Object-Relational Mapping is the Vietnam of Computer Science

http://www.codinghorror.com/blog/archives/000621.html

by wyw • 17 years ago

15 27 17 years ago

codinghorror.com

st3fan• 17 years ago

I love the smell of sql in the morning ...

erlanger• 17 years ago

ORM is a tough problem. News at 11.

Virax• 17 years ago

Talk about making a mountain out of a molehill. At my company, we don't try to turn database rows into objects. Here is how it works:

   * we use Python+MySQL
   * each table has an associated class, a CRUD-API if you will
   * a cursor object accesses a table like this:
    cursor.TableName.select_by_ids(low, high)
   * removing means set time_removed to the current timestamp
   * rows are returned as lists of dicts

So the solution to the object-relational mapping problem is: for each table, there is a class that manages access to it, and a row is represented by a a dict.

wyw10• 17 years ago

How do you map the types if the rows are returned as dicts? Doesn't that become an application responsibility? By using an ORM you would not need to build that functionality, i.e. reinvent the wheel.

Did you consider using SQLObject or SQLAlchemy? If so, I'd be curious to know why you decided against using them?

jhancock8• 17 years ago

There are lots of ways to do this. A most common approach is to have each of those dicts be a first class object in python/blub, and therefore you know the class of the object. Customer.find() returns a customer which contains your dict. No magic going on here. You are not reinventing too much by deciding to use lightweight wrappers and eschew automated SQL generation or ORMS and related DSLs.

rogerthat• 17 years ago

Why reinvent the wheel? An ORM will do all of that for you.

jhancock8• 17 years ago

yes, an ORM will do all that and then some. Sometimes you don't want the extra stuff. I'm not knocking ORMs. They serve their purpose. Sometimes all I need though is a simple lower level driver.

jhancock8• 17 years ago

I agree with Atwood's conclusion here. If you must use both an OO second tier and a RDB 3rd tier, concessions must be made. You can get around most concessions with more code at the hard parts, but sometimes its just not worth the effort. In addition to these concessions, there have been choices of OODBs for quite a while. We are now seeing some new DB choices come on the scene.

My perspective comes from having written what may have been the world's first ORM in Smalltalk in '88. I then spent the next 14 years building them over and over along with a full stack of other frameworks in Smalltalk and Java. Choices must be made. No framework is one size fits all. I generally sided with "make the easy things easy/automated and make the hard things possible/maintainable".

Is it Vietnam? No. You have far easier choices in how to develop your app than soldiers did in choosing wether or not to go to Vietnam. I wrote very hard ORM and other framework code; solved problems that enabled my customer's projects to be far more productive; was paid very well; enjoyed a good life; worked hard and was respected for my contributions. Hardly Vietnam.

wyw10• 17 years ago

I think the comparison to Vietnam makes sense as an analogy.

The basic argument seems to be that each ORM imposes limitations specific to that implementation which may result in the developer's choices being limited later on in the development process - just as early strategic decisions in a war can predetermine the outcome and potentially result in 'quagmire'.

It's an argument that seems to echo Joel's idea of "leaky abstractions", i.e. the ORM is just a big leaky abstraction and you have to deal with the leaks sooner or later.

jhancock8• 17 years ago

The arguments in Atwood's and Neward's articles stand well on their own. The war analogy is there as link bait.

joubert• 17 years ago

I find that Core Data is pretty good. http://developer.apple.com/macosx/coredata.html

russell• 17 years ago

Neward's article is here: http://blogs.tedneward.com/2006/06/26/The+Vietnam+Of+Compute... There is a post elsewhere on HN that links to the wrong article. You have to skip forward a bit to get beyond the history of the Vietnam War to get to the meat.

The gist of the post is that there is a fundamental impedance mismatch between OO and Relational. If you try to apply inheritance to relational databases you get an unholy quagmire. My experience is that this is true. You get a rat's nest of tables and unwieldy joins.

There are more problems with schema ownership, dual schemas, and refactoring.

Coding Horror lifts the summary and leaves behind all the argument

wyw10• 17 years ago

Yes, a mismatch exists and can lead to compromises as you try to use the object paradigm to glue the two layers together. But the benefits are also undeniable, primarily that of productivity gained during development by not having to concern yourself with the job of mapping types between the two layers.

keefe• 17 years ago

What is an object, anyway? An object is a piece of data in memory, a set of fields and a set of pointers to other pieces of data. It's a live object in a small, closed world that is consuming a portion of a finite amount of resources. Consider the data in any kind of database - a traditional RDBMS or a triple store, data in such structures is much larger and cannot be loaded into RAM for querying, so indexes must be formed somewhere for querying. However you slice it, there will be a mismatch because of this scale difference. I recently wrote a utility class to automatically map objects to resources in an arbitrary RDF graph. The insight I gained from this was to consider that there exists a large, universal data model (defined in RDF in this case, could be defined in any way) that contains all the data, but is too large to load into ram. Querying this structure leads to a set of resources that represent the data that needs to be "alive" in objects in ram at the moment and then this data can be loaded into the objects, manipulated and serialized again. This is a bit of a rambling reply for something that is unlikely to be read but the point I am trying to make is that in data modeling we like to think of an open world of data but the reality of writing computer programs is working with a very small closed world of the data in RAM and object mapping highlights this issue, which is a difficult problem in general but which has numerous easy, ad hoc solutions.

wyw10• 17 years ago

Usually the ORM has some notion of "associations" between models so that the data relationships are easier to navigate from the application side.

Object-Relational Mapping is the Vietnam of Computer Science

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