Give more context from `AssociationMismatchError`

The error message that we give today makes this error difficult to debug
if you receive it. I have no clue why we're printing the object ID of
the class (the commit doesn't give context), but I've left it as it was
deliberate.

1 files changed, 2 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/associations/association.rb b/activerecord/lib/active_record/associations/association.rb
index f7edfbfb5f..62e867a353 100644
--- a/activerecord/lib/active_record/associations/association.rb
+++ b/activerecord/lib/active_record/associations/association.rb
@@ -217,7 +217,8 @@ module ActiveRecord
           unless record.is_a?(reflection.klass)
             fresh_class = reflection.class_name.safe_constantize
             unless fresh_class && record.is_a?(fresh_class)
-              message = "#{reflection.class_name}(##{reflection.klass.object_id}) expected, got #{record.class}(##{record.class.object_id})"
+              message = "#{reflection.class_name}(##{reflection.klass.object_id}) expected, "\
+                "got #{record.inspect} which is an instance of #{record.class}(##{record.class.object_id})"
               raise ActiveRecord::AssociationTypeMismatch, message
             end
           end