How to generate json model file for new doctype in new app?

I followed the tutorial:
the tutorial says"When you update the DocType, the JSON model file is updated and whenever bench migrate is executed"
but json files dosen’t appear,how can i generate it?

The developer mode has to be set to 1 for this to happen. A quick search of “Set Developer Mode” in this forum will return a lot of posts to help you.

thank you!