Merge pull request #10566 from neerajdotname/10509d

do not load all child records for inverse case

activerecord/CHANGELOG.md

+*   Do not load all child records for inverse case.
+
+    currently `post.comments.find(Comment.first.id)` would load all
+    comments for the given post to set the inverse association.
+
+    This has a huge performance penalty. Because if post has 100k
+    records and all these 100k records would be loaded in memory
+    even though the comment id was supplied.
+
+    Fix is to use in-memory records only if loaded? is true. Otherwise
+    load the records using full sql.
+
+    Fixes #10509.
+
+    *Neeraj Singh*
+
 *   Fixture setup does no longer depend on `ActiveRecord::Base.configurations`.
     This is relevant when `ENV["DATABASE_URL"]` is used in place of a `database.yml`.
 

activerecord/lib/active_record/associations/collection_association.rb

@@ -81,7 +81,7 @@ def find(*args)
         else
           if options[:finder_sql]
             find_by_scan(*args)
-          elsif options[:inverse_of]
+          elsif options[:inverse_of] && loaded?
             args = args.flatten
             raise RecordNotFound, "Couldn't find #{scope.klass.name} without an ID" if args.blank?
 

activerecord/test/cases/associations/inverse_associations_test.rb

@@ -401,6 +401,14 @@ def test_parent_instance_should_find_child_instance_using_child_instance_id_when
     assert_equal man.name, man.interests.find(interest.id).man.name, "The name of the man should match after the child name is changed"
   end
 
+  def test_find_on_child_instance_with_id_should_not_load_all_child_records
+    man = Man.create!
+    interest = Interest.create!(man: man)
+
+    man.interests.find(interest.id)
+    refute man.interests.loaded?
+  end
+
   def test_raise_record_not_found_error_when_invalid_ids_are_passed
     man = Man.create!