luckyframework/avram

Sub-associations from polymorphic types don't work

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#1,036 opened on May 8, 2024

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Description

It appears we never actually test using polymorphic associations. If you try and call a method on them, you'll get a compile time error because Crystal can't determine the true shape of the object.

table do
  belongs_to user : User
  belongs_to car : Car?
  belongs_to computer : Computer?
  polymorphic purchasable, associations: [:car, :computer]
end

purchase = PurchaseQuery.new.preload_purchasable.user_id(current_user.id).first

# We currently test that this works, and it does...
purchase.purchasable

# This however does not work
purchase.purchasable.owner!
# => Error: undefined method 'owner!' for User (compile-time type is BaseModel+)

In order to do this, you have to cast it to the proper type. purchase.purchasable.as(Car).owner!, but the whole point of the polymorphic association is that it could be any number of types so that gets a bit tricky

Contributor guide

Research direction
Investigate how to resolve the compile time type error for polymorphic associations by using type unions or case statements to narrow the type before calling methods. Consider if the library can provide a helper method that returns the proper type or if a crystal idiom like 'as' with a union is needed.
Tech stack
None
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Mostly clear
Prerequisites
CrystalAvram
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40