What is the best way to compare xml with subsequent save?

Question

Ruby on Rails

I would like to compare two xml files (they are large, about 50MB, 100 thousand lines each) and save everything that has changed in the database. Preferably very quickly. The xml structure is simple, literally 6 nodes.
I just can't figure it out. Line by line is not entirely logical. SAX - so it will give me the data of one line of one file and it is impossible to search for this data in the second file (it is possible, but also not logical). How to do it better here?