We are still working on the API. I have struggled with this issue for some time now and realize I might just ask for help.
Problem is assigning category to product . This gives 200/OK but is not updated in MongoDB.
Do you see what the problem might be?
Testing for this product (ID 5c9c2e851c149e000188462f)
[url=https://ibb.co/fDTf7pZ]

...and this category (ID 5c6697123945712d04312c17)
[url=https://ibb.co/Yykk5rk]

Fiddler POST request gives 200 OK
[url=https://ibb.co/Tc7CK1x]

...but DB is not updated
[url=https://ibb.co/fn88zqx]

Can you see the problem?
Will appreciate any help on this!
Also, some suggestions
1) Article / guide on how to set up sandbox for testing/debugging API requests
2) Possible to define relations to Categories/Pictures etc in "main" JSON for Insert/Update product (I understand it's not possible today)