In Entity-Framework, you can create a DB schema, and then generate your ORM classes.  Is there any software that does this in reverse?  For example you create a few classes that have relationships, and software generates a database schema with the relevant primary and foreign key relationships?    
ORM has schema to generate classes, does the reverse exist?
database;orm
Yes, it is available in EF starting from version EF 4.1.You can do it with Entity Framework, using Code First.EF 4.3 even has Migrations built-in.