I am creating one CRM with customize fields. Can anyone give me an idea how to store that in data base?

Example: I am using a retail Crm , there I am creating a custom model type like Customer, Inside shop I would like to add fields like name, address, mobile etc.

Then user can add records to that customer model. [Remember this customer model can be any thing custom made by user not predefined mongoose schema]

Can any one help me how to solve this. Using: Nodejs, express, mongoose,