Comment by michaelmior

I think most successful ORMs have an additional layer of semantics beyond what can be directly expressed in SQL. For example, Active Record has multiple types of associations (belongs_to, has_one, has_many, etc.) that I would argue align more closely to the ER model than the relational model. Of course you can come up with a set of conventions to go from ER to relational when everything is fed through the ORM, but you are losing some semantic information in the process. (That is, if you just look at the SQL table definitions, you don't have the same information about relationships).