ActiveHash 4.0 Polymorphic Has_many With Source_type Exception

Hey guys,

I wanted to share something I ran into while working with ActiveHash 4.0. It's a bit of a niche case, but if you're using polymorphic has_many associations with a source_type, you might encounter an issue. Let's dive into the details so you know what to look out for!

The Issue: Polymorphic Associations and source_type

The core issue revolves around how ActiveHash 4.0 handles polymorphic has_many associations, specifically when you specify a source_type. If you're not familiar, polymorphic associations in Rails allow a model to belong to different types of other models through a single association. The source_type option further narrows down the possible types of models in this association.

In my case, we had an association set up like this:

has_many :asset_types, through: :assignments, source: :assignable, source_type: "AssetType"

This association essentially says: "A user has many asset_types through the assignments table, where the assignable association points to models of type AssetType." It's a pretty common pattern for us, and it's been working well.

However, after upgrading to ActiveHash 4.0, this started throwing an exception:

/Users/jpcody/.local/share/mise/installs/ruby/3.3.8/lib/ruby/gems/3.3.0/gems/activerecord-7.1.5.1/lib/active_record/reflection.rb:464:in `compute_class`: Polymorphic associations do not support computing the class. (ArgumentError)

 raise ArgumentError, "Polymorphic associations do not support computing the class."
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 from /Users/jpcody/.local/share/mise/installs/ruby/3.3.8/lib/ruby/gems/3.3.0/gems/activerecord-7.1.5.1/lib/active_record/reflection.rb:412:in `klass'
 from /Users/jpcody/.local/share/mise/installs/ruby/3.3.8/lib/ruby/gems/3.3.0/gems/active_hash-4.0.0/lib/associations/associations.rb:14:in `has_many'
 from /Users/jpcody/Projects/CampusOptics/Code/optics-web/app/models/user.rb:38:in `<class:User>`

The error message, "Polymorphic associations do not support computing the class," hints at a change in how ActiveHash handles these associations internally. Specifically, it seems like the gem is trying to determine the class of the associated model in a way that's incompatible with polymorphic associations when source_type is used.

Digging Deeper into the Root Cause

To understand why this is happening, we need to delve a bit into the internals of Active Record and ActiveHash. The error originates from ActiveRecord::Reflection, which is responsible for managing association metadata. When you define an association, Active Record creates a reflection object that stores information about the association, such as the associated class, the foreign key, and the association type.

In the case of polymorphic associations, the associated class isn't known upfront because it can be one of several different types. This is where the source_type option comes in, allowing you to specify a particular type. However, it appears that ActiveHash 4.0's association logic, likely introduced by #329, attempts to compute the class in a way that doesn't account for this polymorphism, leading to the ArgumentError.

It's worth noting that this issue specifically arises when using both source_type and a polymorphic has_many. If you're using a regular polymorphic association without source_type, or a has_many with source_type on a non-polymorphic association, you likely won't encounter this problem. The combination of these two features seems to be the trigger.

This change in behavior could stem from updates in how Active Record handles reflections or from modifications within ActiveHash itself. Without further investigation into the gem's code and the specific changes introduced in #329, it's challenging to pinpoint the exact cause. However, the error message and the context strongly suggest an incompatibility in how the class is being computed for this specific type of association.

The Impact on Your Application

If you're relying on polymorphic has_many associations with source_type, this issue can be a significant roadblock when upgrading to ActiveHash 4.0. It can cause your application to crash or behave unexpectedly, particularly in areas where these associations are heavily used. This was definitely the case in our codebase, as we leverage this pattern in multiple places.

The immediate impact is the inability to use ActiveHash 4.0 without either refactoring your associations or applying a workaround. This can delay upgrades and potentially force you to stick with older versions of the gem, missing out on any new features or bug fixes. It's crucial to identify these associations in your codebase before upgrading to ActiveHash 4.0 to avoid any surprises.

Furthermore, this issue highlights the importance of thorough testing when upgrading dependencies, especially those that deal with core functionalities like associations. While it's impossible to catch every edge case, having a comprehensive test suite can help identify these kinds of regressions early on.

Workaround: Monkey Patching for the Win (for Now)

Given that we were also on the verge of upgrading Rails, I decided to take a pragmatic approach for the time being. I opted to monkey patch ActiveHash to revert to the old behavior. Monkey patching, for those unfamiliar, is a technique where you modify the behavior of existing code at runtime. It's generally discouraged as a long-term solution, as it can make your code harder to understand and maintain, but it can be a lifesaver in situations like this.

By monkey patching, I was able to get our application running with ActiveHash 4.0 without having to make extensive changes to our codebase immediately. This bought us some time to investigate the issue properly and come up with a more sustainable solution.

However, I want to emphasize that monkey patching is a temporary fix. It's essential to address the underlying problem eventually, either by refactoring your associations, contributing a fix to ActiveHash, or finding an alternative gem. Monkey patches can introduce unexpected side effects and make it harder to upgrade dependencies in the future.

The Code for the Monkey Patch

While I won't provide the exact code for our monkey patch (as it's specific to our application and the version of ActiveHash we were using), the general idea is to revert the changes introduced in #329 that are causing the issue. This might involve overriding methods in ActiveHash::Associations or ActiveRecord::Reflection to restore the previous behavior.

If you find yourself in a similar situation, you'll need to carefully examine the changes introduced in #329 and identify the parts that are causing the ArgumentError. Then, you can use Ruby's metaprogramming features to redefine those methods in a way that's compatible with polymorphic associations and source_type.

Remember to thoroughly test your monkey patch to ensure it doesn't introduce any new issues. Monkey patching can be tricky, and it's easy to accidentally break something if you're not careful.

Is Upgrading Rails the Answer?

One of the reasons I opted for a monkey patch was that we were planning to upgrade Rails soon anyway. I was hoping that a newer version of Rails might have addressed this issue, either by fixing the underlying problem in Active Record or by providing better support for polymorphic associations in general.

Unfortunately, I don't have a definitive answer yet on whether upgrading Rails resolves this issue. It's something we're still investigating. However, it's worth considering as a potential solution. Newer versions of Rails often include improvements and bug fixes that can address compatibility issues with gems like ActiveHash.

If you're facing this problem and are also planning a Rails upgrade, it might be worth trying the upgrade first to see if it resolves the issue. If not, you can always resort to monkey patching or other workarounds.

Documenting the Issue and Reaching Out

Even though I had a temporary solution in place, I felt it was important to document the issue and share it with the community. That's why I decided to write this post. I wanted to make sure that others who might be running into the same problem could find a solution or at least understand what's going on.

I also wanted to open a discussion about this issue and see if anyone else had encountered it or had any insights to share. Maybe someone has already found a better solution, or perhaps there's a more elegant way to address this problem.

This is one of the great things about the open-source community: we can all learn from each other and help each other out. By sharing our experiences and documenting issues, we can make the development process smoother for everyone.

Why Openly Documenting Issues Matters

Documenting issues like this is crucial for several reasons. First, it helps others who encounter the same problem find a solution more quickly. Search engines can index these posts, making them discoverable by anyone searching for the error message or the specific scenario.

Second, it helps the maintainers of the gem understand the problem and potentially fix it in a future release. By providing a clear description of the issue, along with any relevant code or error messages, you're making it easier for them to reproduce the problem and develop a fix.

Finally, documenting issues helps build a knowledge base for the community. Over time, these posts can become valuable resources for developers working with the gem, providing insights into common problems and best practices.

Feel Free to Contribute!

If you have any insights into this issue, or if you've encountered it yourself, please feel free to share your thoughts in the comments below. I'm always eager to learn from others, and I'm sure there are many people who would benefit from your knowledge.

Even if you don't have a solution, simply sharing your experience can be helpful. Knowing that others have encountered the same problem can be reassuring, and it can also help the maintainers of ActiveHash prioritize this issue.

I'm also open to any suggestions for improving this post or making it more helpful. If you have any feedback, please let me know.

Next Steps: A Proper Fix

While the monkey patch is working for now, it's not a long-term solution. My next step is to dive deeper into the ActiveHash code and try to understand the root cause of the issue. I want to figure out why the gem is trying to compute the class in a way that's incompatible with polymorphic associations and source_type.

Once I have a better understanding of the problem, I can either contribute a fix to ActiveHash or refactor our associations to avoid this issue. Ideally, I'd like to contribute a fix so that others don't have to encounter this problem in the future.

Contributing to Open Source

Contributing to open-source projects is a great way to give back to the community and improve the tools we all use. It can also be a valuable learning experience, as it forces you to understand the code at a deeper level.

If you're interested in contributing to open source, there are many ways to get involved. You can start by reporting bugs, submitting feature requests, or improving the documentation. Once you're more comfortable, you can try contributing code fixes or new features.

The ActiveHash project is a great place to start, as it's a relatively small and well-maintained gem. If you're facing this issue, contributing a fix would be a fantastic way to help the community.

Alternative Solutions: Refactoring Associations

If contributing a fix to ActiveHash proves too difficult or time-consuming, another option is to refactor our associations to avoid this issue. This might involve changing the way we're using polymorphic associations or finding an alternative way to represent the relationships between our models.

Refactoring can be a good solution in some cases, but it's important to weigh the costs and benefits carefully. Refactoring can be time-consuming and can introduce new bugs if not done correctly. It's also important to consider the impact on the rest of your codebase.

In our case, we'll need to carefully evaluate whether refactoring our associations is the best approach. It's possible that contributing a fix to ActiveHash would be a more efficient and sustainable solution in the long run.

Conclusion: Navigating the Polymorphic has_many ActiveHash 4.0 Challenge

So, that's the story of my encounter with the polymorphic has_many and source_type issue in ActiveHash 4.0. It's been a bit of a journey, but I've learned a lot along the way.

To recap, if you're using polymorphic has_many associations with source_type, you might encounter an ArgumentError when upgrading to ActiveHash 4.0. The error message, "Polymorphic associations do not support computing the class," indicates an incompatibility in how the gem handles these associations.

A temporary workaround is to monkey patch ActiveHash to revert to the old behavior. However, this is not a long-term solution, and you should eventually either contribute a fix to ActiveHash or refactor your associations.

I hope this post has been helpful to you. If you have any questions or comments, please feel free to share them below. Let's work together to make the Ruby on Rails ecosystem even better!