Ecto.Multi error handling

Questions / Help
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1

Hi :wave:,

I want to insert user and company at once (user belongs to company, company can have many users) using Ecto.Multi. Here is my current form:

  <.simple_form :let={f} for={@changeset} action={~p"/users/register"}>
    <.inputs_for :let={company} field={f[:company]}>
      <.input field={company[:name]} type="text" label="Company" />
    </.inputs_for>

    <.input field={f[:email]} type="email" label="Email" required />
    <.input field={f[:password]} type="password" label="Password" required />

    <:actions>
      <.button phx-disable-with="Creating account..." class="w-full">Create an account</.button>
    </:actions>
  </.simple_form>

and Ecto.Multi transaction:

  def register(user_params, company_params) do
    Multi.new()
    |> Multi.run(:company, fn _repo, _changes ->
      case Companies.create_company(company_params) do
        {:ok, %{company: company}} -> {:ok, company}
        {:error, :company, changeset, _} -> {:error, changeset}
      end
    end)
    |> Multi.run(:user, fn _repo, %{company: company} ->
      user_params
      |> Map.put("company_id", company.id)
      |> Accounts.register_user()
    end)
    |> Repo.transaction()
  end

However if something went wrong while creating company (e.g. name was already taken) I got Company changeset, what causes an error: “could not generate inputs for :company from MyApp.Companies” (it expects User changeset). What is the most idiomatic approach in this case? I can’t use cast_assoc, since create_company is Multi transaction itself (it also invokes some additional steps).

2

The error you quoted sounds like a Phoenix HTML error message. Presumably you are using the result of the register function and assigning it some kind of socket.assigns.changeset or something like that? The problem is happening then because you are assigning a Company changeset where it should be a User changeset.

I can suggest a straightforward solution. In your view (is it a LiveView?) where you call the register function and handle the result, pattern match the result to see if it is a Company changeset. Then do something like:

update(socket, :changeset, fn changeset -> Ecto.Changeset.put_change(changeset, :company, company_changeset end)

It is difficult for me to imagine a better approach because there are too many unknowns. It would be cleaner if you could cast_assoc the user on the company or vice-versa but without knowing what the create_company Multi does, and without knowing if a user can have many companies, or vice versa, it is difficult to advise. But cast_assoc works well with associations in forms. It is where the sort_param and drop_param magic live. I strongly advise using cast_assoc if you can.

It might be a good exercise to try to add a @spec to your register function. This will force you to think about the possible outputs.

It might also be worthwhile to write the register function as a single Multi pipeline, without the nested Repo.transaction in create_company. You can keep create_company as it is written but split out the ecto multi part so you have a function like this:

defmodule Companies do
  def create_company(attrs) do
    Multi.new()
    |> create_company_multi(attrs)
    |> Repo.transaction()
  end

  def create_company_multi(multi, attrs) do
    multi
    # |> Multi.run(:insert, ...)
  end
end

Then you can rewrite register to pipe into the create_company_multi function.

3

I would say that there is nothing as strong as an “idiom” for cases like this. Hard to say for sure since there’s a piece of your code missing but the problem appears to be that your register function is missing logic for handling different results of each Multi operation. I think the most important pattern you can/should be taking advantage of there is Ecto’s schemaless changesets to add a layer in between your UI (form) and and BE (context). You would handle any validations there, and then in your register function would no longer need to provide error handling.

4

Thanks for replies. I ended up with reorganising code a bit (I changed order of functions in transactions) and I was able to use cast_assoc :slight_smile:

5

It’s useful for future readers if you show how.

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