To preface I no longer recommend using TypeORM, it's pretty dead & there are one too > many > issues to deal with - though I'm using it in production if I could go back in time I'd totally use something else, Prisma looks pretty sweet though so check that out

polymorphic joins

simplest but not the best in terms of data integrity because no fk constraint, uses 2 columns for an entity type & _id

@Entity()
export class Invoice extends BaseEntity {
  @Column() obj_type: "ticket" | "merch" | "patron";
  @Column() obj_id: string;
}

downside is needing two queries to get the items you want / search

const invoices = await Invoice.find({ type: "ticket" });
const tickets = Ticket.find({ _id: In(invoices.map((i) => i._id)) });

table per type

create table invoice_ticket (
  invoice_id integer not null unique references invoice,
  ticket_id integer not unique null references ticket
);

create table invoice_merch (
  invoice_id integer not null unique references invoice,
  merch_id integer not null unique references merch
);

column per type

keeps fk but no guarantee entity doesn't have more than one relation, can do nested queries

@Entity()
export class Invoice extends BaseEntity {
  @ManyToOne(() => Ticket) ticket: Ticket;
  @ManyToOne(() => Merch) merch: Merch;
  @ManyToOne(() => Patron) patron: Patron;

  get data() {
    return [this.ticket, this.merch, this.patron].find((e) => e !== undefined);
  }
}

const tickets = await Invoice.find({
  where: { ticket: { price: 10 } },
  relations: ["ticket"],
  select: { ticket: { _id: true, price: true } },
}).map((i) => i.ticket);