Can not use mnesia disc_copies

[Yao Bao]

Hello,

I have a web application which uses mnesia as the database.

Everything works fine when I use ram_copies.
However, I would like to use disc_copies for production. Then, I encounter an error when I start the database:
==== begin ====
X = mnesia:create_schema([node()])
X = {error,{nonode@REDACTED,{already_exists,nonode@REDACTED}}}
==== end ====

The output of mnesia:info/0 is (from `$ rebar3 shell`):
==== begin ====
---> Processes holding locks <--- 
---> Processes waiting for locks <--- 
---> Participant transactions <--- 
---> Coordinator transactions <---
---> Uncertain transactions <--- 
---> Active tables <--- 
schema         : with 1        records occupying 428      words of mem
===> System info in version "4.18", debug level = none <===
opt_disc. Directory "/Users/by/projects/mlchat_server/db_data" is NOT used.
use fallback at restart = false
running db nodes   = [nonode@REDACTED]
stopped db nodes   = [] 
master node tables = []
remote             = []
ram_copies         = [schema]
disc_copies        = []
disc_only_copies   = []
[{nonode@REDACTED,ram_copies}] = [schema]
2 transactions committed, 1 aborted, 0 restarted, 0 logged to disc
0 held locks, 0 in queue; 0 local transactions, 0 remote
0 transactions waits for other nodes: []
ok
==== end ====

The "sys.config" for mnesia is:
{mnesia, [{dir, "./db_data"}]}

The documentation says:
mnesia:create_schema/1 fails if any of the Erlang nodes given as DiscNodes are not alive, if Mnesia is running on any of the nodes, or if any of the nodes already have a schema.

Here is the code to start the database:
start() ->
    X = mnesia:create_schema([node()]),
    Y = mnesia:start(),
    Z = mnesia:create_table(chat_room,
                        [
                         {type, ordered_set},
                         {disc_copies, [node()]}, %% If I comment out this line, everything works fine.
                         {attributes, record_info(fields, chat_room)}
                        ]),
    io:format("==== X: ~p, Y: ~p, Z: ~p~n", [X, Y, Z]).

And here is the output:
1> ==== X: {error,{nonode@REDACTED,{already_exists,nonode@REDACTED}}}, Y: ok, Z: {aborted,
                                                                           {bad_type,
                                                                            chat_room,
                                                                            disc_copies,
                                                                            nonode@REDACTED}}

I guess my usage for mnesia might be wrong, but I have no clue.

Cheers,
Yao

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