3 files changed, 38 insertions, 2 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index e5ab6bac58..79247e3028 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,15 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   If inverse_of is true on an association, then when one calls +find()+ on
+    the association, ActiveRecord will first look through the in-memory objects
+    in the association for a particular id. Then, it will go to the DB if it
+    is not found. This is accomplished by calling +find_by_scan+ in
+    collection associations whenever +options[:inverse_of]+ is not nil.
+
+    Fixes #9470.
+
+    *John Wang*
+
 *   `rake db:create` does not change permissions of the MySQL root user.
     Fixes #8079.
 
diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index 906560bd44..73b4a187c8 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -79,7 +79,7 @@ module ActiveRecord
         if block_given?
           load_target.find(*args) { |*block_args| yield(*block_args) }
         else
-          if options[:finder_sql]
+          if options[:finder_sql] || options[:inverse_of]
             find_by_scan(*args)
           else
             scope.find(*args)
@@ -567,7 +567,8 @@ module ActiveRecord
           end
         end
 
-        # If using a custom finder_sql, #find scans the entire collection.
+        # If using a custom finder_sql or if the :inverse_of option has been
+        # specified, then #find scans the entire collection.
         def find_by_scan(*args)
           expects_array = args.first.kind_of?(Array)
           ids           = args.flatten.compact.map{ |arg| arg.to_i }.uniq
diff --git a/activerecord/test/cases/associations/inverse_associations_test.rb b/activerecord/test/cases/associations/inverse_associations_test.rb
index 8c9b4fb921..c8cad84013 100644
--- a/activerecord/test/cases/associations/inverse_associations_test.rb
+++ b/activerecord/test/cases/associations/inverse_associations_test.rb
@@ -278,6 +278,31 @@ class InverseHasManyTests < ActiveRecord::TestCase
     assert interests[1].man.equal? man
   end
 
+  def test_parent_instance_should_find_child_instance_using_child_instance_id
+    man = Man.create!
+    interest = Interest.create!
+    man.interests = [interest]
+
+    assert interest.equal?(man.interests.first), "The inverse association should use the interest already created and held in memory"
+    assert interest.equal?(man.interests.find(interest.id)), "The inverse association should use the interest already created and held in memory"
+    assert man.equal?(man.interests.first.man), "Two inversion should lead back to the same object that was originally held"
+    assert man.equal?(man.interests.find(interest.id).man), "Two inversions should lead back to the same object that was originally held"
+  end
+
+  def test_parent_instance_should_find_child_instance_using_child_instance_id_when_created
+    man = Man.create!
+    interest = Interest.create!(man: man)
+
+    assert man.equal?(man.interests.first.man), "Two inverses should lead back to the same object that was originally held"
+    assert man.equal?(man.interests.find(interest.id).man), "Two inversions should lead back to the same object that was originally held"
+
+    assert_equal man.name, man.interests.find(interest.id).man.name, "The name of the man should match before the name is changed"
+    man.name = "Ben Bitdiddle"
+    assert_equal man.name, man.interests.find(interest.id).man.name, "The name of the man should match after the parent name is changed"
+    man.interests.find(interest.id).man.name = "Alyssa P. Hacker"
+    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_trying_to_use_inverses_that_dont_exist_should_raise_an_error
     assert_raise(ActiveRecord::InverseOfAssociationNotFoundError) { Man.first.secret_interests }
   end