How to store a map using :dets in Elixir?

Question

I want to be able to store a map using :dets

Currently, that is the solution I am trying to implement:

# a list of strings
topics = GenServer.call(MessageBroker.TopicsProvider, {:get_topics})

# a map with each element of the list as key and an empty list as value
topics_map =
  topics
  |> Enum.chunk_every(1)
  |> Map.new(fn [k] -> {k, []} end)

{:ok, table} = :dets.open_file(:messages, type: :set)

# trying to store the map
:dets.insert(table, [topics_map])

:dets.close(table)

However, I get

** (EXIT) an exception was raised:
    ** (ArgumentError) argument error
        (stdlib 3.12) dets.erl:1259: :dets.insert(:messages, [%{"tweet" => [], "user" => []}])

How is it possible to accomplish this?

Answer 1

Chen Yu's solution is good, but before getting it I already found another solution. Basically, you can just add the map to a tuple

:dets.insert(table, {:map, topics_map})

Then, you can get this map by using

:dets.lookup(table, :map)

Answer 2

I have tested by erlang. You should convert the map to list first.

Following from dets:insert_new() doc

insert_new(Name, Objects) -> boolean() | {error, Reason}
Types
Name = tab_name()
Objects = object() | [object()] 
Reason = term()
Inserts one or more objects into table Name. If there already exists some object with a key matching the key of any of the specified objects, the table is not updated and false is returned. Otherwise the objects are inserted and true returned.

test code

dets:open_file(dets_a,[{file,"/tmp/aab"}]).
Map = #{a => 2, b => 3, c=> 4, "a" => 1, "b" => 2, "c" => 4}.
List_a = maps:to_list(Map). %% <----- this line
dets:insert(dets_a,List_a).

Answer 3

As I understood your intent, you want to store users and tweets under separate keys. For that, you need to construct a keyword list, not a map, in the first place.

topics = for topic <- topics, do: {topic, []}
# or topics = Enum.map(topics, &{&1, []})
# or topics = Enum.map(topics, fn topic -> {topic, []} end)

then you might use this keyword list to create dets.

{:ok, table} = :dets.open_file(:messages, type: :set)
:dets.insert(table, topics)
:dets.close(table)