connected_to shouldn't be called on the abstract class that not established connection

Fixed: https://github.com/rails/rails/issues/40559#issuecomment-752056106

When abstract class hasn't own connections, calling `AbstractClass.connection`
returns parent class's connection. We call `AbstractClass.connection.preventing_writes?`
expecting abstract class's state to be returned, but actually it is parent's one.

I think that it isn't expected behavior so I prevents call `connected_to` on the abstract
class that not established the connection.

activerecord/lib/active_record/connection_handling.rb

@@ -143,6 +143,10 @@ module ActiveRecord
         if self != Base && !abstract_class
           raise NotImplementedError, "calling `connected_to` is only allowed on ActiveRecord::Base or abstract classes."
         end
+
+        if name != connection_specification_name && !primary_class?
+          raise NotImplementedError, "calling `connected_to` is only allowed on the abstract class that established the connection."
+        end
       end
 
       unless role || shard

activerecord/test/cases/base_test.rb

@@ -41,6 +41,10 @@ class SecondAbstractClass < FirstAbstractClass
   connects_to database: { writing: :arunit, reading: :arunit }
 end
 
+class ThirdAbstractClass < SecondAbstractClass
+  self.abstract_class = true
+end
+
 class Photo < SecondAbstractClass; end
 class Smarts < ActiveRecord::Base; end
 class CreditCard < ActiveRecord::Base
@@ -1658,6 +1662,14 @@ class BasicsTest < ActiveRecord::TestCase
     end
   end
 
+  test "cannot call connected_to on the abstract class that did not establish the connection" do
+    error = assert_raises(NotImplementedError) do
+      ThirdAbstractClass.connected_to(role: :reading) { }
+    end
+
+    assert_equal "calling `connected_to` is only allowed on the abstract class that established the connection.", error.message
+  end
+
   test "#connecting_to with role" do
     SecondAbstractClass.connecting_to(role: :reading)