I often hear people bashing ORMs for being inflexible and a "leaky abstraction", but you really don't hear why they're problematic. When used properly, what exactly are the faults of ORMs? I'm asking this because I'm working on a PHP orm and I'd like for it to solve problems that a lot of other ORMs fail at, such as lazy loading and the lack of subqueries.
Please be specific with your answers. Show some code or describe a database schema where an ORM struggles. Doesn't matter the language or the ORM.
One of the bigger issues I have noticed with all the ORMs I have used is updating only a few fields without retrieving the object first.
For example, say I have a Project object mapped in my database with the following fields: Id, name, description, owning_user. Say, through ajax, I want to just update the description field. In most ORMs the only way for me to update the database table while only having an Id and description values is to either retrieve the project object from the database, set the description and then send the object back to the database (thus requiring two database operations just for one simple update) or to update it via stored procedures (which is the method I am currently using).