* Schema Definition
ActiveRecord keep all schema definitions in migration files, while DataMapper store most of them in model source code. I prefer DataMapper's way, so you won't need plugins like annotate_models any more. Do you use annotate_models in your rails project?
However, keep model schema in model source code brings more complexity, because sometimes you have tasks which are better written in a migration file instead of model source file. So DataMapper has seperate migrations too, and merb provides more migration rake tasks than rails.
* Many-to-Many Relationships Through Scoped Join Model
Sorry for my poor description, this ticket illustrates the problem well. I don't know why this bug won't be fixed in ActiveRecord - below code works well in DataMapper.
has n, :bookmarks, :through => :subscriptions, :conditions => "subscriptions.notification = 'f'"
ActiveRecord introduced named_scope in since 2.x (can't remember), and add dynamic scope (scope_by_xxx like dynamic finder find_by_xxx) in the latest 2.3 release, so now you can chain up many dynamic scopes, named scope and find now. That's really cool, but why we need two seperate finders, find and scope?
In DataMapper, "find" and "scope" is unified. You can do this
# all the keys and property setup here
all(:open => true)
all(:animal_count.gte => 1000)
big_open_zoos = Zoo.big.open
As you can see, the finder #all is able to be chained up or used independently, smart. And I like tye syntax all(:open => true) than find_all_by_open(true): hash can be auto complete by editors while '_' can not, hash param doesn't require dynamic tricks while find_xxx need. Sometimes I feel rails takes too many cares to his baby programmers, sometimes I feel merb takes too little (not in this case, of course).
* Inheritance vs Include
ActiveRecord model is required to inherit from ActiveRecord::Base, while DataMapper model need to include DataMapper::Resource. Composition is better than inheritance, though module inclusion is in fact inheritance in Ruby.
I didn't realize all the things I written down are votes for DataMapper until I write this sentence .. I want to say ActiveRecord is an excellent ORM framework too, at least it's more mature than DataMapper, I enjoy it at most of the time, it just doesn't work in some few case (I really hope the many-to-many relationship through scope join model problem can be fixed, it's a common pattern I've seen it serveral times). The good news is by Rails and Merb merge, rails will become super flexible and you'll be able to switch between ORMs in Rails 3 easily.