Fix Config.idFor() recursive ID lookup in anonymous fields (fixes #732)

Prefix was not being propagated in second pass so was only correct at
first level where pref == ""

Add a test case to validate the fix

schema/schema.go

@@ -975,7 +975,7 @@ func (c *Config) idFor(rules fieldRules, rt reflect.Type, rv reflect.Value, pref
 		if !fld.Anonymous {
 			continue
 		}
-		id, err = c.idFor(rules, fld.Type, rv.Field(i), fld.Name+".")
+		id, err = c.idFor(rules, fld.Type, rv.Field(i), pref+fld.Name+".")
 		if err != nil || id != nil {
 			return
 		}

schema/schema_test.go

@@ -94,6 +94,11 @@ type subObject struct {
 	Num int `quad:"num"`
 }
 
+type subSubObject struct {
+	subObject
+	Num2 int `quad:"num2"`
+}
+
 func init() {
 	voc.RegisterPrefix("ex:", "http://example.org/")
 	schema.RegisterType(quad.IRI("ex:Coords"), Coords{})
@@ -237,6 +242,26 @@ var testWriteValueCases = []struct {
 		},
 		nil,
 	},
+	{
+		"simple object (embedded multiple levels)",
+		subSubObject{
+			subObject: subObject{
+				genObject: genObject{
+					ID:   "1234",
+					Name: "Obj",
+				},
+				Num: 3,
+			},
+			Num2: 4,
+		},
+		iri("1234"),
+		[]quad.Quad{
+			{iri("1234"), iri("name"), quad.String("Obj"), nil},
+			{iri("1234"), iri("num"), quad.Int(3), nil},
+			{iri("1234"), iri("num2"), quad.Int(4), nil},
+		},
+		nil,
+	},
 	{
 		"required field not set",
 		item2{Name: "partial"},
@@ -636,4 +661,4 @@ func TestSaveNamespaces(t *testing.T) {
 	if !reflect.DeepEqual(expect, q) {
 		t.Fatalf("wrong quads returned: got: %v, expect: %v", q, expect)
 	}
-}
+}