how to make country code _id country schema?
then departure_country: {type: String, ref: 'Country'}
It may be a better design. Country-Schema _id will be small, and country codes should not have collisions. Also in GroupSchema you should use less storage space for the departure_country "foreign key".
then you can sort by departure_country , and there is a chance that the sorting will also be valid for the country name.
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