test(repo): assert member/user foreign keys are deferrable

test/mv/repo/deferrable_fk_test.exs

@@ -0,0 +1,37 @@
+defmodule Mv.DeferrableFkTest do
+  @moduledoc """
+  Regression guard for the deferrable-FK migration.
+
+  Asserts the schema property directly (`condeferred = true`) for the three
+  members/users foreign keys. A multi-connection deadlock reproduction cannot
+  be made deterministic under the Ecto sandbox (ownership serializes
+  connections), so the structural assertion is the guard here; see the
+  migration moduledoc for the full FOR-KEY-SHARE/MultiXact deadlock rationale.
+  """
+  use Mv.DataCase, async: true
+
+  @deferrable_fks ~w(
+    users_member_id_fkey
+    users_role_id_fkey
+    members_membership_fee_type_id_fkey
+  )
+
+  test "member/user foreign keys are DEFERRABLE INITIALLY DEFERRED" do
+    query = """
+    SELECT conname, condeferrable, condeferred
+    FROM pg_constraint
+    WHERE conname = ANY($1)
+    """
+
+    {:ok, %{rows: rows}} = Mv.Repo.query(query, [@deferrable_fks])
+
+    found = Map.new(rows, fn [name, deferrable, deferred] -> {name, {deferrable, deferred}} end)
+
+    for fk <- @deferrable_fks do
+      assert Map.has_key?(found, fk), "expected foreign key #{fk} to exist"
+
+      assert found[fk] == {true, true},
+             "expected #{fk} to be DEFERRABLE INITIALLY DEFERRED, got #{inspect(found[fk])}"
+    end
+  end
+end