Many times, you have an object, and you want to only update a piece of data. While you could use @var.save, that would save the entire model instance, not just a piece.
Say you have a news website that runs a list of articles. You track the number of views. When viewed, you want to increment the number of views.
What do you do? In the show page, you could do this:
@article.views += 1
@article.save
While this works, the problem is that it re-saves the ENTIRE article, NOT just the number of views. If you get thousands of views per hour, this could prove to be a scalability bottleneck! So what do you do?
The answer is that you use update instead of save. Update takes two parameters–the ID of the row/instance to update, and a hash of which values to update. Use this code instead of the above:
Article.update(@article.id, {:views => @article.views + 1}
And that’s it! Rails will increment the view-count without re-saving the entire article! Handy, eh?
