entity-inheritance.md

Entity Inheritance

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• Entity Inheritance

Entity Inheritance

Class inheritance is the great way to reduce code duplication, this is supported by TypeDORM using entity inheritance pattern. Here is the base example of how it works with TypeDORM.

For example, you have Admin, Student and Parent entities with following attributes.

@Entity({
  name: 'admin', // name of the entity that will be added to each item as an attribute
  // primary key
  primaryKey: {
    partitionKey: 'ADMIN#{{id}}',
    sortKey: 'ADMIN#{{id}}',
  }
})
export class Admin {
  @AutoGenerateAttribute({
    strategy: AUTO_GENERATE_ATTRIBUTE_STRATEGY.UUID4,
  })
  id: string;

  @Attribute()
  name: string;

  @Attribute()
  department: string

  @AutoGenerateAttribute({
    strategy: AUTO_GENERATE_ATTRIBUTE_STRATEGY.EPOCH,
    autoUpdate: true
  })
  updatedAt: string
}

@Entity({
  name: 'student', // name of the entity that will be added to each item as an attribute
  // primary key
  primaryKey: {
    partitionKey: 'STUDENT#{{id}}',
    sortKey: 'STUDENT#{{id}}',
  }
})
export class Student {
  @AutoGenerateAttribute({
    strategy: AUTO_GENERATE_ATTRIBUTE_STRATEGY.UUID4,
  })
  id: string;

  @Attribute()
  name: string;

  @Attribute()
  studentId: number

  @AutoGenerateAttribute({
    strategy: AUTO_GENERATE_ATTRIBUTE_STRATEGY.EPOCH,
    autoUpdate: true
  })
  updatedAt: string
}

@Entity({
  name: 'parent', // name of the entity that will be added to each item as an attribute
  // primary key
  primaryKey: {
    partitionKey: 'PARENT#{{id}}',
    sortKey: 'PARENT#{{id}}',
  }
})
export class Parent {
  @AutoGenerateAttribute({
    strategy: AUTO_GENERATE_ATTRIBUTE_STRATEGY.UUID4,
  })
  id: string;

  @Attribute()
  name: string;

  @AutoGenerateAttribute({
    strategy: AUTO_GENERATE_ATTRIBUTE_STRATEGY.EPOCH,
    autoUpdate: true
  })
  updatedAt: string
}

All of our Admin, Student and Parent has attributes id, name, and updatedAt and only few attributes change. To reduce code duplication and have better abstractions, we can rewrite these entities like:

export abstract class User {
  @AutoGenerateAttribute({
    strategy: AUTO_GENERATE_ATTRIBUTE_STRATEGY.UUID4,
  })
  id: string;

  @Attribute()
  name: string;

  @AutoGenerateAttribute({
    strategy: AUTO_GENERATE_ATTRIBUTE_STRATEGY.EPOCH,
    autoUpdate: true
  })
  updatedAt: string
}

@Entity({
  name: 'admin', // name of the entity that will be added to each item as an attribute
  // primary key
  primaryKey: {
    partitionKey: 'ADMIN#{{id}}',
    sortKey: 'ADMIN#{{id}}',
  }
})
export class Admin extends User{
  @Attribute()
  department: string
}

@Entity({
  name: 'student', // name of the entity that will be added to each item as an attribute
  // primary key
  primaryKey: {
    partitionKey: 'STUDENT#{{id}}',
    sortKey: 'STUDENT#{{id}}',
  }
})
export class Student extends User {
  @Attribute()
  studentId: number
}

@Entity({
  name: 'parent', // name of the entity that will be added to each item as an attribute
  // primary key
  primaryKey: {
    partitionKey: 'PARENT#{{id}}',
    sortKey: 'PARENT#{{id}}',
  }
})
export class Parent extends User {}

Much better right? You can also overwrite attributes defined on base classes like this:

@Entity({
  name: 'parent', // name of the entity that will be added to each item as an attribute
  // primary key
  primaryKey: {
    partitionKey: 'PARENT#{{id}}',
    sortKey: 'PARENT#{{id}}',
  }
})
export class Parent extends User {
  @AutoGenerateAttribute({
    strategy: AUTO_GENERATE_ATTRIBUTE_STRATEGY.KSUID,
  })
  id: string;
}

Here parent entity will now have id generated using KSUID strategy and rest of the entities will still use UUID4 strategy derived from base class User.

You can go event further by having a base class like BaseEntity as predefine all the attributes that all other entities in your application must have, common ones being id, createdAt, updatedAt, and make all other entities extend this BaseEntity with their own schema.

Note: you do not ever want to annotate base class with @Entity and should always be marked abstract to force other extend it instead of initializing it, or it can cause many confusion and unwanted behaviors, as TypeDORM is not designed to work with inherited with @Entity annotation.