如何使用dgo api.NQuad删除dgraph中的四边形

go get github.com/dgraph-io/dgo/v210

import (
    "context"
    "github.com/dgraph-io/dgo/v210"
    "github.com/dgraph-io/dgo/v210/protos/api"
    "google.golang.org/grpc"
)

conn, err := grpc.Dial("localhost:9080", grpc.WithInsecure())
if err != nil {
    log.Fatal(err)
}
defer conn.Close()

client := dgo.NewDgraphClient(api.NewDgraphClient(conn))

mutation := &api.Mutation{
    DeleteNquad: []*api.NQuad{
        &api.NQuad{
            Subject:   "_:subject",
            Predicate: "predicate",
            ObjectValue: &api.Value{
                Val: &api.Value_StrVal{
                    StrVal: "object",
                },
            },
        },
    },
}

txn := client.NewTxn()
defer txn.Discard(context.Background())

_, err = txn.Mutate(context.Background(), mutation)
if err != nil {
    log.Fatal(err)
}

err = txn.Commit(context.Background())
if err != nil {
    log.Fatal(err)
}