Overriding prefix from `Repo.default_options/1`

Ash Forum Ash Questions / Help
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1

Hi! First, I’d like to thank the Ash team. Ash solves so many problems! I’m planning to incrementally migrate my whole app to it.

I’ve got one problem, though. My app uses schema-based multitenancy and the Process dictionary to store the tenant_id, and default_options/1 to automatically put the right schema prefix for all queries.

defmodule App.Repo do
  use AshPostgres.Repo,
    otp_app: :app

  @tenant_key {__MODULE__, :tenant_id}

  def default_options(_opts) do
    case get_tenant_id() do
      nil -> []
      tenant_id -> [prefix: "tenant_#{tenant_id}"]
    end
  end

  def put_tenant_id(tenant_id) when is_integer(tenant_id) do
    Process.put(@tenant_key, tenant_id)
  end

  def get_tenant_id do
    Process.get(@tenant_key)
  end
end

This is not a problem for the multitenant Ash Resources. But when I try to create global Ash Resources, they are scoped to the current tenant’s schema. I’ve tried something like this:

  postgres do
    repo App.Repo
    table "settings"
    schema "public"
  end

But schema "public" doesn’t seem to translate to a prefix: "public" passed to Ecto that would override the default option. It also creates a new migration with a “public” prefix.

How could I tell Ash to set a prefix for Ecto’s Repo?

2

We actually intentionally removed the functionality in Ash to do that because there are far too many edge cases and footguns with that logic IMHO. With that said, you ought to be able to add a change/preparation that does this logic.

defmodule DynamicTenant do
  use Ash.Resource.Preparation

  def supports, do: [Ash.Query, Ash.Changeset, Ash.ActionInput]

  def prepare(subject, _, _) do
    tenant = get_tenant_from_pdict()

    case subject do
      %Ash.Changeset{} -> Ash.Changeset.set_tenant(subject, tenant)
      %Ash.Query{} -> Ash.Query.set_tenant(subject, tenant)
      %Ash.ActionInput{} -> Ash.ActionInput.set_tenant(subject, tenant)
    end
  end
end

And then you can do:

preparations do
  prepare DynamicTenant, on: [:create, :read, :update, :destroy, :action]
end

to have it apply to all action types.

One major caveat: the on option did not support :create, :update, and :destroy, so I added it just now, but its only in the main branch. You’ll have to wait for a release to use it.

3

Wow, that was fast! I totally agree that using the process dictionary is not the best pattern. Unfortunately, my codebase is rather big, and I can’t make a big bang change right now.

Thanks for your answer. I’ve tried it using the main branch of Ash, but unfortunately the prefix still wasn’t set. So I ended doing a bigger refactoring: I’ve renamed Repo to TenantRepo and the added a Repo module for Ash and other non-tenant operations. This is much cleaner: no need to pass prefix: "public" as Repo never uses the process dictionary.