Perhaps the most subversive issue I’ve had with ORMs is “attribute creep” or “wide tables”, that is, tables that just keep accruing attributes. As much as I’d like to avoid it, sometimes it becomes necessary (although things like Postgres’ hstore can help). For example, a client may be providing you with lots of data that they want attached to reports based on various business logic. Furthermore, you don’t have much insight into this data; you’re just schlepping it around.

This in and of itself isn’t a terrible thing in a database. It becomes a real pain point with an ORM. Specifically, the problem starts to show up in any query that uses the entity directly to create the query. You may have a Hibernate query like so early on in the project.

query(Foo.class).add(Restriction.eq("x", value))

This may be fine when Foo has five attributes, but becomes a data fire hose when it has a hundred. This is the equivalent of using SELECT *, which is usually saying more than what is intended. ORMs, however, encourage this use and often make writing precise projections as tedious as they are in SQL. (I have optimized such queries by adding the appropriate projection and reduced the run time from minutes to seconds; all the time was spent translating the database row into a Java object.)

Which leads to another bad experience: the pernicious use of foreign keys. In the ORMs I’ve used, links between classes are represented in the data model as foreign keys which, if not configured carefully, result in a large number of joins when retrieving the object. (A recent count of one such table in my work resulted in over 600 attributes and 14 joins to access a single object, using the preferred query methodology.)

Attribute creep and excessive use of foreign keys shows me is that in order to use ORMs effectively, you still need to know SQL. My contention with ORMs is that, if you need to know SQL, just use SQL since it prevents the need to know how non-SQL gets translated to SQL.