More descriptive error message when an associated object class is mis…

…sing

Co-authored-by: Garrett Dimon <email@garrettdimon.com>

lib/active_record/associated_object/object_association.rb

@@ -14,7 +14,10 @@ def extend_source_from(chunks, &block)
   module ClassMethods
     def has_object(*names, **callbacks)
       extend_source_from(names) do |name|
-        "def #{name}; (@associated_objects ||= {})[:#{name}] ||= #{const_get(name.to_s.camelize)}.new(self); end"
+        const_get object_name = name.to_s.camelize
+        "def #{name}; (@associated_objects ||= {})[:#{name}] ||= #{object_name}.new(self); end"
+      rescue NameError
+        raise "The #{self}::#{object_name} associated object referenced from #{self} doesn't exist"
       end
 
       extend_source_from(names) do |name|

test/active_record/associated_object/object_association_test.rb

@@ -18,4 +18,9 @@ class ActiveRecord::AssociatedObject::ObjectAssociationTest < ActiveSupport::Tes
     Post.first.touch
     assert Post.first.mailroom.touched
   end
+
+  test "descriptive error when encountering unknown associated object" do
+    assert_raise { Post.has_object :unknown } => error
+    assert_equal "The Post::Unknown associated object referenced from Post doesn't exist", error.message
+  end
 end