How to left join JSON function in TypeORM

In TypeORM, performing a left join with JSON data typically involves two main steps: configuring entity relationships and using the query builder to execute the join. Here, I will provide a detailed explanation of how to implement this, along with a specific example to clarify the process.

Step 1: Configuring Entity Relationships

First, ensure that relationships between your entities are properly defined. For instance, consider two entities: User and Profile, representing a one-to-one relationship between users and their profiles:

typescript
import {Entity, PrimaryGeneratedColumn, Column, OneToOne, JoinColumn} from "typeorm";
import { Profile } from "./Profile";

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

    @Column()
    name: string;

    @OneToOne(() => Profile)
    @JoinColumn()
    profile: Profile;
}

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

    @Column()
    age: number;

    @Column({ type: "json" })
    details: any;
}

In this example, the Profile entity includes a details column of JSON type.

Step 2: Using the Query Builder to Execute a Left Join

Next, leverage TypeORM's query builder to perform the left join and access JSON data. The critical aspect is using appropriate expressions to handle JSON fields. Suppose you want to retrieve all users along with specific information from their profile details, such as their city of residence:

typescript
import {getManager} from "typeorm";
import {User} from "./User";

async function getUsersWithCity() {
    const entityManager = getManager(); // Alternatively, use getRepository(User)
    const users = await entityManager.createQueryBuilder(User, "user")
        .leftJoinAndSelect("user.profile", "profile")
        .select([
            "user.name",
            "profile.details ->> 'city' AS city"
        ])
        .getRawMany(); // Use getRawMany() for raw results when accessing JSON data

    return users;
}

In this query, leftJoinAndSelect connects the Profile entity, and PostgreSQL's JSON operator ->> is used to extract city information from the JSON object. Note that getRawMany() is employed to handle raw results, as the output includes data parsed from the JSON column.

Summary

By following these steps, you can effectively perform a left join in TypeORM to operate on and access JSON data within related entities. This approach is particularly valuable for managing complex data models with JSON attributes in real-world applications. I hope this example helps you understand how to work with JSON functions and implement advanced queries in TypeORM.