Here’s the slides from a talk about MongoDB at the feb OC Ruby meetup.
There were a few examples from Tim Morgan’s presentation at the LaRubyConf2010 that resonated how ActiveRecord can be a source of many headaches. I wager this is because the ActiveRecord ORM adds complexity to mapping from ruby objects to SQL world. The mapping isn’t one to one and complexity is the unfortunate byproduct.
On the other hand, MongoDB’s data modeling naturally maps to today’s popular programming languages. Essentially, most data are hashes(or associative arrays/dictionaries) and hashes are the basic data model for MongoDB. By having such a similar data representation, the ORM complexity level is extremely minimal compared to a typical ORM designed for SQL.
So use MongoDB because it can reduce the amount of complexity and bugs in your system.