Add tests for Issue #1467: ActiveModel resources support

[takaokouji]

Summary

Adds comprehensive tests for Issue JSONAPI-Resources#1467 and documents the correct approach for using ActiveModel-based models with jsonapi-resources.

Problem

Issue JSONAPI-Resources#1467 reported that ActiveModel-based models fail with undefined method 'where' errors in v0.10.7+. Users were confused about why their non-ActiveRecord models stopped working.

Root Cause

The default JSONAPI::Resource inherits from ActiveRelationResource, which expects ActiveRecord query methods like:

• where, order, limit, offset
• Other ActiveRecord::Relation methods

ActiveModel::Model doesn't provide these methods, causing failures.

Solution

Use JSONAPI::BasicResource for non-ActiveRecord models:

class SimpleModelResource < JSONAPI::BasicResource
  model_name 'SimpleModel'
  
  attribute :name
  attribute :description
  
  # Override methods for custom data access
  def self.find_by_key(key, options = {})
    # Custom implementation
  end
  
  def self.find_fragments(filters, options = {})
    # Custom implementation
  end
end

Tests Added

1. active_model_resource_test.rb - Demonstrates BasicResource working correctly with ActiveModel

   • find_fragments with and without filters
   • find_by_key
   • No ActiveRecord dependencies

2. active_model_broken_test.rb - Shows the problem with ActiveRelationResource

   • Using default JSONAPI::Resource fails with NoMethodError
   • Confirms the error is about missing ActiveRecord methods

Testing

• All 695 tests pass
• New tests verify both working and broken scenarios
• Serves as documentation for future users

Documentation

This PR serves as living documentation showing:

• ✅ How to correctly use BasicResource with ActiveModel
• ❌ Why using Resource (ActiveRelationResource) fails
• 📝 Clear examples for future users

Related

• Upstream issue: https://github.com/cerebris/jsonapi-resources/issues/1467

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