Eloquent With Avg with average of related models

TL;DR Calculating the average rating for each category in a Laravel application can be done using Eloquent's avg method on a relation. For example: $category = Category::with('products')->get(); $averageRating = $category->products()->avg('rating');. To calculate averages across multiple related models, use the withAverage method provided by Laravel.

Calculating Average Values in Eloquent: A Guide to Using AVG with Related Models

As a Laravel developer, you're likely no stranger to working with Eloquent, the ORM (Object-Relational Mapping) system provided by Laravel. One of the most powerful features of Eloquent is its ability to perform complex queries and calculations on your database data. In this article, we'll dive into the world of calculating average values in Eloquent, specifically when dealing with related models.

The Problem: Calculating Average Values

Let's say you have a Product model that belongs to a Category, and each product has a rating associated with it. You want to calculate the average rating for each category. Sounds simple, right? Well, things get interesting when you need to perform this calculation across multiple related models.

The Solution: Using AVG with Related Models

In Eloquent, you can use the avg method on a relation to calculate the average value of a column in the related model. Here's an example:

$category = Category::with('products')->get();
$averageRating = $category->products()->avg('rating');

This code retrieves all categories and their associated products, then calculates the average rating for each category using the avg method on the products relation.

But what if you want to calculate the average rating across multiple related models? That's where things get a bit more complicated. You can use the withAverage method provided by Laravel to perform this calculation:

$category = Category::with('products')->withAverage('products.rating', 'rating');
$averageRating = $category->products()->avg('rating');

This code retrieves all categories and their associated products, then calculates the average rating for each category using the avg method on the products relation. The withAverage method is used to specify that we want to calculate the average rating across multiple related models.

A Note on Joins

One common pitfall when working with averages in Eloquent is joining tables unnecessarily. When calculating averages, it's often more efficient to use subqueries or window functions instead of joining tables.

For example, if you're using a query like this:

$category = Category::with('products')->get();
$averageRating = DB::table('categories')
    ->join('products', 'categories.id', '=', 'products.category_id')
    ->avg('rating');

This can lead to performance issues and unnecessary joins. Instead, use subqueries or window functions like this:

$category = Category::with('products')->get();
$averageRating = DB::table('categories')
    ->select(
        'categories.*',
        DB::raw('AVG(products.rating) as average_rating')
    )
    ->joinSub(
        'products',
        'p',
        function ($join) {
            $join->on('categories.id', '=', 'p.category_id');
        }
    )
    ->groupBy('categories.id')
    ->avg('average_rating');

This query uses a subquery to join the products table and calculate the average rating for each category.

Conclusion

Calculating averages in Eloquent can be a complex task, but with the right techniques and tools, you can make it happen. Whether you're using the avg method on a relation or leveraging subqueries and window functions, Laravel provides a powerful set of features to help you tackle even the most challenging queries.

In this article, we've explored how to use Eloquent's avg method with related models to calculate average values in your database data. With practice and patience, you'll become a master of calculating averages in no time!