Disallow calling connected_to on subclasses of ActiveRecord::Base

Behavior has not changed here but the previous API could be
misleading to people who thought it would switch connections for only
that class. `connected_to` switches the context from which we are
getting connections, not the connections themselves.

Co-authored-by: John Crepezzi <john.crepezzi@gmail.com>

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Disallow calling `connected_to` on subclasses of `ActiveRecord::Base`.
+
+    Behavior has not changed here but the previous API could be misleading to people who thought it would switch connections for only that class. `connected_to` switches the context from which we are getting connections, not the connections themselves.
+
+    *Eileen M. Uchitelle*, *John Crepezzi*
+
 *   Add support for horizontal sharding to `connects_to` and `connected_to`.
 
     Applications can now connect to multiple shards and switch between their shards in an application. Note that the shard swapping is still a manual process as this change does not include an API for automatic shard swapping.

activerecord/lib/active_record/connection_handling.rb

@@ -136,6 +136,8 @@ module ActiveRecord
     #
     # The database kwarg is deprecated and will be removed in 6.2.0 without replacement.
     def connected_to(database: nil, role: nil, shard: nil, prevent_writes: false, &blk)
+      raise NotImplementedError, "connected_to can only be called on ActiveRecord::Base" unless self == Base
+
       if database
         ActiveSupport::Deprecation.warn("The database key in `connected_to` is deprecated. It will be removed in Rails 6.2.0 without replacement.")
       end

activerecord/test/cases/base_test.rb

@@ -1591,6 +1591,14 @@ class BasicsTest < ActiveRecord::TestCase
     end
   end
 
+  test "cannot call connected_to on subclasses of ActiveRecord::Base" do
+    error = assert_raises(NotImplementedError) do
+      Bird.connected_to(role: :reading) { }
+    end
+
+    assert_equal "connected_to can only be called on ActiveRecord::Base", error.message
+  end
+
   test "preventing writes applies to all connections on a handler" do
     conn1_error = assert_raises ActiveRecord::ReadOnlyError do
       ActiveRecord::Base.connection_handler.while_preventing_writes do