To access any relation field you have to define it in ModelResource.
class OrderItemResource(ModelResource):
created_by = fields.ToOneField(UserResource,'created_by')
[...]
class Meta:
queryset = OrderItem.objects.all()
resource_name = 'order'
authorization = Authorization()
always_return_data =True
The reason why you have to do it is that Tastypie has to know ModelResource definition of that model: resource name, query set and authorization access among others. Without it Tastypie will ignore it.
If you haven't defined UserResource yet, you should do it.
I see you are using base model for all models: "SmartModel". I think the best for you will be creation of base model resource to inherit. "SmartModelResource" and you will define needed fields there.
class SmartModelResource(ModelResource):
created_by = fields.ToOneField(UserResource,'created_by')
[...]
class OrderItemResource(SmartModelResource):
[...]
class Meta:
queryset = OrderItem.objects.all()
resource_name = 'order'
authorization = Authorization()
always_return_data =True