Joining relations
Let's say you have the following entities:

import { Entity, PrimaryGeneratedColumn, Column, OneToMany } from "typeorm"
import { Photo } from "./Photo"

@Entity()
export class User {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    name: string

    @OneToMany((type) => Photo, (photo) => photo.user)
    photos: Photo[]
}
import { Entity, PrimaryGeneratedColumn, Column, ManyToOne } from "typeorm"
import { User } from "./User"

@Entity()
export class Photo {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    url: string

    @ManyToOne((type) => User, (user) => user.photos)
    user: User
}
Now let's say you want to load user "Timber" with all of his photos:

const user = await createQueryBuilder("user")
    .leftJoinAndSelect("user.photos", "photo")
    .where("user.name = :name", { name: "Timber" })
    .getOne()
You'll get the following result:

{
    id: 1,
    name: "Timber",
    photos: [{
        id: 1,
        url: "me-with-chakram.jpg"
    }, {
        id: 2,
        url: "me-with-trees.jpg"
    }]
}
As you can see leftJoinAndSelect automatically loaded all of Timber's photos. The first argument is the relation you want to load and the second argument is an alias you assign to this relation's table. You can use this alias anywhere in query builder. For example, let's take all Timber's photos which aren't removed.

const user = await createQueryBuilder("user")
    .leftJoinAndSelect("user.photos", "photo")
    .where("user.name = :name", { name: "Timber" })
    .andWhere("photo.isRemoved = :isRemoved", { isRemoved: false })
    .getOne()

This will generate following SQL query:

SELECT user.*, photo.* FROM users user
    LEFT JOIN photos photo ON photo.user = user.id
    WHERE user.name = 'Timber' AND photo.isRemoved = FALSE

You can also add conditions to the join expression instead of using “where”:

const user = await createQueryBuilder("user")
    .leftJoinAndSelect("user.photos", "photo", "photo.isRemoved = :isRemoved", {
        isRemoved: false,
    })
    .where("user.name = :name", { name: "Timber" })
    .getOne()

This will generate the following SQL query:

SELECT user.*, photo.* FROM users user
    LEFT JOIN photos photo ON photo.user = user.id AND photo.isRemoved = FALSE
    WHERE user.name = 'Timber'