Add modifier for nested type codec constructor

codec.go

@@ -42,6 +42,8 @@ var (
 	MaxBlockSize = int64(math.MaxInt32)
 )
 
+type CodecModifier func(map[string]*Codec)
+
 // Codec supports decoding binary and text Avro data to Go native data types,
 // and conversely encoding Go native data types to binary or text Avro data. A
 // Codec is created as a stateless structure that can be safely used in multiple
@@ -86,7 +88,7 @@ type Codec struct {
 //     if err != nil {
 //             fmt.Println(err)
 //     }
-func NewCodec(schemaSpecification string) (*Codec, error) {
+func NewCodec(schemaSpecification string, modifiers ...CodecModifier) (*Codec, error) {
 	var schema interface{}
 
 	if err := json.Unmarshal([]byte(schemaSpecification), &schema); err != nil {
@@ -96,6 +98,10 @@ func NewCodec(schemaSpecification string) (*Codec, error) {
 	// bootstrap a symbol table with primitive type codecs for the new codec
 	st := newSymbolTable()
 
+	for _, modifier := range modifiers {
+		modifier(st)
+	}
+
 	c, err := buildCodec(st, nullNamespace, schema)
 	if err != nil {
 		return nil, err

record_test.go

@@ -445,6 +445,82 @@ func TestRecordRecursiveRoundTrip(t *testing.T) {
 	}
 }
 
+func TestUnknownTypeCodecError(t *testing.T) {
+	_, err := NewCodec(`
+{
+  "type": "record",
+  "name": "Parent",                  
+  "fields" : [
+    {"name": "child", "type": "Child"}
+  ]
+}
+`)
+	ensureError(t, err, "Record \"Parent\" field 1 ought to be valid Avro named type: unknown type name: \"Child\"")
+}
+
+func TestCodecModifier(t *testing.T) {
+	childCodec, err := NewCodec(`
+{
+  "type": "record",
+  "name": "Child",                  
+  "fields" : [
+    {"name": "age", "type": "int"}
+  ]
+}
+`)
+	ensureError(t, err)
+
+	modifier := func(st map[string]*Codec) {
+		st["Child"] = childCodec
+	}
+
+	codec, err := NewCodec(`
+{
+  "type": "record",
+  "name": "Parent",                  
+  "fields" : [
+    {"name": "child", "type": "Child"}
+  ]
+}
+`, modifier)
+	ensureError(t, err)
+
+	child := map[string]interface{} {
+		"age": 7,
+	}
+	parent := map[string]interface{} {
+		"child": child,
+	}
+
+	// Convert native Go form to binary Avro data
+	buf, err := codec.BinaryFromNative(nil, parent)
+	ensureError(t, err)
+
+	// Convert binary Avro data back to native Go form
+	datum, _, err := codec.NativeFromBinary(buf)
+	ensureError(t, err)
+
+	actual, ok := datum.(map[string]interface{})
+	if !ok {
+		t.Fatalf("origin data contaminated")
+	}
+
+	child, ok = actual["child"].(map[string]interface{})
+	if !ok {
+		t.Fatalf("child type contaminated")
+	}
+
+	age, ok := child["age"].(int32)
+	if !ok {
+		t.Fatalf("child age field contaminated")
+	}
+
+	if age != 7 {
+		t.Fatalf("child age data contaminated")
+	}
+}
+
+
 func ExampleRecordRecursiveRoundTrip() {
 	codec, err := NewCodec(`
 {