Rails, DataMapper and JSON

I have some problems with a very simple Rails configuration using DataMapper. This is my model:

class Capture
  include DataMapper::Resource
  property :id, Serial
  property :identifier, String
  property :caption, Text
 end

Now I add a new capture in the Rails console:

Capture.create (: identifier => '12345' ,: caption => 'Foo bar foo')

If I try to get all the records

Capture.all

... I get

[#<Capture @id=1 @identifier="12345" @caption=<not loaded>>]

First question: what does “not loaded” mean in this case? But the problem is that I cannot convert the result to JSON:

Capture.all.to_json

NoMethodError: undefined method `encode_json' for #<Capture @id=1 @identifier="12345" @caption=<not loaded>>

Is this a DM problem? How to encapsulate such a result in JSON? Thanks a lot in advance ;-) Chris.