From cd915531c2937d16404517861c2e67d0a46d33d7 Mon Sep 17 00:00:00 2001
From: Moritz <moritz.m@local-it.org>
Date: Fri, 12 Dec 2025 20:08:03 +0100
Subject: [PATCH] feat: add validation for same-interval membership fee type
 changes

---
 lib/membership/member.ex                      |   5 +
 .../changes/validate_same_interval.ex         | 119 ++++++++++
 .../changes/validate_same_interval_test.exs   | 220 ++++++++++++++++++
 3 files changed, 344 insertions(+)
 create mode 100644 lib/membership_fees/changes/validate_same_interval.ex
 create mode 100644 test/membership_fees/changes/validate_same_interval_test.exs

diff --git a/lib/membership/member.ex b/lib/membership/member.ex
index b76fb64..4c90e05 100644
--- a/lib/membership/member.ex
+++ b/lib/membership/member.ex
@@ -178,6 +178,11 @@ defmodule Mv.Membership.Member do
         where [changing(:user)]
       end
 
+      # Validate that membership fee type changes only allow same-interval types
+      change Mv.MembershipFees.Changes.ValidateSameInterval do
+        where [changing(:membership_fee_type_id)]
+      end
+
       # Auto-calculate membership_fee_start_date when membership_fee_type_id is set
       # and membership_fee_start_date is not already set
       change Mv.MembershipFees.Changes.SetMembershipFeeStartDate do
diff --git a/lib/membership_fees/changes/validate_same_interval.ex b/lib/membership_fees/changes/validate_same_interval.ex
new file mode 100644
index 0000000..0d067ce
--- /dev/null
+++ b/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
+
diff --git a/test/membership_fees/changes/validate_same_interval_test.exs b/test/membership_fees/changes/validate_same_interval_test.exs
new file mode 100644
index 0000000..7b7a433
--- /dev/null
+++ b/test/membership_fees/changes/validate_same_interval_test.exs
@@ -0,0 +1,220 @@
+defmodule Mv.MembershipFees.Changes.ValidateSameIntervalTest do
+  @moduledoc """
+  Tests for ValidateSameInterval change module.
+  """
+  use Mv.DataCase, async: true
+
+  alias Mv.Membership.Member
+  alias Mv.MembershipFees.MembershipFeeType
+  alias Mv.MembershipFees.Changes.ValidateSameInterval
+
+  # Helper to create a membership fee type
+  defp create_fee_type(attrs) do
+    default_attrs = %{
+      name: "Test Fee Type #{System.unique_integer([:positive])}",
+      amount: Decimal.new("50.00"),
+      interval: :yearly
+    }
+
+    attrs = Map.merge(default_attrs, attrs)
+
+    MembershipFeeType
+    |> Ash.Changeset.for_create(:create, attrs)
+    |> Ash.create!()
+  end
+
+  # Helper to create a member
+  defp create_member(attrs) do
+    default_attrs = %{
+      first_name: "Test",
+      last_name: "Member",
+      email: "test.member.#{System.unique_integer([:positive])}@example.com"
+    }
+
+    attrs = Map.merge(default_attrs, attrs)
+
+    Member
+    |> Ash.Changeset.for_create(:create_member, attrs)
+    |> Ash.create!()
+  end
+
+  describe "validate_interval_match/1" do
+    test "allows change to type with same interval" do
+      yearly_type1 = create_fee_type(%{interval: :yearly, name: "Yearly Type 1"})
+      yearly_type2 = create_fee_type(%{interval: :yearly, name: "Yearly Type 2"})
+
+      member = create_member(%{membership_fee_type_id: yearly_type1.id})
+
+      changeset =
+        member
+        |> Ash.Changeset.for_update(:update_member, %{
+          membership_fee_type_id: yearly_type2.id
+        })
+        |> ValidateSameInterval.change(%{}, %{})
+
+      assert changeset.valid?
+    end
+
+    test "prevents change to type with different interval" do
+      yearly_type = create_fee_type(%{interval: :yearly})
+      monthly_type = create_fee_type(%{interval: :monthly})
+
+      member = create_member(%{membership_fee_type_id: yearly_type.id})
+
+      changeset =
+        member
+        |> Ash.Changeset.for_update(:update_member, %{
+          membership_fee_type_id: monthly_type.id
+        })
+        |> ValidateSameInterval.change(%{}, %{})
+
+      refute changeset.valid?
+      assert %{errors: errors} = changeset
+      assert Enum.any?(errors, fn error ->
+               error.field == :membership_fee_type_id and
+                 error.message =~ "yearly" and
+                 error.message =~ "monthly"
+             end)
+    end
+
+    test "allows first assignment of membership fee type" do
+      yearly_type = create_fee_type(%{interval: :yearly})
+      member = create_member(%{}) # No fee type assigned
+
+      changeset =
+        member
+        |> Ash.Changeset.for_update(:update_member, %{
+          membership_fee_type_id: yearly_type.id
+        })
+        |> ValidateSameInterval.change(%{}, %{})
+
+      assert changeset.valid?
+    end
+
+    test "allows removal of membership fee type" do
+      yearly_type = create_fee_type(%{interval: :yearly})
+      member = create_member(%{membership_fee_type_id: yearly_type.id})
+
+      changeset =
+        member
+        |> Ash.Changeset.for_update(:update_member, %{
+          membership_fee_type_id: nil
+        })
+        |> ValidateSameInterval.change(%{}, %{})
+
+      assert changeset.valid?
+    end
+
+    test "does nothing when membership_fee_type_id is not changed" do
+      yearly_type = create_fee_type(%{interval: :yearly})
+      member = create_member(%{membership_fee_type_id: yearly_type.id})
+
+      changeset =
+        member
+        |> Ash.Changeset.for_update(:update_member, %{
+          first_name: "New Name"
+        })
+        |> ValidateSameInterval.change(%{}, %{})
+
+      assert changeset.valid?
+    end
+
+    test "error message is clear and helpful" do
+      yearly_type = create_fee_type(%{interval: :yearly})
+      quarterly_type = create_fee_type(%{interval: :quarterly})
+
+      member = create_member(%{membership_fee_type_id: yearly_type.id})
+
+      changeset =
+        member
+        |> Ash.Changeset.for_update(:update_member, %{
+          membership_fee_type_id: quarterly_type.id
+        })
+        |> ValidateSameInterval.change(%{}, %{})
+
+      error = Enum.find(changeset.errors, &(&1.field == :membership_fee_type_id))
+      assert error.message =~ "yearly"
+      assert error.message =~ "quarterly"
+      assert error.message =~ "same-interval"
+    end
+
+    test "handles all interval types correctly" do
+      intervals = [:monthly, :quarterly, :half_yearly, :yearly]
+
+      for interval1 <- intervals,
+          interval2 <- intervals,
+          interval1 != interval2 do
+        type1 =
+          create_fee_type(%{
+            interval: interval1,
+            name: "Type #{interval1} #{System.unique_integer([:positive])}"
+          })
+
+        type2 =
+          create_fee_type(%{
+            interval: interval2,
+            name: "Type #{interval2} #{System.unique_integer([:positive])}"
+          })
+
+        member = create_member(%{membership_fee_type_id: type1.id})
+
+        changeset =
+          member
+          |> Ash.Changeset.for_update(:update_member, %{
+            membership_fee_type_id: type2.id
+          })
+          |> ValidateSameInterval.change(%{}, %{})
+
+        refute changeset.valid?,
+               "Should prevent change from #{interval1} to #{interval2}"
+      end
+    end
+  end
+
+  describe "integration with update_member action" do
+    test "validation works when updating member via update_member action" do
+      yearly_type = create_fee_type(%{interval: :yearly})
+      monthly_type = create_fee_type(%{interval: :monthly})
+
+      member = create_member(%{membership_fee_type_id: yearly_type.id})
+
+      # Try to update member with different interval type
+      assert {:error, %Ash.Error.Invalid{} = error} =
+               member
+               |> Ash.Changeset.for_update(:update_member, %{
+                 membership_fee_type_id: monthly_type.id
+               })
+               |> Ash.update()
+
+      # Check that error is about interval mismatch
+      error_message = extract_error_message(error)
+      assert error_message =~ "yearly"
+      assert error_message =~ "monthly"
+      assert error_message =~ "same-interval"
+    end
+
+    test "allows update when interval matches" do
+      yearly_type1 = create_fee_type(%{interval: :yearly, name: "Yearly Type 1"})
+      yearly_type2 = create_fee_type(%{interval: :yearly, name: "Yearly Type 2"})
+
+      member = create_member(%{membership_fee_type_id: yearly_type1.id})
+
+      # Update member with same-interval type
+      assert {:ok, updated_member} =
+               member
+               |> Ash.Changeset.for_update(:update_member, %{
+                 membership_fee_type_id: yearly_type2.id
+               })
+               |> Ash.update()
+
+      assert updated_member.membership_fee_type_id == yearly_type2.id
+    end
+
+    defp extract_error_message(%Ash.Error.Invalid{errors: errors}) do
+      errors
+      |> Enum.filter(&(&1.field == :membership_fee_type_id))
+      |> Enum.map(& &1.message)
+      |> Enum.join(" ")
+    end
+  end
+end