If an Entity is defined without an Id or _id property, and a Shadow Property is defined as part of the OnModelCreating method

modelBuilder.Entity<ModelType>().Property<ObjectId>("Id");

System.NotSupportedException is thrown at runtime 

System.NotSupportedException: 'Unsupported shadow property 'TempId' identified on entity type 'ModelType'.'

It can be desirable to have the entity identifier implemented as a shadow property to enable models to be easily ported between SqlServer and MongoDB, for example.

For MongoDB

modelBuilder.Entity<ModelType>().Property<ObjectId>("Id"); 

For SqlServer

modelBuilder.Entity<ModelType>().Property<uint>("Id");