Warn on performance issues: non-normalized result set

lib/jsonapi/active_relation_resource.rb

@@ -181,6 +181,10 @@ def find_fragments(filters, options = {})
           end
         end
 
+        if JSONAPI.configuration.warn_on_performance_issues && (rows.length > fragments.length)
+          warn "Performance issue detected: `#{self.name.to_s}.records` returned non-normalized results in `#{self.name.to_s}.find_fragments`."
+        end
+
         fragments
       end
 

lib/jsonapi/configuration.rb

@@ -10,6 +10,7 @@ class Configuration
                 :raise_if_parameters_not_allowed,
                 :warn_on_route_setup_issues,
                 :warn_on_missing_routes,
+                :warn_on_performance_issues,
                 :default_allow_include_to_one,
                 :default_allow_include_to_many,
                 :allow_sort,
@@ -60,6 +61,7 @@ def initialize
 
       self.warn_on_route_setup_issues = true
       self.warn_on_missing_routes = true
+      self.warn_on_performance_issues = true
 
       # :none, :offset, :paged, or a custom paginator name
       self.default_paginator = :none
@@ -272,6 +274,8 @@ def allow_include=(allow_include)
 
     attr_writer :warn_on_missing_routes
 
+    attr_writer :warn_on_performance_issues
+
     attr_writer :use_relationship_reflection
 
     attr_writer :resource_cache

test/controllers/controller_test.rb

@@ -3625,6 +3625,29 @@ def test_book_comments_exclude_unapproved_context_based
   end
 end
 
+class Api::V4::PostsControllerTest < ActionController::TestCase
+  def test_warn_on_joined_to_many
+    original_config = JSONAPI.configuration.dup
+
+    JSONAPI.configuration.warn_on_performance_issues = true
+    _out, err = capture_subprocess_io do
+      get :index, params: {fields: {posts: 'id,title'}}
+      assert_response :success
+    end
+    assert_equal(err, "Performance issue detected: `Api::V4::PostResource.records` returned non-normalized results in `Api::V4::PostResource.find_fragments`.\n")
+
+    JSONAPI.configuration.warn_on_performance_issues = false
+    _out, err = capture_subprocess_io do
+      get :index, params: {fields: {posts: 'id,title'}}
+      assert_response :success
+    end
+    assert_empty err
+
+  ensure
+    JSONAPI.configuration = original_config
+  end
+end
+
 class Api::V4::BooksControllerTest < ActionController::TestCase
   def setup
     JSONAPI.configuration.json_key_format = :camelized_key

test/fixtures/active_record.rb

@@ -1989,7 +1989,15 @@ class PreferencesResource < PreferencesResource; end
 
 module Api
   module V4
-    class PostResource < PostResource; end
+    class PostResource < PostResource
+      class << self
+        def records(options = {})
+          # Sets up a performance issue for testing
+          super(options).joins(:comments)
+        end
+      end
+    end
+
     class PersonResource < PersonResource; end
     class ExpenseEntryResource < ExpenseEntryResource; end
     class IsoCurrencyResource < IsoCurrencyResource