How to properly organize the structure of entities?

Question

symfony

How to properly organize the structure?
At the moment there are two
Book entities - booking
Extra - Additional services
when booking a person can add a few additional points to the base price
, because of this the total amount will increase
, you need to get the price of additional services and their names and attach this economy to the Book
I'm a little confused already. As I see the solution:
An additional BookExtra entity is created
in Book via OneToOne and bound to BookExtra
* @ORM\OneToOne(targetEntity="AppBundle\Entity\BookExtra")
* @ORM\JoinColumn(name="bookextra_id", referencedColumnName="id")
everything is clear here
And BookExtra needs to be tied to Extra, it seems to me through the OneToMany relationship.
One BookExtra can have several Extras.
How can I describe the connection in this case?

Answer 1

And why it is impossible to do without the third entity? Wouldn't it be logical to keep the price of the add. services just in one more field of entity Book?

Answer 2

ManyToMany relationship, symfony will automatically create a table with relationships