Relations

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Introduction

Migration from Angel 1.1.x

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:
@orm
@serializable
abstract class _Wheel extends Model {
  @belongsTo
  Car get car;
}
The local key defaults to car_id, and the foreign key defaults to id. You can manually override these:
@BelongsTo(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:
@orm
@serializable
abstract class _Class extends Model {
  String get courseName;
​
  @ManyToMany(_Enrollment)
  List<_Student> get students;
}
​
@orm
@serializable
abstract class _Student extends Model  {
  String get name;
  int get year;
​
  @ManyToMany(_Enrollment)
  List<_Class> get classes;
}
​
@orm
@serializable
abstract class _Enrollment {
    @belongsTo
    _Student get student;
​
    @belongsTo
    _Class get class_;
}

Basic Functionality

Migrations

Last modified 3yr ago

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