Forum: Ruby on Rails Engine: Where there should be a database.yml ?

For developing an Engine.
Where there should be a database.yml file for a configuration of
connection with database? In <engine_dir>/config/database.yml it doesn't
work, also doesn't use a configuration of the main application.

I place into <engine_dir>/test/dummy/config/database.yml
It works.
But why there? It not an obvious place for configuration placement, it
only test-directory.