issue #611 solution

.tool-versions

@@ -1,3 +1,3 @@
 erlang 27.1.2
-elixir 1.18.4-otp-27
+elixir 1.18.1
 pipx 1.8.0

lib/migration_generator/migration_generator.ex

@@ -2154,7 +2154,8 @@ defmodule AshPostgres.MigrationGenerator do
           table: snapshot.table,
           schema: snapshot.schema,
           source: attribute.source,
-          multitenancy: snapshot.multitenancy
+          multitenancy: snapshot.multitenancy,
+          old_multitenancy: old_snapshot.multitenancy
         }
       end)
 
@@ -2663,7 +2664,8 @@ defmodule AshPostgres.MigrationGenerator do
             []
           end
 
-        if Map.get(old_attribute, :references) != Map.get(new_attribute, :references) do
+        if Map.get(old_attribute, :references) != Map.get(new_attribute, :references) and
+             references_differ_beyond_index?(old_attribute, new_attribute) do
           redo_deferrability =
             if has_reference?(old_snapshot.multitenancy, old_attribute) and
                  differently_deferrable?(new_attribute, old_attribute) do
@@ -2771,6 +2773,23 @@ defmodule AshPostgres.MigrationGenerator do
 
   defp differently_deferrable?(_, _), do: false
 
+  # When the only reference change is index? (add/remove index), we should not emit
+  # DropForeignKey + AlterAttribute; the separate AddReferenceIndex/RemoveReferenceIndex
+  # operations handle it. This avoids migrations that drop and re-add the same FK.
+  defp references_differ_beyond_index?(old_attr, new_attr) do
+    old_refs = Map.get(old_attr, :references)
+    new_refs = Map.get(new_attr, :references)
+
+    cond do
+      old_refs == new_refs -> false
+      is_nil(old_refs) or is_nil(new_refs) -> true
+      true ->
+        old_without_index = Map.delete(old_refs, :index?)
+        new_without_index = Map.delete(new_refs, :index?)
+        old_without_index != new_without_index
+    end
+  end
+
   # This exists to handle the fact that the remapping of the key name -> source caused attributes
   # to be considered unequal. We ignore things that only differ in that way using this function.
   defp attributes_unequal?(left, right, repo, _old_snapshot, _new_snapshot, ignore_names?) do

test/migration_generator_test.exs

@@ -1817,6 +1817,130 @@ defmodule AshPostgres.MigrationGeneratorTest do
       assert File.read!(file) =~ ~S{create index(:posts, [:post_id])}
     end
 
+    test "changing only reference index? does not drop and re-add foreign key (issue #611)", %{
+      snapshot_path: snapshot_path,
+      migration_path: migration_path
+    } do
+      # First generate: reference with index?: true
+      defresource PostRefIdx, "posts" do
+        attributes do
+          uuid_primary_key(:id)
+          attribute(:key_id, :uuid, allow_nil?: false, public?: true)
+          attribute(:foobar, :string, public?: true)
+        end
+
+        actions do
+          defaults([:create, :read, :update, :destroy])
+        end
+      end
+
+      defresource Post2RefIdx, "posts" do
+        attributes do
+          uuid_primary_key(:id)
+          attribute(:name, :string, public?: true)
+          attribute(:related_key_id, :uuid, public?: true)
+        end
+
+        relationships do
+          belongs_to(:post, PostRefIdx, public?: true)
+        end
+
+        actions do
+          defaults([:create, :read, :update, :destroy])
+        end
+
+        postgres do
+          references do
+            reference(:post, index?: true)
+          end
+        end
+      end
+
+      defmodule DomainRefIdx do
+        use Ash.Domain
+        resources do
+          resource(PostRefIdx)
+          resource(Post2RefIdx)
+        end
+      end
+
+      AshPostgres.MigrationGenerator.generate(DomainRefIdx,
+        snapshot_path: snapshot_path,
+        migration_path: migration_path,
+        quiet: true,
+        format: false,
+        auto_name: true
+      )
+
+      # Second generate: same reference but index?: false (only index change)
+      defresource PostRefNoIdx, "posts" do
+        attributes do
+          uuid_primary_key(:id)
+          attribute(:key_id, :uuid, allow_nil?: false, public?: true)
+          attribute(:foobar, :string, public?: true)
+        end
+
+        actions do
+          defaults([:create, :read, :update, :destroy])
+        end
+      end
+
+      defresource Post2RefNoIdx, "posts" do
+        attributes do
+          uuid_primary_key(:id)
+          attribute(:name, :string, public?: true)
+          attribute(:related_key_id, :uuid, public?: true)
+        end
+
+        relationships do
+          belongs_to(:post, PostRefNoIdx, public?: true)
+        end
+
+        actions do
+          defaults([:create, :read, :update, :destroy])
+        end
+
+        postgres do
+          references do
+            reference(:post, index?: false)
+          end
+        end
+      end
+
+      defmodule DomainRefNoIdx do
+        use Ash.Domain
+        resources do
+          resource(PostRefNoIdx)
+          resource(Post2RefNoIdx)
+        end
+      end
+
+      AshPostgres.MigrationGenerator.generate(DomainRefNoIdx,
+        snapshot_path: snapshot_path,
+        migration_path: migration_path,
+        quiet: true,
+        format: false,
+        auto_name: true
+      )
+
+      [_, file2] =
+        Path.wildcard("#{migration_path}/**/*_migrate_resources*.exs")
+        |> Enum.reject(&String.contains?(&1, "extensions"))
+        |> Enum.sort()
+
+      content = File.read!(file2)
+
+      # Should only drop the index, not touch the foreign key
+      assert content =~ ~S{drop_if_exists index(:posts, [:post_id])},
+             "migration should drop the reference index when index? changes to false"
+
+      refute content =~ ~S{drop constraint(:posts, "posts_post_id_fkey")},
+             "migration should not drop the foreign key when only index? changed (issue #611)"
+
+      refute content =~ ~S{modify :post_id, references(},
+             "migration should not modify references when only index? changed (issue #611)"
+    end
+
     test "references with deferrable modifications generate changes with the correct schema", %{
       snapshot_path: snapshot_path,
       migration_path: migration_path