I love Datamapper, but there are some aspects of it that throw me because they don’t work the way they should on first glance.

Foreign Constraints

Let’s make a few objects.

class Apple < DataMapper::Base
 
  property :name, :string
 
end
 
class Banana < DataMapper::Base
 
  belongs_to :apple, :class => 'Apple', :foreign_key => 'a1'
  belongs_to :apple, :class => 'Apple', :foreign_key => 'a2'
 
end

And now:

>> b = Banana.find_or_create :a1 => { :name => "Red Delicious" }, :a2 => { :name => "Granny Smith" }
RuntimeError: Don't know how to quote {:name=>"Red Delicious"}

Oof. So we have to hold Datamapper’s hand the whole way.

>> b = Banana.find_or_create :a1 => Apple.find_or_create( :name => "Red Delicious" ), 
                             :a2 => Apple.find_or_create( :name => "Granny Smith")

Updating Properties by Self-Reference

Here’s our test object.

class Apple < DataMapper::Base
 
  property :name, :string
  property :weight, :integer
 
end

And now:

>> a = Apple.find_or_create :name => "Red Delicious"
>> a.update_attributes :weight => 0

So far, so good. But…

>> a.update_attributes :weight => :weight + 1

Will fail horribly. Of course,

>> a.weight = a.weight + 1

will work, but if we have a hashload of attributes we want to update, this becomes a little cumbersome.