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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:
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@orm
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@serializable
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abstract class _Wheel extends Model {
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@belongsTo
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Car get car;
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}
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The local key defaults to
car_id, and the foreign key defaults to id. You can manually override these:1
@BelongsTo(localKey: 'carId', foreignKey: 'licenseNumber')
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Car get car;
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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:
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@orm
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@serializable
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abstract class _Class extends Model {
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String get courseName;
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@ManyToMany(_Enrollment)
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List<_Student> get students;
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}
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@orm
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@serializable
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abstract class _Student extends Model {
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String get name;
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int get year;
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@ManyToMany(_Enrollment)
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List<_Class> get classes;
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}
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@orm
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@serializable
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abstract class _Enrollment {
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@belongsTo
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_Student get student;
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@belongsTo
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_Class get class_;
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}
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Last modified 3yr ago
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Contents
Many-to-many Relationships