Mongoose variable key name
You'll be better off if you avoid dynamic keys in your schema and go with your second idea of:
user_info: [{sessionid: String, value: String}]You can use the $ positional operator to update individual user_info array elements by sessionid.
After testing the above, I found that defining the schema as user_info: { String: String } is a valid way to do this (option 1 specified in the question).