feat: add validation for same-interval membership fee type changes

lib/membership_fees/changes/validate_same_interval.ex

@@ -0,0 +1,119 @@
+defmodule Mv.MembershipFees.Changes.ValidateSameInterval do
+  @moduledoc """
+  Validates that membership fee type changes only allow same-interval types.
+
+  Prevents changing from yearly to monthly, etc. (MVP constraint).
+
+  ## Usage
+
+  In a Member action:
+
+      update :update_member do
+        # ...
+        change Mv.MembershipFees.Changes.ValidateSameInterval
+      end
+
+  The change module only executes when `membership_fee_type_id` is being changed.
+  If the new type has a different interval than the current type, a validation error is returned.
+  """
+  use Ash.Resource.Change
+
+  @impl true
+  def change(changeset, _opts, _context) do
+    if changing_membership_fee_type?(changeset) do
+      validate_interval_match(changeset)
+    else
+      changeset
+    end
+  end
+
+  # Check if membership_fee_type_id is being changed
+  defp changing_membership_fee_type?(changeset) do
+    Ash.Changeset.changing_attribute?(changeset, :membership_fee_type_id)
+  end
+
+  # Validate that the new type has the same interval as the current type
+  defp validate_interval_match(changeset) do
+    current_type_id = get_current_type_id(changeset)
+    new_type_id = get_new_type_id(changeset)
+
+    # If no current type, allow any change (first assignment)
+    if is_nil(current_type_id) do
+      changeset
+    else
+      # If new type is nil, that's allowed (removing type)
+      if is_nil(new_type_id) do
+        changeset
+      else
+        # Both types exist - validate intervals match
+        case get_intervals(current_type_id, new_type_id) do
+          {:ok, current_interval, new_interval} ->
+            if current_interval == new_interval do
+              changeset
+            else
+              add_interval_mismatch_error(changeset, current_interval, new_interval)
+            end
+
+          {:error, _reason} ->
+            # If we can't load the types, allow the change (fail open)
+            # The database constraint will catch invalid foreign keys
+            changeset
+        end
+      end
+    end
+  end
+
+  # Get current type ID from changeset data
+  defp get_current_type_id(changeset) do
+    case changeset.data do
+      %{membership_fee_type_id: type_id} -> type_id
+      _ -> nil
+    end
+  end
+
+  # Get new type ID from changeset
+  defp get_new_type_id(changeset) do
+    case Ash.Changeset.fetch_change(changeset, :membership_fee_type_id) do
+      {:ok, type_id} -> type_id
+      :error -> nil
+    end
+  end
+
+  # Get intervals for both types
+  defp get_intervals(current_type_id, new_type_id) do
+    alias Mv.MembershipFees.MembershipFeeType
+
+    case {Ash.get(MembershipFeeType, current_type_id),
+          Ash.get(MembershipFeeType, new_type_id)} do
+      {{:ok, current_type}, {:ok, new_type}} ->
+        {:ok, current_type.interval, new_type.interval}
+
+      _ ->
+        {:error, :type_not_found}
+    end
+  end
+
+  # Add validation error for interval mismatch
+  defp add_interval_mismatch_error(changeset, current_interval, new_interval) do
+    current_interval_name = format_interval(current_interval)
+    new_interval_name = format_interval(new_interval)
+
+    message =
+      "Cannot change membership fee type: current type uses #{current_interval_name} interval, " <>
+        "new type uses #{new_interval_name} interval. Only same-interval changes are allowed."
+
+    Ash.Changeset.add_error(
+      changeset,
+      field: :membership_fee_type_id,
+      message: message
+    )
+  end
+
+  # Format interval atom to human-readable string
+  defp format_interval(:monthly), do: "monthly"
+  defp format_interval(:quarterly), do: "quarterly"
+  defp format_interval(:half_yearly), do: "half-yearly"
+  defp format_interval(:yearly), do: "yearly"
+  defp format_interval(interval), do: to_string(interval)
+end
+