How to use leftJoinAndSelect query in TypeORM postgres?

In TypeORM, using the leftJoinAndSelect method allows you to conveniently perform join queries and select specific fields from the joined table. This is particularly useful for handling relational data in the database, especially when retrieving data from multiple tables in a single query.

Basic Usage

Assume we have two entities: User and Photo, where each user can have multiple photos. We can use leftJoinAndSelect to achieve this in TypeORM.

Here is an example of how to use leftJoinAndSelect:

typescript
import { getRepository } from "typeorm";
import { User } from "./entity/User";
import { Photo } from "./entity/Photo";

async function getUsersWithPhotos() {
  const userRepository = getRepository(User);
  
  const users = await userRepository.createQueryBuilder("user")
    .leftJoinAndSelect("user.photos", "photo")
    .getMany();

  return users;
}

In the above code, "user.photos" is the property name defined in the User entity that relates to the Photo entity. "photo" is the alias we specify for the joined table, which we can use to select or conditionally filter specific fields.

Selecting Specific Fields

If you don't need all fields from the Photo table, you can specify the exact fields you want to select:

typescript
const users = await userRepository.createQueryBuilder("user")
  .leftJoinAndSelect("user.photos", "photo")
  .select(["user.name", "user.email", "photo.url"])
  .getMany();

This will retrieve only the name and email fields from user and the url field from photo.

Query with Conditions

You can also add conditions to the leftJoinAndSelect query, such as retrieving only verified users' photos:

typescript
const users = await userRepository.createQueryBuilder("user")
  .leftJoinAndSelect("user.photos", "photo")
  .where("user.isVerified = :isVerified", { isVerified: true })
  .getMany();

Here, the .where method adds a condition to select only users where isVerified is true.

Using leftJoinAndMap

If you need more complex operations, such as customizing the returned structure or combining multiple fields, you can use the leftJoinAndMap method. This method allows you to map the selected results to a new object or entity property.

typescript
const users = await userRepository.createQueryBuilder("user")
  .leftJoinAndMapOne("user.profilePhoto", "user.photos", "photo", "photo.isProfile = true")
  .getMany();

In this example, we select the photos that are profile photos (isProfile field is true).

Summary

leftJoinAndSelect is a powerful tool in TypeORM that simplifies managing and querying relational data. It optimizes the data retrieval process and reduces the amount of code you need to write, as well as potential errors, by allowing you to join and select data from different tables in a single query.