Object-relational mapping (ORM)

What an ORM maps: class and table, attribute and column, association and foreign key, inheritance strategies; lazy or eager loading and the N+1 query problem.

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Object-relational mapping (ORM): what is it?
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A technique and tool that map a program's classes and objects to the tables and rows of a relational database, and generate the SQL in the programmer's place.
Hibernate (which implements the JPA specification), SQLAlchemy or Django's ORM are examples. The SQL is still there: the ORM writes it, and you need to know which SQL it writes.
Which JPA annotation, placed on the auteur attribute, puts the foreign key auteur_id in the livre table?

```java
@Entity
class Livre {
@Id Long id;
String titre;
Auteur auteur;
}
`
- @ManyToOne
- @OneToMany
- @ManyToMany
- @Column(name = "auteur_id")
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@ManyToOne
Many books for one author: the "many" side holds the foreign key, with @ManyToOne, and its default name is auteur_id (@JoinColumn picks another one). @ManyToMany would create a join table, and @Column describes a plain column, not an association.
In an ORM, a class corresponds to a …, an attribute to a … and an object to a ….
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In an ORM, a class corresponds to a table, an attribute to a column and an object to a row.
This is the basic correspondence. The rest (associations, inheritance) follows from it.
Entity: what is it?
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A class whose instances correspond to rows of a table, each identified by a primary key that the ORM tracks.
In JPA, it is marked with @Entity and its identifier with @Id. A class without the mark is not stored in the database.

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