Add backward compatibility for 0.9.x result.resource access

Add `resource` and `resources` methods to ResourceSetOperationResult and
ResourcesSetOperationResult for backward compatibility with 0.9.x API.

In 0.9.x, operation results had a `resource` accessor. In 0.10+, resources
are accessed via `resource_set`. These new methods allow existing code
that uses `result.resource` to continue working without modification.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: takaokouji

lib/jsonapi/operation_result.rb

@@ -58,6 +58,32 @@ def to_hash(serializer)
         # :nocov:
       end
     end
+
+    # Returns the first resource from the resource_set for backward compatibility with 0.9.x
+    # In 0.9.x, OperationResult had a `resource` accessor that returned the single resource.
+    # This method provides the same interface for code that relied on `result.resource`.
+    def resource
+      return nil unless resource_set&.resource_klasses
+      resource_set.resource_klasses.each_value do |identities|
+        identities.each_value do |data|
+          return data[:resource] if data[:resource]
+        end
+      end
+      nil
+    end
+
+    # Returns all resources from the resource_set for backward compatibility with 0.9.x
+    # Useful when the result contains multiple resources.
+    def resources
+      return [] unless resource_set&.resource_klasses
+      result = []
+      resource_set.resource_klasses.each_value do |identities|
+        identities.each_value do |data|
+          result << data[:resource] if data[:resource]
+        end
+      end
+      result
+    end
   end
 
   class ResourcesSetOperationResult < OperationResult
@@ -80,6 +106,29 @@ def to_hash(serializer)
         # :nocov:
       end
     end
+
+    # Returns the first resource from the resource_set for backward compatibility with 0.9.x
+    def resource
+      return nil unless resource_set&.resource_klasses
+      resource_set.resource_klasses.each_value do |identities|
+        identities.each_value do |data|
+          return data[:resource] if data[:resource]
+        end
+      end
+      nil
+    end
+
+    # Returns all resources from the resource_set for backward compatibility with 0.9.x
+    def resources
+      return [] unless resource_set&.resource_klasses
+      result = []
+      resource_set.resource_klasses.each_value do |identities|
+        identities.each_value do |data|
+          result << data[:resource] if data[:resource]
+        end
+      end
+      result
+    end
   end
 
   class RelatedResourcesSetOperationResult < ResourcesSetOperationResult

test/unit/resource/backward_compatibility_test.rb

@@ -0,0 +1,87 @@
+require File.expand_path('../../../test_helper', __FILE__)
+
+# Tests for backward compatibility with 0.9.x API
+class BackwardCompatibilityTest < ActiveSupport::TestCase
+  # Test find_by_key with old signature: find_by_key(id, context)
+  # New signature: find_by_key(key, options = {})
+  def test_find_by_key_with_old_signature
+    # 0.9.x style: find_by_key(id, context)
+    # In 0.9.x, context was passed directly as second argument
+    # In 0.10+, context should be passed as options[:context]
+    # For backward compatibility, if second arg is a Hash without :context key,
+    # it should still work (context will be nil in resource)
+    context = { current_user: 'test_user' }
+    resource = PostResource.find_by_key(1, context)
+
+    assert_not_nil resource
+    assert_equal 1, resource.id
+  end
+
+  def test_find_by_key_with_new_signature
+    # 0.10+ style: find_by_key(key, options = {})
+    context = { current_user: 'test_user' }
+    resource = PostResource.find_by_key(1, context: context)
+
+    assert_not_nil resource
+    assert_equal 1, resource.id
+  end
+
+  def test_find_by_key_without_context
+    # find_by_key without any context
+    resource = PostResource.find_by_key(1)
+
+    assert_not_nil resource
+    assert_equal 1, resource.id
+  end
+end
+
+# Tests for ResourceSetOperationResult backward compatibility
+class ResourceSetOperationResultBackwardCompatibilityTest < ActiveSupport::TestCase
+  def setup
+    # Create a helper to build ResourceSet with resources
+  end
+
+  def test_resource_method_returns_first_resource
+    # Create a ResourceSet with a single resource using the proper API
+    post = Post.find(1)
+    resource = PostResource.new(post, nil)
+
+    # Use the ResourceSet with a resource (not nil)
+    resource_set = JSONAPI::ResourceSet.new(resource)
+    resource_set.mark_populated!
+
+    result = JSONAPI::ResourceSetOperationResult.new(:ok, resource_set)
+
+    # 0.9.x style: result.resource
+    assert result.respond_to?(:resource), "ResourceSetOperationResult should respond to :resource for 0.9.x compatibility"
+    assert_equal resource, result.resource
+  end
+
+  def test_resource_method_returns_nil_for_empty_resource_set
+    # Create an empty resource set by passing an empty array
+    resource_set = JSONAPI::ResourceSet.new([])
+    result = JSONAPI::ResourceSetOperationResult.new(:ok, resource_set)
+
+    assert_nil result.resource
+  end
+
+  def test_resources_method_returns_all_resources
+    # Create a ResourceSet with multiple resources
+    post1 = Post.find(1)
+    post2 = Post.find(2)
+    resource1 = PostResource.new(post1, nil)
+    resource2 = PostResource.new(post2, nil)
+
+    # Use the ResourceSet with an array of resources
+    resource_set = JSONAPI::ResourceSet.new([resource1, resource2])
+    resource_set.mark_populated!
+
+    result = JSONAPI::ResourceSetOperationResult.new(:ok, resource_set)
+
+    # 0.9.x style: result.resources (for collections)
+    assert result.respond_to?(:resources), "ResourceSetOperationResult should respond to :resources for 0.9.x compatibility"
+    assert_equal 2, result.resources.length
+    assert_includes result.resources, resource1
+    assert_includes result.resources, resource2
+  end
+end