How to save and load trace

@junpenglao, After I restart my computer and re-open my jupyter notebook, I tried loading the saved trace:

with model_1:
    trace_1 = pm.load_trace(".pymc_1.trace")

But it says it doesn’t recognize the model. Do I have to re-run my model? If so, what is save_trace usually used for, considering that you have to run the whole model again?