Merge pull request #11051 from neerajdotname/10509f

do not load all child records for inverse case

3 files changed, 25 insertions, 1 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index b24222b700..608b5b64cb 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,19 @@
+*   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*
+
 *   `inspect` on Active Record model classes does not initiate a
     new connection. This means that calling `inspect`, when the
     database is missing, will no longer raise an exception.
diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index efd7ecb97c..9833822f8f 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -81,7 +81,7 @@ module ActiveRecord
         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?
 
diff --git a/activerecord/test/cases/associations/inverse_associations_test.rb b/activerecord/test/cases/associations/inverse_associations_test.rb
index d961ceca1d..71cf1237e8 100644
--- a/activerecord/test/cases/associations/inverse_associations_test.rb
+++ b/activerecord/test/cases/associations/inverse_associations_test.rb
@@ -401,6 +401,14 @@ class InverseHasManyTests < ActiveRecord::TestCase
     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
     # delete all interest records to ensure that hard coded invalid_id(s)
     # are indeed invalid.