hakarabakara
Checking if records exist with Memento
I am using memento to wrap Mnesia functionality into my application.
I want to check if a record with a certain id exists and if not perform certain actions.
Documentation says read should return nil when record doesn’t exist. Howerver, the Memento.transation wrapper aborts with a :not_exists exception therefore my application cannot proceed.
Here’s how my function looks like:
def find_conversation(user_id) do
Memento.transaction! fn ->
user_id
|> read
|> to_conversation
end
end
defp to_conversation(nil), do: nil
defp to_conversation(%@store{} = conversation) do
struct(Conversation, Map.from_struct(conversation))
end
Most Liked
Exadra37
I cannot test it now, but I think you need to call to_conversation/1 outside the Memento transaction.
This is how I do in my app…
To get a resource:
def get_todo!(uid) do
_read_transaction(uid)
end
To read it from Mnesia:
defp _read_transaction(uid) do
result = Memento.transaction fn -> Memento.Query.read(Todo, uid) end
_parse_fetch_result(result)
end
To parse the result of reading it from Mnesia:
defp _parse_fetch_result(result) do
case result do
{:ok, nil} ->
[]
{:ok, []} ->
[]
{:ok, todo} ->
todo
{:error, error} ->
Logger.error("_parse_resul/1: #{error}")
Todo.new_changeset()
end
end
To note that the returned values foreach case are specific to my app.
Also bear in my mind that is app serves as a Learning path for me to know more about Mnesia and Elixir, thus I may also not doing it in the best way 
Popular in Questions
Other popular topics
Latest on Elixir Forum
Sponsor Spotlight
Producing high quality Elixir screencasts since 2017.
Categories:
Sub Categories:
Forums
Our Sponsors
Build Elixir applications with speed and confidence.
Supporting innovation across the BEAM ecosystem.
We build reliable cloud platforms for business-critical systems.
Catch errors, track performance, monitor hosts and more.
Practical resources that improve the lives of professional developers.
Develop your skills with books, videos, and courses.
Producing high quality Elixir screencasts since 2017.
Courses that'll move you from confusion to "Aha, now I get it!"