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The case of ActiveRecord vs. CouchDB

March 31, 2009 by alex

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.