如何使用:dets在药剂中存储地图？

Question

我希望能够使用:dets存储地图。

目前，这是我正在努力实现的解决方案：

# 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)

然而，我得到

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

如何才能做到这一点？

Answer 1

陈郁的解决方案是好的，但在得到之前，我已经找到了另一个解决方案。基本上，您可以将地图添加到元组中。

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

然后，您可以使用

:dets.lookup(table, :map)

Answer 2

我被erlang测试过了。你应该先把地图转换成列表。

新()医生

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.

测试代码

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

正如我理解您的意图一样，您希望将users和tweets存储在单独的密钥下。为此，首先需要构造关键字列表，而不是地图。

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

然后您可以使用这个关键字列表来创建dets。

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