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Postgresql – How do I create a new database record that has a many-to-one relation?

I’m learning NestJS with TypeORM (PostgreSQL) and trying to create a library website for practice.

Here are my entities, Book and Author. An author can have multiple books, but each book can belong only to one author.

import { Author } from 'src/author/entities/author.entity';
import { Column, Entity, ManyToOne, PrimaryGeneratedColumn } from 'typeorm';

@Entity()
export class Book {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  title: string;

  @ManyToOne(() => Author, (author) => author.books)
  author: Author;

  @Column()
  summary: string;

  @Column()
  isbn: string;
}

import { Book } from 'src/book/entities/book.entity';
import { Column, Entity, OneToMany, PrimaryGeneratedColumn } from 'typeorm';

@Entity()
export class Author {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  firstName: string;

  @Column()
  lastName: string;

  @OneToMany(() => Book, (book) => book.author)
  books: Book[];
}

Here are the DTOs:

export class CreateBookDto {
  title: string;
  author: number;
  summary: string;
  isbn: string;
}

export class CreateAuthorDto {
  firstName: string;
  lastName: string;
}

And here’s the method that adds a new Book to the database:

import { Injectable } from '@nestjs/common';
import { CreateBookDto } from './dto/create-book.dto';
import { InjectRepository } from '@nestjs/typeorm';
import { Book } from './entities/book.entity';
import { Repository } from 'typeorm';

@Injectable()
export class BookService {
  constructor(
    @InjectRepository(Book) private readonly bookRepository: Repository<Book>,
  ) {}

  create(createBookDto: CreateBookDto) {
    return this.bookRepository.save(createBookDto);
  }
}

A typical request would look like this:

{
  title: "Example Book Title",
  author: 1,
  summary: "Example Book Summary",
  isbn: "978-3-16-148410-0"
}

The author should already be in the database and would be selected with a prefilled <select> tag, for example. The number is that author’s id in the database table.

I think there’s something wrong with my DTOs or the way I’m creating a new Book, because typescript is giving me this error for this.bookRepository.save(createBookDto);:

No overload matches this call.
  Overload 1 of 4, '(entities: DeepPartial<Book>[], options?: SaveOptions | undefined): Promise<(DeepPartial<Book> & Book)[]>', gave the following error.
    Argument of type 'CreateBookDto' is not assignable to parameter of type 'DeepPartial<Book>[]'.
      Type 'CreateBookDto' is missing the following properties from type 'DeepPartial<Book>[]': length, pop, push, concat, and 29 more.
  Overload 2 of 4, '(entity: DeepPartial<Book>, options?: SaveOptions | undefined): Promise<DeepPartial<Book> & Book>', gave the following error.
    Argument of type 'CreateBookDto' is not assignable to parameter of type 'DeepPartial<Book>'.
      Type 'CreateBookDto' is not assignable to type '{ id?: number | undefined; title?: string | undefined; author?: DeepPartial<Author> | undefined; summary?: string | undefined; isbn?: string | undefined; }'.
        Types of property 'author' are incompatible.
          Type 'number' is not assignable to type 'DeepPartial<Author> | undefined'.

The error goes away if I do this:

create(createBookDto: CreateBookDto) {
  const book = Object.assign(new Book(), createBookDto)
  return this.bookRepository.save(book);
}

However, I’m not sure if that’s the correct way to create something. I haven’t seen such a pattern yet.

What should I do? How are relations usually handled when creating a new record? Maybe there’s something wrong with author: number in CreateBookDto?

Tags:

2

Answers


  1. 0

    Typeorm relationships – save by id

    author: 1 as any

    Try the author in this way

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  2. 0

    The author property in CreateBookDto is of type number, while the author property in Book is of type Author. TypeScript sees that you’re trying to assign a number (the author’s ID) where it expects an instance of Author, leading to a type mismatch error.

    Here is the updated code

    @Entity()
export class Book {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  title: string;
  
  @Column()
  authorId: number;
  
  @Column()
  summary: string;

  @Column()
  isbn: string;

  @ManyToOne(() => Author, (author) => author.books)
  author: Author;
  @JoinColumn({
    name: "authorId",
  })
}
    export class CreateBookDto {
  title: string;
  authorId: number;
  summary: string;
  isbn: string;
}

    updated create method

    @Injectable()
export class BookService {
  constructor(
    @InjectRepository(Book) private readonly bookRepository: Repository<Book>,
  ) {}

  create(createBookDto: CreateBookDto) {

    // Create a new Book instance
    const book = new Book();
    book.title = "Example Book Title";
    book.authorId = 1;
    book.summary = "Example Book Summary";
    book.isbn = "978-3-16-148410-0";

    // Save the new Book
    return this.bookRepository.save(book);
  }
}
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