TypeOrm QueryBuilder: Add a New Field If a Relation Exists

Welcome to this comprehensive guide on using TypeOrm QueryBuilder to add a new field if a relation exists. In this article, we’ll dive deep into the world of TypeOrm and explore how to harness the power of QueryBuilder to create complex queries that can handle relationships between entities.

What is TypeOrm?

TypeOrm is a TypeScript-based ORM (Object-Relational Mapping) tool that allows developers to interact with databases using a simple and intuitive API. It supports a wide range of databases, including PostgreSQL, MySQL, Microsoft SQL Server, and more. TypeOrm provides a powerful way to define entities, perform CRUD operations, and execute complex queries.

What is QueryBuilder?

QueryBuilder is a powerful tool in TypeOrm that allows developers to build complex queries using a fluent API. It provides a way to construct queries by chaining methods together, making it easy to read and maintain. QueryBuilder is particularly useful when working with complex queries that involve multiple joins, subqueries, and aggregations.

The Problem: Adding a New Field If a Relation Exists

Sometimes, when working with entities that have relationships, we need to conditionally add a new field to our query results based on whether the relationship exists. For example, let’s say we have an `Order` entity that has a one-to-one relationship with a `Customer` entity. We want to retrieve all orders along with the customer’s name, but only if the customer exists. If the customer doesn’t exist, we want to return a default value, such as “Unknown Customer”.

This is where QueryBuilder comes in. We can use QueryBuilder to construct a query that adds a new field to our results based on whether the customer relationship exists.

The Solution: Using QueryBuilder’s `addSelect` Method

To add a new field to our query results, we can use the `addSelect` method provided by QueryBuilder. This method allows us to specify a new field to be added to our results, along with an optional alias.

import { getRepository } from 'typeorm';
import { Order } from './Order.entity';

const orderRepository = getRepository(Order);

const query = orderRepository
  .createQueryBuilder('order')
  .leftJoinAndSelect('order.customer', 'customer')
  .addSelect('CASE WHEN customer.id IS NOT NULL THEN customer.name ELSE \'Unknown Customer\' END AS customerName')
  .getMany();

In this example, we’re using the `createQueryBuilder` method to create a new QueryBuilder instance. We’re then using the `leftJoinAndSelect` method to join the `Order` entity with the `Customer` entity. Finally, we’re using the `addSelect` method to add a new field called `customerName` to our results. The field is calculated using a CASE expression that checks whether the customer relationship exists. If it does, we return the customer’s name; otherwise, we return “Unknown Customer”.

Using QueryBuilder’s `addSelect` Method with Aggregations

Sometimes, we need to perform aggregations on our query results, such as summing or averaging values. We can use QueryBuilder’s `addSelect` method in combination with aggregations to add new fields to our results.

import { getRepository } from 'typeorm';
import { Order } from './Order.entity';

const orderRepository = getRepository(Order);

const query = orderRepository
  .createQueryBuilder('order')
  .leftJoinAndSelect('order.customer', 'customer')
  .addSelect('SUM(order.total) AS totalAmount')
  .addGroupBy('customer.name')
  .addSelect('CASE WHEN customer.id IS NOT NULL THEN customer.name ELSE \'Unknown Customer\' END AS customerName')
  .getMany();

In this example, we’re using the `addSelect` method to add a new field called `totalAmount` to our results, which is calculated by summing the `total` field of each order. We’re also using the `addGroupBy` method to group our results by the customer’s name. Finally, we’re using the `addSelect` method again to add a new field called `customerName` to our results, which is calculated using a CASE expression that checks whether the customer relationship exists.

Using QueryBuilder’s `addSelect` Method with Subqueries

Sometimes, we need to perform subqueries to retrieve additional data. We can use QueryBuilder’s `addSelect` method in combination with subqueries to add new fields to our results.

import { getRepository } from 'typeorm';
import { Order } from './Order.entity';

const orderRepository = getRepository(Order);

const subquery = orderRepository
  .createQueryBuilder('order')
  .select('order.id')
  .where('order.total > 100');

const query = orderRepository
  .createQueryBuilder('order')
  .leftJoinAndSelect('order.customer', 'customer')
  .addSelect('EXISTS(' + subquery.getQuery() + ') AS isHighValueOrder')
  .addSelect('CASE WHEN customer.id IS NOT NULL THEN customer.name ELSE \'Unknown Customer\' END AS customerName')
  .getMany();

In this example, we’re using the `addSelect` method to add a new field called `isHighValueOrder` to our results, which is calculated by checking whether the order exists in a subquery that retrieves all orders with a total value greater than 100. We’re also using the `addSelect` method again to add a new field called `customerName` to our results, which is calculated using a CASE expression that checks whether the customer relationship exists.

Conclusion

In this article, we’ve explored how to use TypeOrm QueryBuilder to add a new field to our query results if a relation exists. We’ve seen how to use the `addSelect` method to add new fields to our results, along with examples of using aggregations and subqueries. By mastering QueryBuilder, you can take your TypeOrm skills to the next level and create complex queries that meet your business requirements.

Method	Description
addSelect	Adds a new field to the query results.
leftJoinAndSelect	Performs a left join with another entity and adds the joined entity to the query results.
addGroupBy	Groups the query results by one or more fields.
getMany	Executes the query and returns an array of results.

Best Practices

1. Use QueryBuilder to construct complex queries in a fluent and readable way.
2. Use the `addSelect` method to add new fields to your query results.
3. Use aggregations and subqueries to perform complex calculations and retrieve additional data.
4. Use the `getMany` method to execute the query and return an array of results.
5. Test your queries thoroughly to ensure they return the expected results.

Common Errors and Solutions

• Error: Unknown column in the `addSelect` method.
  
  Solution: Make sure the column exists in the entity or join the correct entity using the `leftJoinAndSelect` method.
• Error: The subquery returns multiple rows.
  
  Solution: Use the `EXISTS` or `IN` operator to check if the subquery returns at least one row.
• Error: The `addSelect` method is not supported in the current query mode.
  
  Solution: Check the query mode and ensure it supports the `addSelect` method.

By following these best practices and troubleshooting tips, you’ll be well on your way to mastering TypeOrm QueryBuilder and creating complex queries that meet your business requirements.

Frequently Asked Question

TypeORM QueryBuilder is a powerful tool for querying databases, but sometimes we need to add an extra layer of complexity to our queries. One such scenario is when we want to add a new field to our query result if a certain relation exists.