When working with a model, say a User, that has a has_many association with another model, say a Post, you can create a new post for a user like so:

u1 = User.first
=> #<User:0x...>
u1.posts.create(title: "Some Title", content: "...")
=> #<Post:0x...>

What about with a has_one association? Consider a Customer that has a has_one association with an Account. Rails provides this method for you:

c1.create_account(account_number: 123, ...)
=> #<Account:0x...>

Rails also gives you a similar build method:

c1.build_account(account_number: 123, ...)
=> #<Account:0x...>