move functionality to Connection#db, set up default database name to use when absolutely nothing is provided

lib/mongo/connection.rb

@@ -34,6 +34,7 @@ module Mongo
 
     DEFAULT_HOST = 'localhost'
     DEFAULT_PORT = 27017
+    DEFAULT_DB_NAME = 'test'
     GENERIC_OPTS = [:ssl, :auths, :pool_size, :pool_timeout, :timeout, :op_timeout, :connect_timeout, :safe, :logger, :connect]
     CONNECTION_OPTS = [:slave_ok]
 
@@ -312,7 +313,13 @@ module Mongo
     # @return [Mongo::DB]
     #
     # @core databases db-instance_method
-    def db(db_name, opts={})
+    def db(db_name=nil, opts={})
+      if !db_name && uri = ENV['MONGODB_URI']
+        db_name = uri[%r{/([^/\?]+)(\?|$)}, 1]
+      end
+
+      db_name ||= DEFAULT_DB_NAME
+
       DB.new(db_name, self, opts)
     end
 

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
+      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
+      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
+      db = con.db
+      assert_equal db.name, Mongo::Connection::DEFAULT_DB_NAME
+    ensure
+      ENV['MONGODB_URI'] = old_mongodb_uri
+    end
+  end
+
   def test_server_version
     assert_match(/\d\.\d+(\.\d+)?/, @conn.server_version.to_s)
   end