sdb_dal is an object to database mapping layer.  It enables you to create persistable domain model objects stored in Amazon's SDB (Simple DB).  A typical domain model class will look like this. 

class StorageItem  < SdbDal::DomainObject
    
    data_attribute :id ,:string,:is_primary_key=>true,:unique=>true
    data_attribute :path,:string,:unique=>true
    data_attribute :uploaded_date,:date
    data_attribute :login,:string
    data_attribute :description,:string
    data_attribute :title,:string     
    data_attribute :has_audio,:boolean
    data_attribute :has_video,:boolean
    data_attribute :has_image,:boolean
    data_attribute :metadata,:string,:is_collection=>true #unlike relation DB columns, sdb attributes can have multiple values
    
    has_many :StorageFile,:storage_item_id,:storage_files
    belongs_to :User,:login,:user
	many_to_many :Album, :AlbumItems, :storage_item_id,:album_id, :items

    index :login_and_has_audio ,[:login,:has_audio]
    index :login_and_has_video ,[:login,:has_video]
    index :login_and_has_image ,[:login,:has_image]
end     

SDB does not have a schema the way relational databases do.  SDB_DAL forces you to create a schema in your code. You this in your class using the data_attribute method.   Relationships and indexes can also be created.   

The resulting class will expose many helper methods.  Some examples...
   
item=StorageItem.find(some_id)
item.title='new title'
item.save!
user=item.user                
albums=item.albums                               
storage_item=StorageItem.find_by_path('some path')
storage_items=StorageItem.find_by_title('some title') 
storage_items=StorageItem.find_by_login_and_has_audio('david',true)