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.

2 files changed, 3 insertions, 2 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
diff --git a/activerecord/test/cases/associations/belongs_to_associations_test.rb b/activerecord/test/cases/associations/belongs_to_associations_test.rb
index eef70f5691..9dadd114a1 100644
--- a/activerecord/test/cases/associations/belongs_to_associations_test.rb
+++ b/activerecord/test/cases/associations/belongs_to_associations_test.rb
@@ -168,7 +168,7 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
     e = assert_raise(ActiveRecord::AssociationTypeMismatch) {
       Admin::RegionalUser.new(region: 'wrong value')
     }
-    assert_match(/^Region\([^)]+\) expected, got String\([^)]+\)$/, e.message)
+    assert_match(/^Region\([^)]+\) expected, got "wrong value" which is an instance of String\([^)]+\)$/, e.message)
   ensure
     Admin.send :remove_const, "Region" if Admin.const_defined?("Region")
     Admin.send :remove_const, "RegionalUser" if Admin.const_defined?("RegionalUser")