Lets say you have a many-to-many relationship between two types of objects, which are connected by a join table. Lets call them Celebrities and Followers, and lets call the join table Followings.

If you want to add a bunch of followers to a celebrity, you need to create a Following object for each new follower, like so:

Following.create!(user_id: 1, follower_id: 4)
Following.create!(user_id: 1, follower_id: 5)
Following.create!(user_id: 1, follower_id: 6)
Following.create!(user_id: 1, follower_id: 7)
Following.create!(user_id: 1, follower_id: 8)
Following.create!(user_id: 1, follower_id: 9)

This is repetitive and unintuitive. It would be nice if you could do something like user.followers = # *list of followers*. Rails actually provides the functionality to do just that. Here's the syntax:

user.follower_ids = [4, 5, 6, 7, 8, 9]

For each follower id in the list, this will automatically create a new Following entry that joins user with the follower having that id. It will also delete any Following entries joining user to any followers that are not on the list you provide.