firestore_doc_field.go

package main

import (
	"context"
	"encoding/json"
	"flag"
	"log"
	"os"

	"cloud.google.com/go/firestore"
)

func main() {
	var project, collection, docID, field string
	flag.StringVar(&project, "project", os.Getenv("PROJECT_ID"), "GCP project ID")
	flag.StringVar(&collection, "collection", "", "Firestore collection")
	flag.StringVar(&docID, "doc", "", "document id")
	flag.StringVar(&field, "field", "", "field path to extract")
	flag.Parse()

	if project == "" || collection == "" {
		flag.Usage()
		os.Exit(1)
	}

	ctx := context.Background()

	db, err := firestore.NewClient(ctx, project)
	if err != nil {
		log.Fatal(err)
	}
	defer db.Close()

	doc, err := db.Collection(collection).Doc(docID).Get(ctx)
	if err != nil {
		log.Fatal(err)
	}

	val, err := doc.DataAt(field)
	if err != nil {
		log.Fatal(err)
	}

	switch v := val.(type) {
	case []byte:
		os.Stdout.Write(v)
	default:
		if err := json.NewEncoder(os.Stdout).Encode(val); err != nil {
			log.Fatal(err)
		}
	}
}