# Mastering TypeORM: Solutions to Limiting Relation Join and Agregation Operation

TypeORM is a powerful Object-Relational Mapping (ORM) library for TypeScript and JavaScript applications. It simplifies the interaction between your application and the underlying database, allowing you to work with entities and relationships instead of raw SQL queries. However, as your application grows in complexity, you may encounter scenarios where you need to perform more advanced queries or manipulations.

In this article, we'll explore two common questions from Stack Overflow related to TypeORM and provide detailed explanations and solutions to help you master these scenarios.

## Question 1: How to Limit Children While Left Joining in TypeORM?

One common requirement in many applications is to fetch data from multiple related entities while limiting the number of child entities returned. This is especially useful when dealing with scenarios like displaying a limited number of comments for a blog post or showing a preview of recent activities.

Here's a solution to this problem using TypeORM:

```typescript
// 1. Fetch all users
const users = await this.userRepository
  .createQueryBuilder("user")
  .getMany();

// 2. Loop through users and fetch photos with limit
for (const user of users) {
  user.photos = await this.photoRepository
    .createQueryBuilder("photo")
    .where("photo.userId = :userId", { userId: user.id })
    .limit(5) // Set the desired limit here
    .getMany();
}

// Return users with limited photos
```

In this example, we first fetch all users using the `userRepository`. Then, we loop through each user and fetch the associated photos using the `photoRepository`. The key step here is the `.limit(5)` clause, which limits the number of photos returned to 5 for each user.

This approach involves two separate queries: one to fetch all users, and another to fetch the limited photos for each user. While this solution works, it may not be the most efficient approach, especially if you have a large number of users or photos.

An alternative approach could be to use a single query with a subquery to fetch the limited photos for each user:

```typescript
const users = await this.userRepository
  .createQueryBuilder("user")
  .leftJoinAndSelect(
    "user.photos",
    "photo",
    "photo.id IN (SELECT p.id FROM Photo p WHERE p.userId = user.id ORDER BY p.createdAt DESC LIMIT 5)"
  )
  .getMany();
```

In this example, we use the `leftJoinAndSelect` method to fetch the users and their associated photos in a single query. The subquery `SELECT` [`p.id`](http://p.id) `FROM Photo p WHERE p.userId =` [`user.id`](http://user.id) `ORDER BY p.createdAt DESC LIMIT 5` selects the top 5 most recent photos for each user based on the `createdAt` column. The `IN` clause then filters the photos to only include those returned by the subquery.

This approach can be more efficient as it reduces the number of database queries, but it may be more complex to write and maintain, especially for more intricate scenarios.

## Question 2: How to Use TypeORM's SUM Operator on a Relation's Field?

Another common requirement in applications is to perform aggregations, such as calculating the sum, average, or count of values from related entities. For example, you might want to calculate the total duration of all songs in a playlist.

Here's how you can achieve this using TypeORM:

```typescript
const playlist = await this.playlistRepository
  .createQueryBuilder("playlist")
  .where("playlist.id = :playlistId", { playlistId })
  .leftJoinAndSelect("playlist.songs", "songs")
  .addSelect("SUM(songs.duration)", "totalDuration")
  .getRawOne();

console.log(playlist.totalDuration);
```

In this example, we use the `createQueryBuilder` to build a query for the `playlistRepository`. We start by filtering for a specific playlist using the `where` clause.

Next, we use the `leftJoinAndSelect` method to include the related `songs` entities in the query. This allows us to access the `duration` field of each song.

The key step here is the `addSelect("SUM(songs.duration)", "totalDuration")` clause. This tells TypeORM to calculate the sum of the `duration` field from all related songs and alias the result as `totalDuration`.

Finally, we use the `getRawOne` method to execute the query and retrieve the result as a raw object. The `totalDuration` property of this object will contain the calculated sum of all song durations for the specified playlist.

It's important to note that `getRawOne` and `getRawMany` are used when you need to perform aggregations or other advanced SQL operations that cannot be easily represented using TypeORM's entity model.

## Conclusion

TypeORM is a powerful tool that simplifies database interactions in TypeScript and JavaScript applications. However, as your application's requirements grow more complex, you may encounter scenarios where you need to perform advanced queries or manipulations.

In this article, we explored two common questions from Stack Overflow related to TypeORM: limiting the number of child entities returned while left joining, and using the SUM operator on a relation's field. We provided detailed explanations and solutions to help you master these scenarios and unlock the full potential of TypeORM in your applications.

Remember, while TypeORM abstracts away much of the complexities of working with databases, it's still essential to have a solid understanding of SQL and query optimization techniques. By combining the power of TypeORM with your SQL knowledge, you can build robust and efficient applications that meet even the most demanding requirements.