How to move attributes in Ruby hash "up" one level

Try this:

x = x.merge(x.delete(:data))

I'd go after it this way:

x = {:name => "John", :data => {:physical => {:age => 25, :weight => 150}}}

x[:physical] = x.delete(:data)[:physical]

pp x #=> {:name=>"John", :physical=>{:age=>25, :weight=>150}}