Function upsert_command 

async fn upsert_command<U, T: Serialize>(
    store: &MockSuotarStore,
    generation: &str,
    hash: EntityHash,
    result_key: &'static str,
    items: Vec<U>,
    build: impl Fn(U) -> T,
    key_of: impl Fn(&T) -> String,
) -> Result<Value, CommandError>

The shared body behind every Upsert* command. key_of reads the id off the built entity rather than the wire type, so a derived id is what comes back under result_key.