fix: resolve translated post slugs (D-58)

GET /_journal/api/v1/posts/{slug} now looks the slug up in translate's
slug index for the request locale after the default-locale slug and
before the numeric id. TestTranslatedSlug covers the translated slug,
the default slug, the id and an unknown slug.

controllers/api/posts.go

@@ -128,7 +128,7 @@ func Show(app *backpack.App) http.HandlerFunc {
 		}
 		db = db.WithContext(r.Context())
 		slug := strings.TrimSpace(r.PathValue("slug"))
-		post, err := findPostBySlugOrID(db, slug)
+		post, err := findPostBySlugOrID(db, slug, requestLocale(r))
 		if err != nil || post == nil {
 			writeAPIError(w, http.StatusNotFound, errPostNotFound)
 			return

controllers/api/posts_helpers.go

@@ -16,8 +16,10 @@ import (
 	"git.golem15.com/golem15/sm-journal-plugin/classes"
 	"git.golem15.com/golem15/sm-journal-plugin/models"
 	translate "git.golem15.com/golem15/sm-translate-plugin/classes"
+	translatemodels "git.golem15.com/golem15/sm-translate-plugin/models"
 	"git.golem15.com/golem15/summercms/modules/bouncer"
 	"git.golem15.com/golem15/summercms/modules/cabana"
+	"git.golem15.com/golem15/summercms/modules/towel"
 	"gorm.io/gorm"
 )
 
@@ -52,12 +54,29 @@ func ctypeDigit(s string) bool {
 	return true
 }
 
-func findPostBySlugOrID(db *gorm.DB, slug string) (*models.Post, error) {
+// findPostBySlugOrID finds a post by its default-locale slug, then by its
+// slug in locale (the request locale, D-58: translate's slug index), then
+// by numeric id. Drafts are found too; the caller decides who may see them.
+func findPostBySlugOrID(db *gorm.DB, slug, locale string) (*models.Post, error) {
 	q := postQuery(db)
 	var post models.Post
 	if err := q.Where("slug = ?", slug).Take(&post).Error; err == nil {
 		return &post, nil
 	}
+	if locale != "" && slug != "" {
+		var ids []string
+		err := db.Session(&gorm.Session{NewDB: true, Context: db.Statement.Context}).
+			Table(translatemodels.Index{}.TableName()).
+			Where("model_type = ? AND locale = ? AND item = ? AND value = ?", models.Post{}.MorphName(), locale, "slug", slug).
+			Order("id ASC").
+			Pluck("model_id", &ids).Error
+		if err == nil && len(ids) > 0 {
+			var translated models.Post
+			if err := postQuery(db).Where("CAST(id AS TEXT) IN ?", ids).Order("id ASC").Take(&translated).Error; err == nil {
+				return &translated, nil
+			}
+		}
+	}
 	if ctypeDigit(slug) {
 		id, err := strconv.ParseUint(slug, 10, 64)
 		if err == nil && id > 0 {
@@ -780,3 +799,21 @@ func syncTranslations(ctx context.Context, db *gorm.DB, post *models.Post, raw a
 	}
 	return nil
 }
+
+// requestLocale is the request's locale (towel, set by the router from the
+// locale resolver or Accept-Language) as a lower-case language code: the
+// first language of an Accept-Language list, without its region.
+func requestLocale(r *http.Request) string {
+	if r == nil {
+		return ""
+	}
+	raw, _ := towel.Locale(r.Context())
+	raw = strings.TrimSpace(raw)
+	if i := strings.IndexAny(raw, ",;"); i >= 0 {
+		raw = raw[:i]
+	}
+	if i := strings.IndexAny(raw, "-_"); i >= 0 {
+		raw = raw[:i]
+	}
+	return strings.ToLower(strings.TrimSpace(raw))
+}