Merge pull request rails#50284 from seanpdoyle/close-49574

Infer `:inverse_of` for `has_many ..., through:`

Author: byroot

activerecord/lib/active_record/reflection.rb

@@ -708,9 +708,10 @@ def inverse_name
         def automatic_inverse_of
           if can_find_inverse_of_automatically?(self)
             inverse_name = ActiveSupport::Inflector.underscore(options[:as] || active_record.name.demodulize).to_sym
+            plural_inverse_name = ActiveSupport::Inflector.pluralize(inverse_name)
 
             begin
-              reflection = klass._reflect_on_association(inverse_name)
+              reflection = klass._reflect_on_association(inverse_name) || klass._reflect_on_association(plural_inverse_name)
             rescue NameError => error
               raise unless error.name.to_s == class_name
 
@@ -720,7 +721,7 @@ def automatic_inverse_of
             end
 
             if valid_inverse_reflection?(reflection)
-              inverse_name
+              reflection.name
             end
           end
         end

activerecord/test/cases/associations/has_one_through_disable_joins_associations_test.rb

@@ -8,6 +8,7 @@
 require "models/developer"
 require "models/company"
 require "models/computer"
+require "models/contract"
 require "models/club"
 require "models/membership"
 

activerecord/test/cases/associations/inverse_associations_test.rb

@@ -24,6 +24,7 @@
 require "models/room"
 require "models/contract"
 require "models/subscription"
+require "models/subscriber"
 require "models/book"
 require "models/branch"
 require "models/cpk"
@@ -199,6 +200,16 @@ def test_has_many_and_belongs_to_automatic_inverse_shares_objects_on_comment
     assert_equal comment.body, rating.comment.body, "Changing the original Comment's body should change the Comment's body on the association"
   end
 
+  def test_belongs_to_should_find_inverse_has_many_automatically
+    book = Book.create!
+    subscriber = book.subscribers.new nick: "Nickname"
+
+    subscriber.save!
+
+    assert_equal [subscriber], book.reload.subscribers
+    assert_equal 1, book.reload.subscribers.count
+  end
+
   def test_polymorphic_and_has_many_through_relationships_should_not_have_inverses
     sponsor_reflection = Sponsor.reflect_on_association(:sponsorable)