fix: handle removal of :context multitenancy in migrations

lib/migration_generator/migration_generator.ex

@@ -3109,11 +3109,39 @@ defmodule AshPostgres.MigrationGenerator do
     not is_nil(Map.get(attribute, :references)) and
       !(attribute.references.multitenancy &&
           attribute.references.multitenancy.strategy == :context &&
-          (is_nil(multitenancy) || multitenancy.strategy == :attribute))
+          (is_nil(multitenancy) || multitenancy.strategy == :attribute)) and
+      !(multitenancy && multitenancy.strategy == :context &&
+          attribute.references.multitenancy &&
+          attribute.references.multitenancy.strategy == :context)
   end
 
-  def get_existing_snapshot(snapshot, opts) do
-    folder = get_snapshot_folder(snapshot, opts)
+  defp get_snapshot_folder(snapshot, opts) do
+    base =
+      opts
+      |> snapshot_path(snapshot.repo)
+      |> Path.join(repo_name(snapshot.repo))
+
+    if snapshot.multitenancy.strategy == :context do
+      Path.join(base, "tenants")
+    else
+      base
+    end
+  end
+
+  defp get_alternate_snapshot_folder(snapshot, opts) do
+    base =
+      opts
+      |> snapshot_path(snapshot.repo)
+      |> Path.join(repo_name(snapshot.repo))
+
+    if snapshot.multitenancy.strategy == :context do
+      base
+    else
+      Path.join(base, "tenants")
+    end
+  end
+
+  defp load_snapshot_from_folder(folder, snapshot, opts) do
     snapshot_path = get_snapshot_path(snapshot, folder)
 
     if File.exists?(snapshot_path) do
@@ -3138,16 +3166,13 @@ defmodule AshPostgres.MigrationGenerator do
     end
   end
 
-  defp get_snapshot_folder(snapshot, opts) do
-    if snapshot.multitenancy.strategy == :context do
-      opts
-      |> snapshot_path(snapshot.repo)
-      |> Path.join(repo_name(snapshot.repo))
-      |> Path.join("tenants")
+  def get_existing_snapshot(snapshot, opts) do
+    result = load_snapshot_from_folder(get_snapshot_folder(snapshot, opts), snapshot, opts)
+
+    if is_nil(result) do
+      load_snapshot_from_folder(get_alternate_snapshot_folder(snapshot, opts), snapshot, opts)
     else
-      opts
-      |> snapshot_path(snapshot.repo)
-      |> Path.join(repo_name(snapshot.repo))
+      result
     end
   end
 

test/migration_generator_test.exs

@@ -1264,6 +1264,90 @@ defmodule AshPostgres.MigrationGeneratorTest do
     end
   end
 
+  describe "removing :context multitenancy" do
+    setup %{snapshot_path: snapshot_path, migration_path: migration_path} do
+      defposts do
+        multitenancy do
+          strategy(:context)
+        end
+
+        attributes do
+          uuid_primary_key(:id)
+          attribute(:title, :string, public?: true)
+        end
+      end
+
+      defcomments do
+        multitenancy do
+          strategy(:context)
+        end
+
+        attributes do
+          uuid_primary_key(:id)
+          attribute(:post_id, :uuid, public?: true)
+        end
+
+        relationships do
+          belongs_to(:post, Post, public?: true)
+        end
+      end
+
+      defdomain([Post, Comment])
+
+      AshPostgres.MigrationGenerator.generate(Domain,
+        snapshot_path: snapshot_path,
+        migration_path: migration_path,
+        quiet: true,
+        format: false,
+        auto_name: true
+      )
+
+      :ok
+    end
+
+    test "when removing :context multitenancy, no invalid drop constraint is generated", %{
+      snapshot_path: snapshot_path,
+      migration_path: migration_path
+    } do
+      defposts do
+        attributes do
+          uuid_primary_key(:id)
+          attribute(:title, :string, public?: true)
+        end
+      end
+
+      defcomments do
+        attributes do
+          uuid_primary_key(:id)
+          attribute(:post_id, :uuid, public?: true)
+        end
+
+        relationships do
+          belongs_to(:post, Post, public?: true)
+        end
+      end
+
+      defdomain([Post, Comment])
+
+      AshPostgres.MigrationGenerator.generate(Domain,
+        snapshot_path: snapshot_path,
+        migration_path: migration_path,
+        quiet: true,
+        format: false,
+        auto_name: true
+      )
+
+      migration_files = Enum.sort(Path.wildcard("#{migration_path}/**/*_migrate_resources*.exs")) |> Enum.reject(&String.contains?(&1, "extensions"))
+
+      all_contents = Enum.map_join(migration_files, "", &File.read!/1)
+
+      [up_contents] =Regex.run(~r/def up do(.+)def down do/s, all_contents, capture: :all_but_first)
+
+
+      refute up_contents =~ ~S{drop constraint(:comments, "comments_post_id_fkey")}
+    end
+  end
+
   describe "creating follow up migrations" do
     setup %{snapshot_path: snapshot_path, migration_path: migration_path} do
       :ok