Mnesia Complex Queries

Hi! I’m using @sheharyarn’s Memento for integrating Mnesia for my Phoenix App. Having little to no experience in erlang, where should I look in the documentation for complex queries, I am storing a map in one of my column and I need to fetch by querying in that map, I can do this in Ecto using fragments but I’m wondering how to do the same using Mnesia.

Usually for complicated queries we would use qlc but I’ve not use it in combination with a map. YMMV.

Please update this thread with the results of your investigation.


Thanks for the suggestion but I lack understanding in Erlang stuffs. Haven’t found any leads in the meantime, I’ll be sticking with the elixir way of doing things and learn Erlang in the process. As a workaround I just stored the nested data I needed to query as a field in my Mnesia table.

Hey @jbrb. It’s absolutely possible doing that with the Erlang Matchspec, but fair warning, the syntax is pretty confusing. Take this table as example:

defmodule Nested do
  use Memento.Table, type: :ordered_set, attributes: [:id, :data]

  def seed do
    nested = [
      %Nested{id: 1, data: %{title: "Elixir",  type: :language,  stars: 15000}},
      %Nested{id: 2, data: %{title: "Phoenix", type: :framework, stars: 13000}},
      %Nested{id: 3, data: %{title: "Memento", type: :library,   stars: 160}},
      %Nested{id: 4, data: %{title: "Ecto",    type: :library,   stars: 4000}},
      %Nested{id: 5, data: %{title: "Ruby",    type: :language,  stars: 16000}},

    Memento.transaction(fn -> Enum.each(nested, &Memento.Query.write/1) end)


This is how you’d match a nested map’s values using the matchspec in Memento:

match_head = {Nested, :"$1", %{title: :"$2", type: :"$3", stars: :"$4"}} 
guards = [{:==, :"$3", :language}]
result = [:"$_"]

Query.select_raw(Nested, [{ match_head, guards, result }])
# => [
#  %Nested{id: 1, data: %{title: "Elixir", type: :language, stars: 15000}},
#  %Nested{id: 5, data: %{title: "Ruby", type: :language, stars: 16000}},

I created an issue in the project github with a lot more examples. You can check them out here:

Reply to this thread on ElixirForum if you have more questions.


@sheharyarn This solved my problem. I quite understand the syntax due to this ElixirSchool example I have read earlier.

Thank you!

Also if you have any lisp experience it becomes trivial, just replace { } with ( ) and your there. :grin: :wink:


Also it is possible to use :ets.fun2ms(...) function, which which translates an Elixir function to match spec expression - and Mnesia too, supports this, since it uses ets/dets:

iex(1)> p = :ets.fun2ms(fn ({{1,_,x,_},_} = rec) when 3 < x and x < 7 -> rec end)
  {{{1, :_, :"$1", :_}, :_}, [{:andalso, {:<, 3, :"$1"}, {:<, :"$1", 7}}],