add Connection#db_from_uri to round out MONGODB_URI support

lib/mongo/connection.rb

@@ -327,6 +327,22 @@ module Mongo
       DB.new(db_name, self)
     end
 
+    # Return the database specified in ENV['MONGODB_URI']
+    #
+    # @param [String] uri the uri to use
+    # @param [Hash] opts options to be passed to the DB constructor.
+    #
+    # @return [Mongo::DB]
+    #
+    # @core databases db_from_uri-instance_method
+    def db_from_uri(uri=ENV['MONGODB_URI'], opts={})
+      if db_name = uri[%r{/([^/\?]+)(\?|$)}, 1]
+        DB.new(db_name, self, opts)
+      else
+        raise ArgumentError.new("No database name found in #{uri}")
+      end
+    end
+
     # Drop a database.
     #
     # @param [String] name name of an existing database.

test/connection_test.rb

@@ -82,6 +82,46 @@ class TestConnection < Test::Unit::TestCase
     end
   end
 
+  def test_db_from_uri_exists_no_options
+    begin
+      db_name = "_database"
+
+      old_mongodb_uri = ENV['MONGODB_URI']
+      ENV['MONGODB_URI'] = "mongodb://#{host_port}/#{db_name}"
+      con = Connection.from_uri
+      db = con.db_from_uri
+      assert_equal db.name, db_name
+    ensure
+      ENV['MONGODB_URI'] = old_mongodb_uri
+    end
+  end
+
+  def test_db_from_uri_exists_options
+    begin
+      db_name = "_database"
+
+      old_mongodb_uri = ENV['MONGODB_URI']
+      ENV['MONGODB_URI'] = "mongodb://#{host_port}/#{db_name}?"
+      con = Connection.from_uri
+      db = con.db_from_uri
+      assert_equal db.name, db_name
+    ensure
+      ENV['MONGODB_URI'] = old_mongodb_uri
+    end
+  end
+
+  def test_db_from_uri_exists_no_db_name
+    begin
+      old_mongodb_uri = ENV['MONGODB_URI']
+      ENV['MONGODB_URI'] = "mongodb://#{host_port}/"
+      con = Connection.from_uri
+
+      assert_raise ArgumentError do con.db_from_uri end
+    ensure
+      ENV['MONGODB_URI'] = old_mongodb_uri
+    end
+  end
+
   def test_server_version
     assert_match(/\d\.\d+(\.\d+)?/, @conn.server_version.to_s)
   end