurances with regards to the output's order (a Redis Hash may undergo rehashing during its lifecycle). Look into using Redis' Sorted Sets instead perhaps.