Fixing Ash 3.0: %Ash.NotLoaded{} Errors In Calculations

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Hey folks! Ever run into a snag where your Ash calculations are spitting out %Ash.NotLoaded{} instead of the values you expect? Yeah, it's a common headache, especially after upgrading to Ash 3.0. This article dives deep into why this happens, how to fix it, and how to prevent it in the future. We'll cover the root cause, the broken code, the fixed code, and the steps to reproduce the issue. Let's get started!

The Problem: %Ash.NotLoaded{} and Calculation Failures

So, what's the deal with %Ash.NotLoaded{}? In Ash, this indicates that a related record hasn't been loaded. This typically happens when a calculation tries to access a field from a related resource that hasn't been explicitly loaded during the query. Specifically, in Ash 3.0, the load/3 callback's default behavior changed. It now only loads the primary keys for relationships unless you tell it otherwise. This strict loading can be a gotcha if your calculations rely on fields beyond those primary keys. When Ash 3.0 was released, this change in behavior was part of an effort to improve performance by reducing the amount of data loaded by default. However, it also meant that developers needed to be more explicit about what fields they needed in their calculations.

The Anatomy of the Error

Let's break down the error. The framework throws an "Assumption failed" error. The error message explicitly states the problem: "Invalid return from calculation, expected a value, got %Ash.NotLoaded}`". This tells us precisely where the problem lies a calculation is trying to use a field that hasn't been loaded. This often occurs when your calculation depends on related data. The framework is designed to be efficient, and if you haven't specified that you need the related data, it won't load it. When your calculation tries to access that unloaded data, it gets `%Ash.NotLoaded{. The specific example given involves a :iconcalculation within theCulturalismo.Places.Placeresource, which tries to access theslugfrom a relatedrouteand itsparent`.

Where It Goes Wrong

The load/3 callback in the original code snippet only specified the routes relationship. However, the calculation itself tried to access the slug of the route and the slug of the route.parent. Since the fields were not explicitly loaded, accessing them resulted in the error. This is a crucial detail because it illustrates the need for meticulous field selection in the load/3 callback. Without specifying the required fields, you're setting yourself up for this error.

Deep Dive into the Root Cause: Ash 3.0 Strict Loading

Alright, let's get into the nitty-gritty of the problem. The core issue stems from the strict loading behavior introduced in Ash 3.0. Before this, when you specified a relationship in the load/3 callback, Ash might have loaded more data than you strictly needed. This was often convenient, but it also meant that queries could be less efficient because Ash might load unnecessary data. The change to strict loading was a trade-off. It improves performance by default, but it makes developers responsible for explicitly specifying the fields required by their calculations. This requires a shift in how you think about data loading. You now have to be more deliberate about what you load and where. This change is not necessarily a bug, but rather a change in how the framework operates. The goal is to provide a more efficient and predictable way to load related data. But it also means that you need to understand this new behavior and adjust your code accordingly.

Understanding the load/3 Callback

The load/3 callback is your primary tool for controlling which fields are loaded for related resources. You declare what related data you need here. The callback takes the query, options, and context as arguments. Inside this callback, you specify the relationships and the fields you need from those relationships. If you don't specify fields, only the primary keys are loaded. Understanding how to use this callback effectively is key to avoiding the %Ash.NotLoaded{} error.

The Impact of Strict Loading

With strict loading, every field in a related record that your calculation needs must be specified in the load/3 callback. The consequence of not doing this is Ash.NotLoaded{} values, which breaks your calculation. This can lead to unexpected behavior and hard-to-debug issues. This means that if your calculation depends on the slug of a route or its parent, you need to explicitly tell Ash to load those fields.

Fixing the Problem: Explicitly Loading Required Fields

Okay, so we know why we're seeing the error. Now, let's discuss how to fix it! The solution is straightforward: modify the load/3 callback to explicitly load the required fields. Instead of just loading the relationship, you specify which fields you need from that relationship. This tells Ash to fetch those fields when loading the related data.

The Corrected Code

Here's the corrected code snippet that addresses the problem:

defmodule Culturalismo.Places.Place.Calculations.Icon do
  use Ash.Resource.Calculation

  @impl true
  def load(_query, _opts, _context) do
    [routes: [:slug, parent: [:slug]]]
  end

  # ... rest unchanged
end

In this fixed code, the load/3 callback has been updated to explicitly load the slug field from the routes relationship and the slug field from the parent relationship. This ensures that the slug fields are available when the calculation runs. This is the core of the fix. By specifying [:slug, parent: [:slug]], you are telling Ash to load the slug field of the route and the slug of the route's parent. This ensures that your calculation has access to the data it needs.

The Role of Field Specification

The key takeaway here is explicit field specification. When you are using calculations that rely on relationships, you must explicitly load the fields you'll need within those calculations. This includes any fields used in the calculation itself and any fields used in the get_top_level_route helper function. Without this explicit specification, you'll run into the %Ash.NotLoaded{} error. The code above is the perfect demonstration of that. It corrects the load/3 method to load the fields that are required by the calculation.

Steps to Reproduce the %Ash.NotLoaded{} Error

To solidify your understanding and prevent this issue from biting you again, here's how to reproduce the error:

  1. Set Up the Scenario: Create an Ash resource with relationships. Let's say, a Place resource with a relationship to a Route resource. The Route resource might have a parent relationship to another Route (creating a hierarchy). Ensure that your Place has a calculation that depends on the slug of the Route and the slug of the Route's parent.
  2. Improper load/3: In your Place resource's calculation (e.g., :icon), create a load/3 callback. This is the crucial step. Make sure that your load/3 callback only specifies the relationships without explicitly including the necessary fields (slug in this example). This emulates the broken code.
  3. Run the Calculation: Trigger the calculation. This could be by fetching the Place records through a list_map_places action. This will trigger the calculation.
  4. Observe the Error: You should see the Framework Error * Assumption failed: Invalid return from calculation, expected a value, got %Ash.NotLoaded{} message, and your calculation will fail, demonstrating the issue. You should also observe that you will get the %Ash.NotLoaded{} values in the response. This shows the calculation is failing because it's trying to access the fields that aren't loaded.

By following these steps, you can easily reproduce the error and confirm that the fix (explicitly loading the fields in the load/3 callback) resolves it.

The Environment

For reference, the specific environment where this issue was encountered is:

  • Ash version: 3.5.33
  • Elixir version: 1.18.3
  • OTP version: 27

While these versions are specific, the core problem is related to Ash 3.0's strict loading behavior and will likely affect you regardless of your exact versions as long as you're using Ash 3.0 or later.

Conclusion: Navigating Ash 3.0's Data Loading

Alright, folks, that wraps up our deep dive into the %Ash.NotLoaded{} error in Ash calculations! Remember, the key to avoiding this issue is understanding Ash 3.0's strict loading behavior and being explicit about what data you need in your calculations.

  • Always Explicitly Load Fields: When your calculations depend on related data, make sure to specify the fields you need in the load/3 callback. Don't rely on implicit loading. Use the [relationship: [:field1, :field2]] syntax.
  • Review Your Calculations: Audit your calculations to identify any dependencies on related data. Make sure that the corresponding fields are loaded in your load/3 callbacks.
  • Understand the Trade-Offs: Strict loading provides performance benefits but requires you to be more diligent. Take this into consideration when designing your resources and calculations.

By following these guidelines, you can write more reliable and efficient Ash applications. Happy coding!