Assigning user to a territory

Question

I am working on a scenario where a user fills a form containing the field 'Area Code'. Based on the value of Area Code , the user has to be assigned to a territory(by matching the Area Code with a custom field on Territory object). So my initial understanding is to query on Territory object and get the territoryId of the Area Code. And then create (by inserting)a UserTerritory record whose TerritoryId is achieved above. But I am getting an error 'DML not allowed on User Territory' on doing an insert. I googled to find there is a create function for this. How would this work?

Below is the code snippet

public string AssignTerritory(String AreaCode){
    List<UserTerritory> lstUserTerritory = new list<UserTerritory>();
    List<Territory> lstTerritory = new List<Territory>();
    lstTerritory = [Select id from Territory WHERE ISO_Country_Code =: AreaCode];
    if(lstTerritory.size()>0 && lstTerritory!=null){
        //List<UserTerritory> lstUserTerritory = [Select UserId, Territory.Name from UserTerritory where TerritoryId =: lstTerritory[0].id];
    }
    if(lstUserTerritory==null && lstUserTerritory.size()==0){
        UserTerritory UTerritory = new UserTerritory();
        UTerritory.UserId = UserInfo.getUserId();
        UTerritory.TerritoryId = lstTerritory[0].id;
        //UTerritory.IsActive = true;
        //lstUserTerritory.add(UTerritory);
        create(UTerritory); 
    }

}

Answer 1

This one is problematic but there's a workaround...

UserTerritory is not permitted for DML from Apex. This is true for several SObjects as documented in the Apex Developer's Doc

However, the UserTerritory object can be created using the API as per the Apex SObject Reference

So, your VF controller is going to have to do a callout to the REST API using the same session ID as the running user to do the insert. There's an example here (using DatedConversionRates but the example can be adapted for UserTerritory)