I've changed my model from 

```
class Place
  include DataMapper::Resource
  has n, :trails

  property :id,           Serial
  property :name,         String,         :length => 140
  property :tag,          String,         :required => true
  timestamps :at 
end

```

to

```
class Place
  include DataMapper::Resource
  has n, :trails

  property :id,           Serial
  property :name,         String,         :length => 140
  property :tag,          String,         :required => true
  property :trail_count,  Integer,        :default => 0
  timestamps :at 
end

```

I just added "property :trail\_count, Integer, :default => 0"

and i want to migrate the existing appengine table to have the extra field "trail\_count"
i've read that DataMapper.auto\_upgrade! should do it.

but i get an error "undefined method `auto\_upgrade!' for DataMapper:Module"

can you please help How do i migrate the DM models?