Rails has_one vs has_many association

I've a model say Book and another model named say Title. The associations are simply defined below.

class Book < ApplicationRecord
    has_one :title
end

class Title < ApplicationRecord
    belongs_to :book
end

Now, if I generate a schema with a foreign key of book_id in Title and create a new record for Book, I am not being able to create a new Title record via book.title.create! as book.title returns nil.

But, if I change the association to

 class Book < ApplicationRecord
    has_many :titles
 end

then I am clearly able to create a new Title record using book.title.create!. So, what I have to do to make it work with the has_one association is do something like book.title = Title.create!(foo: "bar", book_id: book.id).

When I checked the values for book.title in the latter case, the console returns <ActiveRecord::Associations::CollectionProxy []> but for the former, book.title returns nil and hence the NoMethod Error for nil class.

I tried following the official docs and some other posts in stack regarding this. But somehow, its still unclear to me.

The reason of this behaviour is that Rails creates methods in a form of create_#{association_name} for every has_one association_name. There are docs for this, if you need extra information.

Thus, to create a title for a book, you'll need something like:

book.create_title

It will perform the trick, you're expecting