fix: skip URL rewriting for non-relation field values

A "?fields=" query selecting scalar fields returned corrupted values:
plain strings were percent-encoded ("Book 1" -> "Book%201") and numbers
were coerced to strings (2019 -> "2019"). The header form was unaffected
because it never rewrites the response body.

The relation handler ran for every scalar leaf, including pure field
projections that are not relations. Those values were parsed with
url.Parse and re-serialized via url.URL.String(), which percent-encodes
the path and stringifies numbers.

Guard the handler so a node with no relation to push and no child
selector to propagate is left untouched. Besides fixing the corruption,
this skips url.Parse, the query re-encoding and the json.Marshal for
every projected scalar, cutting allocations on the fields path by ~30%.

Fixes #96

Author: Nayte91

fixtures/api/jsonld.go

@@ -73,6 +73,7 @@ func (h *JSONLDHandler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	"@id": `+string(encodedURI)+`,
 	"title": "Book 1",
 	"description": "A good book",
+	"year": 2019,
 	"author": "/authors/1.jsonld",
 	"related": "/books/99.jsonld"
 	}`); err != nil {

server_test.go

@@ -202,6 +202,28 @@ func TestFieldsQuery(t *testing.T) {
 	assert.Equal(t, `{"@id":"/books.jsonld"}`, string(b))
 }
 
+func TestFieldsQueryDoesNotEncodePlainValues(t *testing.T) {
+	upstream, gateway := createServers(-1, false)
+	defer upstream.Close()
+	defer gateway.Close()
+
+	resp, _ := http.Get(gateway.URL + `/books/1.jsonld?fields="/title","/description"`)
+	b, _ := io.ReadAll(resp.Body)
+
+	assert.Equal(t, `{"title":"Book 1","description":"A good book"}`, string(b))
+}
+
+func TestFieldsQueryPreservesNumberType(t *testing.T) {
+	upstream, gateway := createServers(-1, false)
+	defer upstream.Close()
+	defer gateway.Close()
+
+	resp, _ := http.Get(gateway.URL + `/books/1.jsonld?fields="/year"`)
+	b, _ := io.ReadAll(resp.Body)
+
+	assert.Equal(t, `{"year":2019}`, string(b))
+}
+
 func TestFieldsHeader(t *testing.T) {
 	upstream, gateway := createServers(-1, false)
 	defer upstream.Close()

vulcain.go

@@ -212,6 +212,14 @@ func (v *Vulcain) Apply(req *http.Request, rw http.ResponseWriter, responseBody
 		oaRouteTested, usePreloadLinks bool
 	)
 	newBody := v.traverseJSON(currentBody, tree, len(f) > 0, func(n *node, val string) string {
+		// A pure field projection (a "fields" leaf with no relation to push and no child
+		// selector to propagate) is not a relation: leave its value untouched. Routing it
+		// through the URL machinery below would percent-encode plain strings and coerce
+		// numbers to strings (https://github.com/dunglas/vulcain/issues/96).
+		if !n.preload && !n.hasChildren(preload) && !n.hasChildren(fields) {
+			return ""
+		}
+
 		var (
 			u        *url.URL
 			useOA    bool