compute_class raise when klass is not ActiveRecord::Base subclass

Changes `ActiveRecord::Reflection#compute_class` in order to raise
an `ArgumentError` when the computed class is not a subclass
of `ActiveRecord::Base`.

Raising a meaningful error ease the user interaction in the following
scenarios:

- Rails couldn't compute correctly the `class_name`
- User sets as `:class_name` an invalid class

Closes #15811.

activerecord/lib/active_record/reflection.rb

@@ -406,7 +406,16 @@ module ActiveRecord
         if polymorphic?
           raise ArgumentError, "Polymorphic associations do not support computing the class."
         end
-        active_record.send(:compute_type, name)
+
+        active_record.send(:compute_type, name).tap do |klass|
+          unless klass < ActiveRecord::Base
+            raise ArgumentError, <<-MSG.squish
+              Rails couldn't find a valid model for #{name} association.
+              Please provide the :class_name option on the association declaration.
+              If :class_name is already provided make sure is an ActiveRecord::Base subclass.
+            MSG
+          end
+        end
       end
 
       attr_reader :type, :foreign_type

activerecord/test/cases/reflection_test.rb

@@ -26,6 +26,7 @@ require "models/department"
 require "models/cake_designer"
 require "models/drink_designer"
 require "models/recipe"
+require "models/user_with_invalid_relation"
 
 class ReflectionTest < ActiveRecord::TestCase
   include ActiveRecord::Reflection
@@ -137,6 +138,19 @@ class ReflectionTest < ActiveRecord::TestCase
     assert_equal "PluralIrregular", reflection.class_name
   end
 
+  def test_reflection_klass_is_not_ar_subclass
+    [:account_invalid,
+     :account_class_name,
+     :info_invalids,
+     :infos_class_name,
+     :infos_through_class_name,
+    ].each do |rel|
+      assert_raise(ArgumentError) do
+        UserWithInvalidRelation.reflect_on_association(rel).klass
+      end
+    end
+  end
+
   def test_aggregation_reflection
     reflection_for_address = AggregateReflection.new(
       :address, nil, { mapping: [ %w(address_street street), %w(address_city city), %w(address_country country) ] }, Customer

activerecord/test/models/user_with_invalid_relation.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+class UserWithInvalidRelation < ActiveRecord::Base
+  has_one :account_invalid
+
+  has_one :account_class_name, class_name: "AccountInvalid"
+
+  has_many :user_info_invalid
+  has_many :info_invalids, through: :user_info_invalid
+
+  has_many :infos_class_name, through: :user_info, class_name: "InfoInvalid"
+
+  has_many :user_infos_class_name, class_name: "UserInfoInvalid"
+  has_many :infos_through_class_name, through: :user_infos_class_name, class_name: "InfoInvalid"
+end
+
+class AccountInvalid; end
+
+class InfoInvalid; end
+
+class UserInfoInvalid < ActiveRecord::Base
+  belongs_to :info_invalid
+  belongs_to :user_invalid
+end