2 files changed, 27 insertions, 20 deletions
diff --git a/skate/cluster.go b/skate/cluster.go
index 7fc4e1b..3421a0b 100644
--- a/skate/cluster.go
+++ b/skate/cluster.go
@@ -1,6 +1,7 @@
 package skate
 
 import (
+	"fmt"
 	"regexp"
 	"strings"
 
@@ -109,6 +110,30 @@ func KeyTitleSandcrawler(p []byte) (ident string, key string, err error) {
 	return ident, sandcrawlerSlugify(key), nil
 }
 
+// CreateFixedFieldFunc creates an extractor function given a json path.
+// Currently only top level key is supported.
+func CreateFixedFieldFunc(path string) IdentifierKeyFunc {
+	f := func(p []byte) (ident string, key string, err error) {
+		var doc map[string]interface{}
+		if err = json.Unmarshal(p, &doc); err != nil {
+			return
+		}
+		v, ok := doc[path]
+		if !ok {
+			return "", "", nil
+		}
+		switch t := v.(type) {
+		case string:
+			return "", t, nil
+		case int, int64, float32, float64:
+			return "", fmt.Sprintf("%v", t), nil
+		default:
+			return "", "", nil
+		}
+	}
+	return f
+}
+
 // sandcrawlerSlugify normalizes a string.
 func sandcrawlerSlugify(s string) string {
 	slug := strings.ToLower(strings.TrimSpace(s))
diff --git a/skate/cmd/skate-derive-key/main.go b/skate/cmd/skate-derive-key/main.go
index 61c79e1..05e1261 100644
--- a/skate/cmd/skate-derive-key/main.go
+++ b/skate/cmd/skate-derive-key/main.go
@@ -29,7 +29,6 @@ import (
 
 	"git.archive.org/martin/cgraph/skate"
 	"git.archive.org/martin/cgraph/skate/parallel"
-	json "github.com/segmentio/encoding/json"
 )
 
 var (
@@ -40,7 +39,7 @@ var (
 	verbose       = flag.Bool("verbose", false, "show progress")
 	bestEffort    = flag.Bool("B", false, "best effort")
 	logFile       = flag.String("log", "", "log filename")
-	skipEmptyKeys = flag.Bool("skip-empty-keys", false, "omit docs without keys")
+	skipEmptyKeys = flag.Bool("skip-empty-keys", false, "omit docs with empty keys")
 
 	wsReplacer = strings.NewReplacer("\t", "", "\n", "")
 	keyOpts    = map[string]skate.IdentifierKeyFunc{
@@ -58,24 +57,7 @@ func main() {
 	if *fixedField != "" {
 		// We want this, because from biblioref we wanted source_release_ident,
 		// from refs release_ident, etc.
-		keyFunc = func(p []byte) (id string, key string, err error) {
-			var doc map[string]interface{}
-			if err = json.Unmarshal(p, &doc); err != nil {
-				return
-			}
-			v, ok := doc[*fixedField]
-			if !ok {
-				return "", "", nil
-			}
-			switch t := v.(type) {
-			case string:
-				return "", t, nil
-			case int, int64, float32, float64:
-				return "", fmt.Sprintf("%v", t), nil
-			default:
-				return "", "", nil
-			}
-		}
+		keyFunc = skate.CreateFixedFieldFunc(*fixedField)
 	} else {
 		if keyFunc, ok = keyOpts[*keyFuncName]; !ok {
 			log.Fatal("invalid key func")