Validate Schema on App level

[danielAsaboro]

Is your feature request related to a problem? Please describe.
I came from the Nodejs world, and I remember with the Mongoosejs library
for MongoDB that we could validate our schema on the app level
e.g ensuring that only one user is associated with one email

Describe the solution you'd like
I don't know if this can be done through a struct tag in the Model

Describe alternatives you've considered
I tried using a bson:"unique" on the email, it failed