Data Models and Relations

A few notes about ORMs:

  • Object relational mappers are used to represent data entities and their relations.
  • They are the workhorses of your backend services, especially with CRUD.
  • A manner of representing SQL-related processes in an object-oriented fashion.
  • Helps to abstract complexity of SQL interfacing with the database.
  • A double-edged sword.

1. Create objection classes for person and pet

An objection class representing a standalone person has been created (see below)

export class Person extends Model {
// every column (property) in the table should be typed
id!: number;
name!: string;
age!: number;
surname!: string;
pets: Array<Pet>;
static get tableName() {
return 'person';
}
static get relationMappings() {
return {
name: {
relation: Model.HasManyRelation,
modelClass: Pet,
join: {
from: 'person.id',
//knexSnakeCaseMappers plugin enables snakecase
to: 'pet.personId',
},
},
};
}
}
  • Create another file for Pet and specify a Pet's relation to it's owner.
  • Create another class representing a pet (you can do it in the same file to spend less time importing).
  • Set up the pets relation for the user class by specifying relationMappings.

Notes

  • Not all relations have to be explicitly declared. If you only cared about querying the pets of specific owners (and not the owner of pets, declaring the Pet to Owner relation would not have been necessary).
  • Objection automatically assumes the name of your table is the name of your class, unless indicated otherwise.
  • Relations can be singular, plural, defined, or undefined depending on the relationships.
  • Objection is (mostly) smart enough to tell you when you've specified the wrong types of relationships, but this is can be a likely bug source.
  • This page always comes in handy when specifying relations between objection classes.
  • Types of all relations have to be added to the top of the class.

References

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