Relations & QueryBuilder — Series 2

Preview — 3 of 10 questions

What does @JoinColumn declare?

javascript

@Entity()
export class Cat {
  @ManyToOne(() => Owner, owner => owner.cats)
  @JoinColumn({ name: 'owner_id' })
  owner: Owner;
}

AThat the relation should be loaded eagerly whenever a Cat is read

BThat this side owns the relation and therefore carries the foreign-key column — cat.owner_id — which is what makes Cat the side whose changes actually update the link

CThat a join table should be created to connect the two entities

DThat the relation is read-only from this side

How do these two options differ?

javascript

@OneToMany(() => Comment, comment => comment.post, { cascade: ['insert'] })
comments: Comment[];

@ManyToOne(() => Post, post => post.comments, { onDelete: 'CASCADE' })
post: Post;

AThey are synonyms, expressed on the two sides of the relation

Bcascade applies to reads and onDelete to writes

ConDelete is an ORM-level setting and cascade a database constraint

Dcascade is an ORM behaviour — saving a Post also inserts its new Comments in the same operation — while onDelete: 'CASCADE' is emitted into the foreign-key constraint, so the database removes children when a parent row is deleted, including by raw SQL

What is the difference?

javascript

qb.leftJoinAndSelect('cat.owner', 'owner')  // (1)
qb.leftJoin('cat.owner', 'owner').where('owner.city = :city', { city }) // (2)

ABoth join the table, but only (1) adds the joined entity's columns to the SELECT list and hydrates cat.owner; (2) joins purely so the relation can be filtered on, leaving cat.owner undefined

B(2) produces an inner join despite its name

C(1) issues two queries and (2) one

DThey are equivalent; AndSelect is a readability suffix

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