7 files changed, 30 insertions, 10 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 325c4abfc8..36bb5e1daf 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,11 @@
 ## Rails 6.0.0.alpha (Unreleased) ##
 
+*   Make `reflection.klass` raise if `polymorphic?` not to be misused.
+
+    Fixes #31876.
+
+    *Ryuta Kamizono*
+
 *   Rails 6 requires Ruby 2.4.1 or newer.
 
     *Jeremy Daer*
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index da75344bde..71afbc1041 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -416,6 +416,9 @@ module ActiveRecord
     # Active Record class.
     class AssociationReflection < MacroReflection #:nodoc:
       def compute_class(name)
+        if polymorphic?
+          raise ArgumentError, "Polymorphic association does not support to compute class."
+        end
         active_record.send(:compute_type, name)
       end
 
@@ -608,13 +611,7 @@ module ActiveRecord
           if can_find_inverse_of_automatically?(self)
             inverse_name = ActiveSupport::Inflector.underscore(options[:as] || active_record.name.demodulize).to_sym
 
-            begin
-              reflection = klass._reflect_on_association(inverse_name)
-            rescue NameError
-              # Give up: we couldn't compute the klass type so we won't be able
-              # to find any associations either.
-              reflection = false
-            end
+            reflection = klass._reflect_on_association(inverse_name)
 
             if valid_inverse_reflection?(reflection)
               return inverse_name
@@ -626,9 +623,6 @@ module ActiveRecord
         # +automatic_inverse_of+ method is a valid reflection. We must
         # make sure that the reflection's active_record name matches up
         # with the current reflection's klass name.
-        #
-        # Note: klass will always be valid because when there's a NameError
-        # from calling +klass+, +reflection+ will already be set to false.
         def valid_inverse_reflection?(reflection)
           reflection &&
             klass <= reflection.active_record &&
@@ -732,6 +726,9 @@ module ActiveRecord
       end
 
       private
+        def can_find_inverse_of_automatically?(_)
+          !polymorphic? && super
+        end
 
         def calculate_constructable(macro, options)
           !polymorphic?
diff --git a/activerecord/test/cases/associations/inverse_associations_test.rb b/activerecord/test/cases/associations/inverse_associations_test.rb
index bad1bcdb67..896bf574f4 100644
--- a/activerecord/test/cases/associations/inverse_associations_test.rb
+++ b/activerecord/test/cases/associations/inverse_associations_test.rb
@@ -190,6 +190,16 @@ class InverseAssociationTests < ActiveRecord::TestCase
     assert_nil belongs_to_ref.inverse_of
   end
 
+  def test_polymorphic_associations_dont_attempt_to_find_inverse_of
+    belongs_to_ref = Sponsor.reflect_on_association(:sponsor)
+    assert_raise(ArgumentError) { belongs_to_ref.klass }
+    assert_nil belongs_to_ref.inverse_of
+
+    belongs_to_ref = Face.reflect_on_association(:human)
+    assert_raise(ArgumentError) { belongs_to_ref.klass }
+    assert_nil belongs_to_ref.inverse_of
+  end
+
   def test_this_inverse_stuff
     firm = Firm.create!(name: "Adequate Holdings")
     Project.create!(name: "Project 1", firm: firm)
diff --git a/activerecord/test/models/face.rb b/activerecord/test/models/face.rb
index 948435136d..e900fd40fb 100644
--- a/activerecord/test/models/face.rb
+++ b/activerecord/test/models/face.rb
@@ -2,6 +2,7 @@
 
 class Face < ActiveRecord::Base
   belongs_to :man, inverse_of: :face
+  belongs_to :human, polymorphic: true
   belongs_to :polymorphic_man, polymorphic: true, inverse_of: :polymorphic_face
   # Oracle identifier length is limited to 30 bytes or less, `polymorphic` renamed `poly`
   belongs_to :poly_man_without_inverse, polymorphic: true
diff --git a/activerecord/test/models/man.rb b/activerecord/test/models/man.rb
index 3acd89a48e..e26920e951 100644
--- a/activerecord/test/models/man.rb
+++ b/activerecord/test/models/man.rb
@@ -11,3 +11,6 @@ class Man < ActiveRecord::Base
   has_many :secret_interests, class_name: "Interest", inverse_of: :secret_man
   has_one :mixed_case_monkey
 end
+
+class Human < Man
+end
diff --git a/activerecord/test/models/sponsor.rb b/activerecord/test/models/sponsor.rb
index f190860fd1..18ff103ffe 100644
--- a/activerecord/test/models/sponsor.rb
+++ b/activerecord/test/models/sponsor.rb
@@ -3,6 +3,7 @@
 class Sponsor < ActiveRecord::Base
   belongs_to :sponsor_club, class_name: "Club", foreign_key: "club_id"
   belongs_to :sponsorable, polymorphic: true
+  belongs_to :sponsor, polymorphic: true
   belongs_to :thing, polymorphic: true, foreign_type: :sponsorable_type, foreign_key: :sponsorable_id
   belongs_to :sponsorable_with_conditions, -> { where name: "Ernie" }, polymorphic: true,
              foreign_type: "sponsorable_type", foreign_key: "sponsorable_id"
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 7d008eecd5..3f4fe16678 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -814,6 +814,7 @@ ActiveRecord::Schema.define do
   create_table :sponsors, force: true do |t|
     t.integer :club_id
     t.references :sponsorable, polymorphic: true, index: false
+    t.references :sponsor, polymorphic: true, index: false
   end
 
   create_table :string_key_objects, id: false, force: true do |t|
@@ -951,6 +952,7 @@ ActiveRecord::Schema.define do
     t.string  :poly_man_without_inverse_type
     t.integer :horrible_polymorphic_man_id
     t.string  :horrible_polymorphic_man_type
+    t.references :human, polymorphic: true, index: false
   end
 
   create_table :interests, force: true do |t|