How to return only some columns of a relations with Typeorm

In TypeORM, to return only specific columns from a table, you can use the select method in your query. Here are several different ways to use the select method to return specified columns:

Using Repository or Entity Manager

Example 1: Using QueryBuilder

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

async function selectSpecificColumns() {
  const userRepository = getRepository(User);
  const users = await userRepository
    .createQueryBuilder('user')
    .select(['user.id', 'user.name', 'user.email'])
    .getMany();

  return users;
}

In this example, the createQueryBuilder method creates an SQL query, and the select method specifies the columns to retrieve. user is the alias used in the query.

Example 2: Using find method

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

async function selectSpecificColumns() {
  const userRepository = getRepository(User);
  const users = await userRepository.find({
    select: ['id', 'name', 'email']
  });

  return users;
}

In this example, the select option within the find method is used to specify the columns to query.

Using Query Builder

Example 3: Using QueryBuilder with select method

typescript
import { createQueryBuilder } from 'typeorm';
import { User } from '../entity/User';

async function selectSpecificColumns() {
  const users = await createQueryBuilder(User, 'user')
    .select(['user.id', 'user.name', 'user.email'])
    .getMany();

  return users;
}

In this example, the createQueryBuilder method is used to create and execute the query. The first parameter is the entity class, and the second parameter is the query alias. The select method then specifies the columns to return.

Notes

• For the select method, provide an array of the fields you want to select. These fields must match the property names defined in your entity class.
• When using the select method to return related entities, ensure you also select their relevant fields.
• If using the find method, the selected fields must correspond to the entity's property names.
• If your query involves multiple tables and join operations, use the correct aliases in the select method to distinguish fields from different tables.

By using these methods, you can effectively control the fields returned in TypeORM queries, improving application performance by transmitting only necessary data.