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/*
This example demonstrates a 3-level hierarchy (Author -> Books -> Reviews)
with data prefetching at each level to avoid N+1 query problems.
To run the example, execute:
go run example/prefetch/main.go
Then send a query like this (using curl or any GraphQL client):
curl -X POST http://localhost:8080/query \
-H 'Content-Type: application/json' \
-d '{"query":"query GetAuthors($top:Int!,$last:Int!){ authors { id name books(top:$top){ id title reviews(last:$last){ id content rating } } }}","variables":{"top":2,"last":2}}'
*/
package main
import (
"context"
_ "embed"
"log"
"net/http"
"github.com/graph-gophers/graphql-go"
"github.com/graph-gophers/graphql-go/relay"
)
//go:embed schema.graphql
var sdl string
type Author struct {
ID string
Name string
}
type Book struct {
ID string
AuthorID string
Title string
}
type Review struct {
ID string
BookID string
Content string
Rating int32
}
var (
allAuthors = []Author{{"A1", "Ann"}, {"A2", "Bob"}}
allBooks = []Book{{"B1", "A1", "Go Tips"}, {"B2", "A1", "Concurrency"}, {"B3", "A2", "GraphQL"}}
allReviews = []Review{{"R1", "B1", "Great", 5}, {"R2", "B1", "Okay", 3}, {"R3", "B3", "Wow", 4}}
)
type root struct {
booksByAuthor map[string][]Book
reviewsByBook map[string][]Review
}
func (r *root) Authors(ctx context.Context) ([]*authorResolver, error) {
authors := allAuthors
// level 1 prefetch: authors already available
if graphql.HasSelectedField(ctx, "books") {
// level 2 prefetch: books for selected authors only
authorSet := make(map[string]struct{}, len(authors))
for _, a := range authors {
authorSet[a.ID] = struct{}{}
}
booksByAuthor := make(map[string][]Book)
// capture potential Top argument once (shared across authors)
var topLimit int
var booksArgs struct{ Top int32 }
if ok, _ := graphql.DecodeSelectedFieldArgs(ctx, "books", &booksArgs); ok && booksArgs.Top > 0 {
topLimit = int(booksArgs.Top)
}
for _, b := range allBooks {
if _, ok := authorSet[b.AuthorID]; ok {
list := booksByAuthor[b.AuthorID]
if topLimit == 0 || len(list) < topLimit {
list = append(list, b)
booksByAuthor[b.AuthorID] = list
}
}
}
if graphql.HasSelectedField(ctx, "books.reviews") {
var lastLimit int
var rvArgs struct{ Last int32 }
if ok, _ := graphql.DecodeSelectedFieldArgs(ctx, "books.reviews", &rvArgs); ok && rvArgs.Last > 0 {
lastLimit = int(rvArgs.Last)
}
bookSet := map[string]struct{}{}
for _, slice := range booksByAuthor {
for _, b := range slice {
bookSet[b.ID] = struct{}{}
}
}
reviewsByBook := make(map[string][]Review)
for _, rv := range allReviews {
if _, ok := bookSet[rv.BookID]; ok {
grp := reviewsByBook[rv.BookID]
grp = append(grp, rv)
if lastLimit > 0 && len(grp) > lastLimit {
grp = grp[len(grp)-lastLimit:]
}
reviewsByBook[rv.BookID] = grp
}
}
r.reviewsByBook = reviewsByBook
}
r.booksByAuthor = booksByAuthor
}
res := make([]*authorResolver, len(authors))
for i, a := range authors {
res[i] = &authorResolver{root: r, a: &a}
}
return res, nil
}
type authorResolver struct {
root *root
a *Author
}
func (ar *authorResolver) ID() graphql.ID { return graphql.ID(ar.a.ID) }
func (ar *authorResolver) Name() string { return ar.a.Name }
func (ar *authorResolver) Books(ctx context.Context, args struct{ Top int32 }) ([]*bookResolver, error) {
// books already limited during prefetch phase (Authors resolver)
books := ar.root.booksByAuthor[ar.a.ID]
out := make([]*bookResolver, len(books))
for i, b := range books {
out[i] = &bookResolver{root: ar.root, b: &b}
}
return out, nil
}
type bookResolver struct {
root *root
b *Book
}
func (br *bookResolver) ID() graphql.ID { return graphql.ID(br.b.ID) }
func (br *bookResolver) Title() string { return br.b.Title }
func (br *bookResolver) Reviews(ctx context.Context, args struct{ Last int32 }) ([]*reviewResolver, error) {
revs := br.root.reviewsByBook[br.b.ID]
if take := int(args.Last); take > 0 && take < len(revs) {
start := len(revs) - take
if start < 0 {
start = 0
}
revs = revs[start:]
}
out := make([]*reviewResolver, len(revs))
for i, r := range revs {
out[i] = &reviewResolver{r: &r}
}
return out, nil
}
type reviewResolver struct{ r *Review }
func (rr *reviewResolver) ID() graphql.ID { return graphql.ID(rr.r.ID) }
func (rr *reviewResolver) Content() string { return rr.r.Content }
func (rr *reviewResolver) Rating() int32 { return rr.r.Rating }
func main() {
schema := graphql.MustParseSchema(sdl, &root{})
http.Handle("/query", &relay.Handler{Schema: schema})
log.Println("Prefetch example listening on :8080 -> POST /query")
log.Fatal(http.ListenAndServe(":8080", nil))
}
|
