How to pattern match list of structs from mysql results for multiple conditions?

Questions / Help
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1

I’m trying to build a simple function to see if a user is authorized to join a conversation. Basically, it needs to query the mysql database to get a list of all users in the conversation, and then compare the userid on the socket to the results list.

This is what I have so far:

 def join("conv:" <> convid, _message, socket) do
    if authorized?(convid, socket) do
      IO.inspect("join conversation " <> convid)
      send(self(), :after_join)
      {:ok, socket}
    else
      IO.inspect("not authorized to join room")
      :error
    end
  end

 defp authorized?(convid, socket) do
    IO.inspect("run conv auth function")
    results =
    from(m in GetUsersInConv, # use lib/app_web/schemas/usersinconv.ex
      where: m.id == ^convid,
      select: %{userid: m.userid, status: m.status}
    )
    |> Repo.all()
    userIdInSocket = socket.assigns.user_id
    IO.inspect("socket userid: " <> userIdInSocket)
    IO.inspect(results)
    # Need to add matching function next...
  end

These are the results I get, a list of all userids in the conversation and their status:

"run conv auth function"
[debug] QUERY OK source="conversations" db=0.7ms idle=1289.4ms
SELECT p0.`userid`, p0.`status` FROM `conversations` AS p0 WHERE (p0.`id` = ?) [107876]
"socket userid: 1"
[
  %{status: 0, userid: 1},
  %{status: 0, userid: 299},
  %{status: 0, userid: 664},
  %{status: 0, userid: 782},
  %{status: 0, userid: 1035},
  %{status: 0, userid: 4609},
  %{status: 0, userid: 4684},
  %{status: 0, userid: 5974},
  %{status: 0, userid: 6755},
  %{status: 0, userid: 6755},
  %{status: 0, userid: 8980},
  %{status: 0, userid: 9246},
  %{status: 0, userid: 10759},
  %{status: 0, userid: 15517},
  %{status: 0, userid: 16502},
  %{status: 0, userid: 32428},
  %{status: 0, userid: 61240},
  %{status: 0, userid: 79131},
  %{status: 0, userid: 98521},
  %{status: 3, userid: 145222}
]

If the userid of the socket is “1”, how can I build an Enum iteration function to match the following conditions:

  1. userid “1” is found within the result list of userids
  2. status != 3 AND status != 7 for that user as well.

If all those conditions are met, return “true”, otherwise “false” for the entire “authorized?” function.

I’m getting better and better at figuring out elixir, but syntax for enum and pattern matching is still a little tricky.

Thanks!

2

Both of these are expressible in SQL - for instance, you could rephrase the above as:

“does a row exist with user_id = 1 and status != 3 and status != 7”

Check out Repo.exists?.

2 Likes
3

Thanks for that function, that is actually pretty useful and I didn’t about it. I’ll definitely be using that for later purposes.

I thought about simply putting the entire conditions into the mysql query, but I’d have to index all three of those columns in mysql (id,userid,status). I was hoping strengthen my knowledge a little bit more with elixir pattern matching, as I have even more complicated things to implement later on, and some that can’t access the database.

4

Well, I’ve been studying all night and hit a dead end. This is about as far as I could get:

userIdInSocket = 1
results = 
[
  %{status: 0, userid: 1}, #this should cause function to return true: userid matched, status not in [3,7]
  %{status: 0, userid: 299},
  %{status: 0, userid: 664},
  %{status: 0, userid: 782},
  %{status: 0, userid: 1035},
  %{status: 0, userid: 4609},
  %{status: 0, userid: 4684},
  %{status: 0, userid: 5974},
  %{status: 0, userid: 6755},
  %{status: 0, userid: 6755},
  %{status: 0, userid: 8980},
  %{status: 0, userid: 9246},
  %{status: 0, userid: 10759},
  %{status: 0, userid: 15517},
  %{status: 0, userid: 16502},
  %{status: 0, userid: 32428},
  %{status: 0, userid: 61240},
  %{status: 0, userid: 79131},
  %{status: 0, userid: 98521},
  %{status: 3, userid: 145222}
]

isFound = Enum.any?(results, fn (x) -> match?(%{status: x, userid: userIdInSocket} when x not in [3,7], x) end)

isFound keeps coming up true even if I edit the condition to: when x not in [0].

Basically x in this function is the user’s status. When I get a list of users in the conversation, I want to run the function to detect if the user is added to the conversation - AND - that the user’s status is not 3 or 7 (which means they are blocked).

The function should return true only if that userid is found in the results list, and their status anything except 3 or 7.

5

You’re close and the compiler should’ve warned you about the actual issue:

warning: variable “userIdInSocket” is unused (there is a variable with the same name in the context, use the pin operator (^) to match on it or prefix this variable with underscore if it is not meant to be used)

Pattern matching for %{status: x, userid: userIdInSocket} means that you match for a map with key status and assign the status value to variable x and a key of userid and assign the userid value to userIdInSocket. That pattern works for all of your inputs elements.

You wanted to match only rows where userids value matches your existing value in the variable userIdInSocket, which you denote with the pin operator like so:

%{status: x, userid: ^userIdInSocket} …

6

While I am not convinced that I understood your requirements well, this works:

defmodule UserChecks do
  def authorized?(user_id, %{userid: user_id, status: status}) when status not in [3, 7], do: true
  def authorized?(_user_id, %{userid: _other_user_id, status: _status}), do: false

  def any_authorized?(user_id, user_ids_and_statuses) when is_list(user_ids_and_statuses) do
    Enum.any?(user_ids_and_statuses, &authorized?(user_id, &1))
  end

  def test() do
    user_ids_and_statuses = [
      %{status: 0, userid: 1},
      %{status: 3, userid: 100},
      %{status: 7, userid: 200},
      %{status: 0, userid: 299},
      %{status: 0, userid: 664}
    ]

    IO.inspect(user_ids_and_statuses, label: "userids and statuses")
    IO.inspect(any_authorized?(1, user_ids_and_statuses), label: "Is userid 1 authorized?")
    IO.inspect(any_authorized?(2, user_ids_and_statuses), label: "Is userid 2 authorized?")
    IO.inspect(any_authorized?(100, user_ids_and_statuses), label: "Is userid 100 authorized?")
    IO.inspect(any_authorized?(200, user_ids_and_statuses), label: "Is userid 200 authorized?")
    IO.inspect(any_authorized?(299, user_ids_and_statuses), label: "Is userid 299 authorized?")
    IO.inspect(any_authorized?(664, user_ids_and_statuses), label: "Is userid 664 authorized?")
  end
end

authorized? basically emulates the match? you are trying to do. IMO a function with multiple pattern-matching heads reads better. Notice how user_id is specified twice in the first head; this a neat trick that matches only when there’s the same value in the first function argument and the userid field of the map.

But if you really must know how does it look with match?, here you go:

  def authorized?(searched_user_id, %{userid: _user_id, status: _status} = userid_and_status) do
    match?(
      %{userid: ^searched_user_id, status: status} when status not in [3, 7],
      userid_and_status
    )
  end
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