It is possible in Andromeda to define a foreign key and then allow inserts that do not match in the parent table. This may seem strange or unnecessary, but there are benefits. Andromeda has many user interface features that are driven by foreign keys. If you want to use these features between two tables that sometimes match, then you can use the unenforced foreign key.
Use this feature by putting the "allow_orphan" property into a foreign key defintion:
table example_child: module: example foreign_key customers: allow_orphan: Y