add data_mapper example

models/data_mapper.md

@@ -0,0 +1,47 @@
+DataMapper
+----------
+
+Start out by getting the DataMapper gem if you don't already have it, and then
+making sure it's in your application. A call to `setup` as usual will get the
+show started, and this example will include a 'Post' model.
+
+    require 'rubygems'
+    require 'sinatra'
+    require 'data_mapper' # metagem, requires common plugins too.
+
+    # need install dm-sqlite-adapter
+    DataMapper::setup(:default, "sqlite3://#{Dir.pwd}/blog.db")
+
+    class Post
+        include DataMapper::Resource
+        property :id, Serial
+        property :title, String
+        property :body, Text
+        property :created_at, DateTime
+    end
+
+    # Perform basic sanity checks and initialize all relationships
+    # Call this when you've defined all your models
+    DataMapper.finalize
+
+    # automatically create the post table
+    Post.auto_upgrade!
+
+Once that is all well and good, you can actually start developing your
+application!
+
+    get '/' do
+        # get the latest 20 posts
+        @posts = Post.all(:order => [ :id.desc ], :limit => 20)
+        erb :index
+    end
+
+Finally, the view at `./view/index.html`:
+
+    <% @posts.each do |post| %>
+        <h3><%= post.title %></h3>
+        <p><%= post.body %></p>
+    <% end %>
+
+For more information on DataMapper, check out the [project
+documentation](http://datamapper.org/docs/ "DataMapper").