Ensuring that flush() will raise on rollback if called from change/0.

lib/ecto/migration.ex

@@ -1096,14 +1096,15 @@ defmodule Ecto.Migration do
     struct(%Constraint{table: table, name: name}, opts)
   end
 
-  @doc """
-  Executes queue migration commands.
-
-  Reverses the order in which commands are executed when doing a rollback
-  on a `change/0` function and resets the commands queue.
-  """
-  def flush do
-    Runner.flush
+  @doc "Executes queue migration commands."
+  defmacro flush do
+    quote do
+      if direction() == :down and not function_exported?(__MODULE__, :down, 0) do
+        raise "calling flush() inside change when doing rollback is not supported."
+      else
+        Runner.flush()
+      end
+    end
   end
 
   # Validation helpers

test/ecto/migrator_test.exs

@@ -121,6 +121,19 @@ defmodule Ecto.MigratorTest do
     use Ecto.Repo, otp_app: :ecto_sql, adapter: EctoSQL.TestAdapter
   end
 
+  defmodule EmptyUpDownMigration do
+    use Ecto.Migration
+
+    def up, do: flush()
+    def down, do: flush()
+  end
+
+  defmodule EmptyChangeMigration do
+    use Ecto.Migration
+
+    def change, do: flush()
+  end
+
   Application.put_env(:ecto_sql, MigrationSourceRepo, [migration_source: "my_schema_migrations"])
 
   setup do
@@ -156,6 +169,18 @@ defmodule Ecto.MigratorTest do
     """
   end
 
+  @tag :current
+  test "flush" do
+    num = System.unique_integer([:positive])
+    assert :ok == up(TestRepo, num, EmptyUpDownMigration, log: false)
+    assert :ok == down(TestRepo, num, EmptyUpDownMigration, log: false)
+    assert :ok == up(TestRepo, num, EmptyChangeMigration, log: false)
+    message = "calling flush() inside change when doing rollback is not supported."
+    assert_raise(RuntimeError, message, fn ->
+      down(TestRepo, num, EmptyChangeMigration, log: false)
+    end)
+  end
+
   test "custom schema migrations table is right" do
     assert SchemaMigration.get_source(TestRepo) == "schema_migrations"
     assert SchemaMigration.get_source(MigrationSourceRepo) == "my_schema_migrations"