After my Ruby sittin’ on the Couch talk about Ruby Frameworks and CouchDB at Scotland on Rails last week a bit of a debate started. In the talk I did compare the available Ruby Frameworks and discussed how well they fit the CouchDB way of doing things. In my conclusion I recommended using either CouchRest or RelaxDB for development and at the same time urged people not to use one of the ActiveRecord like libraries like CouchFoo.

As it seems to me the intention of my talk hasn’t reached all of its audience yet I’ll try to make my point again using this blog post.

The frameworks I looked at could be divided into two classes: the ones using CouchDB semantics (e.g. CouchRest/RelaxDB) and the ones trying to provide an ActiveRecord like interface for the applications (e.g. CouchFoo). The reasons I recommended not to use ActiveRecord semantics are:

CouchDB is not about relations

In ActiveRecord we have to model our domain models so they fit into a relational schema. That means flat tables and relationships between two tables, through a third and fourth table etc. To get results from the database we usually join a few tables and get back the resulting rows, nicely converted into Ruby objects for us. That is (sort of) fine for a relational database because all it has are those tables but it doesn’t work at all with CouchDB.

To use CouchDB to its full potential you need to understand and use its views

CouchDB doesn’t have a concept of tables at all. And the way you pull your data from CouchDB is fundamentally different. Instead of joining data from different tables via an SQL query you procedurally build up an index of data by providing the map and reduce functions which you then query.

In order to fully use CouchDB you have to write custom map/reduce functions and abstracting that into an API that was designed for generating SQL queries doesn’t allow you that.

I could go into more details but that really is my main point. I know from my experience that changing the wiring in your head after too many years of ActiveRecord is hard so I don’t expect anyone to immediately agree with me but I do believe that the only viable way of using CouchDB is through the interface that was designed for it and not by an abstractions that just happened to be there first.

I’m looking forward to a lively discussion, either in the comments or on other blogs.

That couch was our intro picture at the workshop.
(picture from gizmodo.com)

Monday was the first event at our new upstream office, starring @janl and me presenting an introduction to CouchDB – including a new hands on examples part – and afterwards an overview of what can be done with CouchDB in Ruby so far. The talks were followed by a discussion that gave (hopefully not only) me a couple of new insights I want to share here – after summarizing the evening for the people who couldn’t make it. There will also be a video recording with synchronized slides of both talks be available in the next days (thanks @klimpong).

What is CouchDB

CouchDB ist the new cool kid on the block. It’s a document oriented database that has replication built in, can scale massively and uses an HTTP REST interface to query it. Documents are stored as JSON constructs and can be queried with views that are built using Map-Reduce (a smaller company called google has had a bit of success with that recently). Oh and it’s written in Erlang. Jan has given a number of talks on numerous events already, so there are already a couple of videos and slides available – not from the hands on part though For that you should watch his blog I guess.
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On a recent Rails project I had to use an old database schema that had been designed years ago for some php app. The two main entities in the application were companies and their projects. For whatever reason the designer had created one huge table called organizations with all the companies and projects inside. A row in the database marked a company when the is_company column was 1 and a project when the is_project column was 1.

|----------------------------------------------------|
| id | name                | is_company | is_project |
|----------------------------------------------------|
| 1  | google              | 1          | 0          |
| 2  | world domination    | 0          | 1          |
|----------------------------------------------------|

In my rails app I created two ActiveRecord models: Company and Project and set the table name for both to organizations:

class Company
set_table_name ‘organizations’
end

class Project
set_table_name ‘organizations’
end

Now the real problem was his: when I did a Company.find :all I got all the companies and all the projects, so I had to do this instead: Company.find :all, :conditions => {:is_company => true} – throughout the project, and whenever I forgot the condition somewhere all the projects showed up in the wrong place. So what I needed was a generic solution.

To make a longer story short I played with scope_out defining a projects scope on the Company model, so I could at least do Company.find_all_companies and didn’t have to specify the conditions over and over again – but that still wasn’t DRY at all. One morning short after waking up (the time I usually have the best ideas) it dawned on me how simple the solution actually is:

class Company
def self.find(*params)
with_scope(:find => {:conditions => {:is_company => true}}) do
super
end
end

def self.count(*params)
with_scope(:find => {:conditions => {:is_company => true}}) do
super
end
end
end

I added a scope around all find and count calls of my model and now I can just do whatever find operations I want and don’t have to think about scopes and tables ever again. Sweet object encapsulation.

We recently introduced a new feature in autoki called the social feed. It’s basically a yellow box displaying any events on the platform relevant to the current user, like a friend has posted a new photo, or a new interesting car was uploaded. The data model behind this is pretty straightforward, we have a FeedEvent class and all kinds of subclasses, e.g. a MessageReceivedEvent. Each event belongs to a user and an event source, in this example the user would be the user who received the message and the event source would be the message itself. For each user, we simply display all the events that belong to him or her.

Now the question was this: How do we create these events? The most straightforward way would probably have been to create them in the models, so the Message model would have an after_create callback that created the event. What we didn’t like about this solution was that we would put a whole bunch of logic into the models that didn’t really belong there. Why would a Message care if there was some kind of event feed? Plus these events would be all around in our unit tests and make the bloated and probably sloooow (again). So we wanted to use the observer pattern to remove the creation of the event from the models.

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