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Comment by mjallday | Hacker News Reader

Can you give an example? I don't generally have these problems and would like to see if it's my way of thinking or if you're working with vastly different data to me.

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Since the relational model is very set-oriented, it has always had a particularly hard time dealing with ordered lists. But that's really a minor thing, and is actually something an ORM can do well to abstract.

My real headache with ORM has always been to get it to perform efficiently. For example, say I want to iterate through the GPA's of all the students. To do so, the ORM usually pulls in the full object model for each student - multiplying the cost of the desired operation by 10x or 100x. Now, each ORM tool usually has some tweaky knob or special declaration you can use to have it limit the fields queried, but by the time you figure out the right incantation you may as well have written the SQL yourself.

But wait! The ORM tool often saves you from this sort of overhead by caching the object model in memory. Alas, therein lies my biggest headache with ORM. "There are only two hard things in Computer Science: cache invalidation, naming things, and off-by-one errors."

I should acknowledge that these issues apply just as much to a document store as to a relational database. However, you tend to interact with a document store differently, since there is no need for a mapping abstraction. You often just use the database primitives provided, instead of going through a 3rd party. It's akin to manually using SQL in your code, where the database naturally knows what you're asking for.

I've used ORM tools such as the one in Django, Active::Record and DBIx::Class.

I do not have the issues you mentioned ... I always remember the API calls I need to make and performance has not been an issue (granted, I'm fairly familiar with all issues that can come up, so I know when or where to optimize in general).

    the ORM usually pulls in the full object model for each student

Here's how to do it in Django (and note this is off the top of my head):

    for row in Student.objects.values_list("gpa"):
        print row[0]

You're really talking about shitty ORMs or APIs you haven't had the patience to become familiar with.

    by the time you figure out the right
    incantation you may as well have written the 
    SQL yourself

That's not true. On complex filtering, you often want to add or subtract filters based on certain conditions. With plain SQL you end up doing really ugly string concatenations, whereas with a good ORM the queries are composable.

> http://www.postgresql.org/docs/9.1/static/hstore.html

A clearer version would probably be:

    for student in Student.objects.only('gpa'):
        print student.gpa

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