Make a user watch a tag using rails

Is there a way of making a user watch a tag using the rails console so that they receive notifications whenever something is posted with that tag? For instance, I am looking for something like:

u=User.find(10)
u.watched_tags = [“tag1”, “tag2”]

I gave up on rails and decided to do it with the API, however, sending a put request to “watching_first_post_tags” doesn’t change the variable. Is it possible to do it with the API or rails?