Relations

Relational modeling is one of the most commonly-used features of sql databases - after all, it is the namesake of the term "relational database."

Angel supports the following kinds of relations by means of annotations on fields:

  • @hasOne (one-to-one)

  • @hasMany (one-to-many)

  • @belongsTo (one-to-one)

  • @manyToMany (many-to-many)

By default, the keys for columns are inferred automatically. In the following case:

abstract class _Wheel extends Model {
Car get car;
}

The local key defaults to car_id, and the foreign key defaults to id. You can manually override these:

(localKey: 'carId', foreignKey: 'licenseNumber')
Car get car;

The ORM computes relationships by performing JOINs, so that even complex relationships can be fetched using just one query, rather than multiple.

Many-to-many Relationships

A very common situation that occurs when using relational databases is where two tables may be bound to multiple copies of each other. For example, in a school database, each student could be registered to multiple classes, and each class could have multiple students taking it.

This is typically handled by creating a third table, which joins the two together. In the Angel ORM, this is relatively straightforward:

abstract class _Class extends Model {
String get courseName;
(_Enrollment)
List<_Student> get students;
}
abstract class _Student extends Model {
String get name;
int get year;
(_Enrollment)
List<_Class> get classes;
}
abstract class _Enrollment {
_Student get student;
_Class get class_;
}