Add FK between Adjustments and Adjustment Reasons

This migration will fail if there are any adjustments with invalid
adjustment reason IDs. The suggested path of action for store owners is
to change those adjustment reason IDs to NULL, because we don't want to
accidentally lose adjustments.

Author: mamhoff

core/db/migrate/20250708120317_add_adjustment_reason_foreign_keys.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+class AddAdjustmentReasonForeignKeys < ActiveRecord::Migration[7.0]
+  def up
+    # Uncomment the following code to remove orphaned records if this migration fails
+    #
+    # say_with_time "Removing invalid adjustment reason IDs from adjustments table" do
+    #   Spree::Adjustment.where.not(adjustment_reason_id: nil).left_joins(:adjustment_reason).where(spree_adjustment_reasons: { id: nil }).update_all(adjustment_reason_id: nil)
+    # end
+
+    add_foreign_key :spree_adjustments, :spree_adjustment_reasons, column: :adjustment_reason_id, null: true, on_delete: :restrict
+  rescue ActiveRecord::StatementInvalid => e
+    if e.cause.is_a?(PG::ForeignKeyViolation) || e.cause.is_a?(Mysql2::Error) || e.cause.is_a?(SQLite3::ConstraintException)
+      Rails.logger.warn <<~MSG
+        ⚠️ Foreign key constraint failed when adding :spree_adjustments => :spree_adjustment_reasons.
+        To fix this:
+          1. Uncomment the code that removes invalid adjustment reason IDs from the spree_adjustments table.
+          2. Rerun the migration.
+        Offending error: #{e.cause.class} - #{e.cause.message}
+      MSG
+    end
+    raise
+  end
+
+  def down
+    remove_foreign_key :spree_adjustments, :spree_adjustment_reasons, column: :adjustment_reason_id, null: true, on_delete: :restrict
+  end
+end