A relationship between two ActiveRecord models can be established with a has_one or has_many association. This relationship has some implications. By default, saving a record will also save the associated records that have since been built. Consider this example of users that have many posts (has_many posts).

> u = User.first
#=> #<User ...>
> u.posts
#=> []
> u.posts.build(title: "Some Title", content: "This is a post")
#=> #<Post ...>
> u.save
#=> true
> u.posts(reload: true)
#=> [#<Post ...>]

When the user is saved, the associated post that was built for that user also gets saved to the database.

If the association is instead defined with the autosave option set to false, then saving a record will not cause associated records to also be saved. The associated records will need to be saved explicitly. Consider the same example from above, but with has_many posts, autosave: false.

> u = User.first
#=> #<User ...>
> u.posts
#=> []
> u.posts.build(title: "Some Title", content: "This is a post")
#=> #<Post ...>
> u.save
#=> true
> u.posts(reload: true)
#=> []

The post wasn’t saved with the user and it wasn’t saved explicitly, so it isn’t persisted to the database.