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Managing Polymorphic Relations in API Output

The Question​

Introducing the Challenge​

In a Laravel project based on RESTful APIs, there is a challenge in the process of transforming and standardizing API output, especially when models have Polymorphic Relations.

Problem Description​

When an entity or model includes a polymorphic relation, the API output can have different structures in various scenarios. This causes the client side (referred to here as Jupiter) to be forced to check multiple conditions or if/else statements to implement the appropriate behavior based on the type of entity received (e.g., if it's of type X).

Core Problem

The presence of conditional logic in the client to detect and manage different types of polymorphic entities leads to high coupling and unmaintainable code.

Proposed Solutions​

To overcome this challenge, two main proposals have been presented, and a combination of both can be used. However, the optimal implementation approach for these solutions is still unclear and ambiguous.

First Proposal: Define Separate Relations​

This approach is based on defining separate relations. According to this proposal, for each entity that consumes the polymorphic relation in the target table, a separate relation should be defined using the name of those entities.

Second Proposal: Output Standardization (Minimal Output)​

This approach focuses on output standardization. It is suggested that the transformer class output, for the field that creates the polymorphic relation, should be a minimal output with standard fields.

Standard Output

This output should always include two specific fields:

  • An Identifier field
  • A Display field

Ambiguity in Optimal Implementation​

Despite these proposals, the main question or ambiguity is: how can we implement this standardization (especially for minimal output) in the best way?

Three main approaches have been proposed for implementation:

First Approach: Decision-Making in the Transformer Layer

The transformer layer, which has access to polymorphic entity data, directly decides which output field to place this data in.

Second Approach: Standardization in the Repository Layer

In the repository layer, queries are written using aliases to standardize the output.

Third Approach: Using Model Methods and Contracts
  • For the identifier field, use Laravel's getPrimaryKey() method
  • For the display field, a method should be defined or contracted on all models

Key Questions​

Important Notes

In your response, consider the following:

  • If you state that something is or isn't business logic, explain why
  • If you state that something is an anti-pattern, explain why

Regarding the third approach, there is an important ambiguity:

Is it correct to do this (define a Show Field method in the model) or not?

This hesitation arises because the display field fundamentally has a presentation purpose.

The Answer​

Subject: Architectural Analysis and Decision-Making for Managing Polymorphic Relations in API Output​

Alright, technical team. The issue that has been raised is one of the most classic challenges in RESTful API design and the boundary between server and client. Let's dissect this topic carefully and based on software engineering principles.

1. Problem Statement and Constraints (Context)​

We have a Laravel API. In this API, Polymorphic Relations have caused our JSON output to lack a consistent structure. As a result, our client (the Jupiter team) has been forced to implement if/else logic to detect the type of data received and behave accordingly.

Initial Crisis Analysis​

Clear Anti-Pattern

This situation is a clear anti-pattern and a direct red line in our architecture. We are leaking business logic to the client side.

Why is this Business Logic?

The fact that a Comment can belong to a Post or Video is a rule and reality in the business domain. The client (Jupiter) should not and cannot be responsible for understanding and managing this internal server complexity.

Principle Violations​

Coupling (Dependency):

We have created high coupling between the internal server implementation and the client implementation. Every time we add a new type (e.g., Article) to this polymorphic relation in the server, the client team must modify their code and add a new else if.

OCP Violation

This is a direct violation of the Open/Closed Principle (OCP).

SoC (Separation of Concerns):

The responsibility of "managing types of commentable entities" has been mistakenly transferred from the server to the client.

Our Goal​

Our goal should be to eliminate conditional logic in the client for managing this relation. The client should receive a fixed and standard contract from the server.

2. Analysis of Proposed Solutions​

Let's examine the proposed options.

First Proposal: Define Separate Relations​

Definitive Rejection

I categorically reject this proposal that "a separate relation should be defined for each entity."

This is erasing the problem statement and completely ignoring the benefit of using Polymorphism. This design:

Violates the OCP principle:

  • With the addition of each new entity, you must modify the main model and add a new relation

Violates the DRY principle:

  • Query logic will be repeated

Violates the SRP principle:

  • Our model becomes heavy and aware of all its consumers

Second Proposal: Output Standardization (Minimal Output)​

Correct Direction

This is the correct architectural direction.

We need to abstract the output. The client should not know Post or Video; the client should know a "Displayable Entity" that has id and name (or title).

Now let's examine three implementation approaches for this proposal:

3. Comparative Analysis of Implementation Options​

How do we produce this "standard output"?

Approach 1: Decision-Making in the Transformer Layer​

Description:

In the transformer itself (e.g., Fractal or Eloquent API Resources), we check with if/else or instanceof what type the model is and based on that, place the title or name field in the output.

Analysis:

This approach is slightly better than leaking logic to the client because at least it keeps the logic on the server (in the Presentation layer).

Code Smell

However, this is a Code Smell and an explicit violation of OCP.

If tomorrow an Article entity is added, you'll be forced to return to this transformer and add an else if ($model instanceof Article). The system is open for extension (adding Article), but not closed for modification (changing the Transformer).

Tell, Don't Ask Violation:

We are "asking" the model what you are (instanceof), instead of "telling" it what to do.

Conclusion:

danger

Rejected. This is a brittle and unmaintainable solution.

Approach 2: Standardization in the Repository Layer (Query Alias)​

Description:

In the database query, use AS (e.g., SELECT title AS display_name) to standardize the output.

Analysis:

Dangerous Anti-Pattern

This is a dangerous anti-pattern and a severe violation of SoC.

Why is this an Anti-Pattern?

The repository layer (or Data Access layer) is responsible for fetching domain entities or pure Data Transfer Objects (DTOs). This layer should not have any knowledge about "how to display" data in the API.

When you alias title to display_name in the repository, you're coupling the data layer to the presentation layer. If tomorrow we want to change the field name in the API output to display_label, we'd have to change the database query! This is a disaster.

Conclusion:

danger

Categorically rejected. This destroys architectural layering.

Approach 3: Using Model Methods (Interface Contract)​

Description:

Define an Interface (contract) in PHP (e.g., DisplayableContract). This interface forces models to implement two methods: getDisplayIdentifier() and getDisplayName().

Analysis:

Correct Solution

This is the most correct, stable, and professional solution.

Answer to the Ambiguity (Is this Presentation Logic?):

Domain Logic, Not Presentation Logic

No. The fact that "which field" represents the textual representation of an entity is a "Domain Rule" and part of that entity's identity, not presentation logic.

Conceptual Distinction:

  • Presentation Logic means: "Display this text in blue in an H1 tag."
  • Domain Logic means: "A Post is identified by its title and a User by its full_name." This knowledge belongs to the model itself.

Alignment with SOLID Principles:

OCP (Open/Closed)

Our transformer only depends on DisplayableContract. If tomorrow Article is added, it's sufficient for Article to implement this interface. The transformer will never change.

LSP (Liskov Substitution)

The transformer can work with any object that implements DisplayableContract (whether Post or Video) without its behavior breaking.

DIP (Dependency Inversion)

The high-level module (Transformer) doesn't depend on low-level modules (Post, Video). Both depend on an Abstraction (i.e., DisplayableContract).

Alignment with Other Principles:

Tell, Don't Ask:

  • The transformer no longer "asks" the model who you are, but "tells" it to give me your display name ($model->getDisplayName()).

SoC:

  • Each part does its own job. The model knows its display name. The transformer knows how to package it in JSON. The client only displays it.

4. Final Recommendation​

Definitive Solution

The definitive solution is to implement "option three" (using Interface Contract).

Any other solution will lead to Technical Debt and system fragility in the future. We will not tolerate if/else code (either in the client or in the server transformer).

5. Step-by-Step Action Plan​

Step 1: Define the Contract (Interface)​

Define an interface in the Domain or Application layer (depending on project structure):

app/Contracts/DisplayableContract.php
namespace App\Contracts;

interface DisplayableContract
{
    /**
     * Returns the unique identifier of the entity.
     * @return int|string
     */
    public function getDisplayIdentifier();

    /**
     * Returns the display name of the entity.
     * @return string
     */
    public function getDisplayName(): string;
}

Step 2: Implement the Contract in Models​

All models that use the polymorphic relation (Post, Video, ...) must implement this interface.

app/Models/Post.php
use App\Contracts\DisplayableContract;

class Post extends Model implements DisplayableContract
{
    // ... (existing code)

    public function getDisplayIdentifier()
    {
        return $this->id; // or getRouteKey()
    }

    public function getDisplayName(): string
    {
        return $this->title;
    }
}
app/Models/Video.php
use App\Contracts\DisplayableContract;

class Video extends Model implements DisplayableContract
{
    // ... (existing code)

    public function getDisplayIdentifier()
    {
        return $this->id;
    }

    public function getDisplayName(): string
    {
        return $this->name;
    }
}

Step 3: Modify the Transformer (API Resource)​

The transformer (or the class responsible for converting the model to an array) should not have any conditional logic.

app/Http/Resources/CommentResource.php
// Assume we're in a model's transformer that has
// a 'commentable' polymorphic relation.

public function toArray($request)
{
    /** @var \App\Contracts\DisplayableContract $commentable */
    $commentable = $this->whenLoaded('commentable'); 

    // Robustness check: Make sure it's loaded and of the contract type
    if (!($commentable instanceof DisplayableContract)) {
        // Fast Fail: If not loaded or wrong type, error or return null
        return [
            // ...
            'related_item' => null,
        ];
    }

    return [
        'id' => $this->id,
        'body' => $this->body,
        // ...
        'related_item' => [
            'id' => $commentable->getDisplayIdentifier(),
            'name' => $commentable->getDisplayName(),
            // 'type' => $this->commentable_type, // (optional, if client really needs it)
        ],
    ];
}
Note

The getPrimaryKey() method mentioned in the question is good, but getDisplayIdentifier() in the interface is more explicit.

Step 4: Client Cleanup (Jupiter Team)​

The client team must remove all if/else blocks related to type detection and only use the standard related_item object and the id and name fields.

6. Risk Identification and Fallback Plan​

Risk 1: The number of models involved in this polymorphic relation is large and implementing the interface is time-consuming​

Response:

info

This refactoring is mandatory. This time is an investment to prevent future maintenance costs.

Risk 2: Some models don't have a clear title or name field​

Response:

This is a problem in domain modeling. The business analyst should be consulted about what the "textual representation" of that entity is. It might be a combination of two fields. This should be encapsulated in the getDisplayName() method of that model.

Fallback Plan​

Warning

No fallback plan that violates architectural principles (like solutions 1 and 2) is acceptable.

The only acceptable fallback is phased implementation. However, the final API output must be standardized from day one, even if interface implementations are done in phases.

Conclusion​

Use this analysis as the basis for work and begin step-by-step execution.