The State of Ruby ORM | Hacker News Reader

The State of Ruby ORM

solnic.eu

50 pointschrismealy18 comments

The thing that really grates me is that Datamapper (the ORM) is that all their examples tightly couples the models to the database in direct contravention of the data mapper pattern that gives it its name.

After all the entire point of data-mapper is that your objects don't know the details about how they're stored.

look at it as hints. It's not actually tightly coupled, as m of that stuff is optional sugar that allows for better optimization.

That "sugar" is in the wrong place for a project that names itself after a pattern intended to keep the models and storage separate. If Datamapper actually tried to implement the datamapper pattern, you would've put it in separate mapper classes, not littered around the models.

Fair point. Why don't you create a gist with an example of what the model + hint declarations would look like in separate classes. I'm not convinced that this distinction matters, but I'm sure the DM developers would respectfully take a close look at your insights. They are a really great group, very helpful on IRC, etc.

I would if I had time. Or they could just read the chapter of Patterns of Enterprise Application Architecture that they took the name from - it's the chapter right after Active Record :) Page 165. Martin Fowler went into a decent amount of depth in describing the pattern. His site also has a decent summary available for free.

I don't think this is exactly is lack of awareness on their part, though - as someone else has pointed out, Datamapper actually implements the Active Record pattern. Structurally the two are very different.

Datamapper implements the active-record pattern. It's the last thing you would expect.

His example

User.where(:active => true).kind_of?(Enumerable)

What's the issue here? If you change it to

User.where(:active => true).all.kind_of?(Enumerable)

Yeah, I'm not sure I understand that either. The whole point is that with the first example it is NOT an Enumerable yet. You are building, what is essentially, a query object and until you access something that query will not be executed at which point it will be an Enumerable as you point to with the "all" method.

I guess TFAA would like the query under construction to be Enumerable, and Enumerating it to serialize it without needing the `all` indirection. Something similar to LINQ.

In fact, some of Enumerable's messages could even be used to further build the query (e.g. `Enumerable#drop(Integer)` simply bumps the query's offset)

That makes some sense to me, but technically that object at that point is not an "Enumerable" so you are essentially faking it knowing that it would be once executed.

Is there some significant benefit to it returning true for Enumerable that I'm just missing?

> That makes some sense to me, but technically that object at that point is not an "Enumerable"

Why not? An enumerable is anything that can be enumerated, the query object holds the potential of being enumerated, it's just an actual underlying query away but that's of no relevance to the interface. The only reason why it's not "technically Enumerable" at that point is... that it does not implement Enumerable's interface.

> so you are essentially faking it knowing that it would be once executed.

Why would you be faking anything? Is LINQ faking anything when it produces IEnumerable<T> after each "operation"?

> Is there some significant benefit to it returning true for Enumerable that I'm just missing?

Interface simplicity? Workflow simplicity? I see no reason for it not to? Get rid of the useless `all` method?

I don't think there is much difference between what ActiveRecord is doing and what LINQ does.

The main LINQ object is an IQueryable object that implements IEnumerable. However, IEnumerable is not the object that has all the enumerator methods on it. That is IEnumerator. The only method IEnumerable has is the GetEnumerator method which returns the IEnumerator object.

So, in actuality, the "all" method in ActiveRecord kind of acts like the IEnumerable interface in returning the actual enumerator object.

Strictly speaking, you're not actually enumerating for a lot of the potential use-cases of enumerable methods.

For example, say #reject were implemented:

    User.where(admin: true).reject(office: current_user.office)

There are two ways to implement this: One by literally iterating over the result set and filtering it; and Two by adding a projection to the query builder.

In the second case, it is small-e enumerable, but not strictly Enumerable, since Enumerable very explicitly defines each method in terms of #each. That doesn't make sense for a query builder.

It could implement Enumerable and override every single method, but the point a few people are making here and in the post comments is that checking kind_of?(Enumerable) is non-idiomatic. Including modules for semantic meaning is appealing to programmers from certain backgrounds for completely understandable reasons, but simply checking for the presence of the method needed is more clear -- ie. respond_to?(:each) is the intended solution.

TL;DR: Modules aren't Interfaces.

His example is really just silly. ActiveRecord is doing the right thing here, it lets you chain your methods until you do something that requires it to hit the database and actually fetch the data.

What do you mean?

Three years ago the state was much worse, so I ended up writing my own ORM, which tries to address some of the issues pointed out here.

My primary need was to be able to map the same models to different schemas, including very badly designed legacy ones, so the mapper layer had to be clearly separated from the model and very hackable. Also, the query syntax had to be powerful enough to avoid SQL whenever possible, so that the same queries could be applied to different environments.

Right now it implements Units Of Work and Identity Mappers; it has deep querying and multilevel strategic loading (not limited to one level as with DataMapper); functions and aggregates; and many other features, wrapped in a familiar easy syntax.

If someone wants to try it, I'd love to hear some feedback: it's at https://github.com/me/spider.

I find Datamapper's codebase quite easy to follow. It uses a modular architecture, separating different concerns in different modules, which can be hard to follow if you are new to Ruby.

About the raw SQL: Sequel excels at this. And you can combine easily both.

I found the post interesting, but would have loved to see more examples to the arguments. Specially in the frameworks the author gives so much praise.

interesting topic, lame explanation

Reply on news.ycombinator.com