From b3e1355a466d62bdb678216ce292a14115641f81 Mon Sep 17 00:00:00 2001
From: Jack Christensen <jack@jackchristensen.com>
Date: Tue, 12 May 2020 16:58:16 -0500
Subject: [PATCH] CompositeType can assign to struct via reflection

---
 composite_type.go      | 76 +++++++++++++++++++++++++++++++++++-------
 composite_type_test.go | 17 ++++++++++
 2 files changed, 81 insertions(+), 12 deletions(-)

diff --git a/composite_type.go b/composite_type.go
index c37aef27..7f5ae694 100644
--- a/composite_type.go
+++ b/composite_type.go
@@ -2,6 +2,7 @@ package pgtype
 
 import (
 	"encoding/binary"
+	"reflect"
 	"strings"
 
 	"github.com/jackc/pgio"
@@ -102,24 +103,19 @@ func (src CompositeType) AssignTo(dst interface{}) error {
 					continue
 				}
 
-				assignToErr := src.fields[i].AssignTo(v[i])
-				if assignToErr != nil {
-					// Try to use get / set instead -- this avoids every type having to be able to AssignTo type of self.
-					setSucceeded := false
-					if setter, ok := v[i].(Value); ok {
-						err := setter.Set(src.fields[i].Get())
-						setSucceeded = err == nil
-					}
-					if !setSucceeded {
-						return errors.Errorf("unable to assign to dst[%d]: %v", i, assignToErr)
-					}
+				err := assignToOrSet(src.fields[i], v[i])
+				if err != nil {
+					return errors.Errorf("unable to assign to dst[%d]: %v", i, err)
 				}
-
 			}
 			return nil
 		case *[]interface{}:
 			return src.AssignTo(*v)
 		default:
+			if isPtrStruct, err := src.assignToPtrStruct(dst); isPtrStruct {
+				return err
+			}
+
 			if nextDst, retry := GetAssignToDstType(dst); retry {
 				return src.AssignTo(nextDst)
 			}
@@ -131,6 +127,62 @@ func (src CompositeType) AssignTo(dst interface{}) error {
 	return errors.Errorf("cannot decode %#v into %T", src, dst)
 }
 
+func assignToOrSet(src Value, dst interface{}) error {
+	assignToErr := src.AssignTo(dst)
+	if assignToErr != nil {
+		// Try to use get / set instead -- this avoids every type having to be able to AssignTo type of self.
+		setSucceeded := false
+		if setter, ok := dst.(Value); ok {
+			err := setter.Set(src.Get())
+			setSucceeded = err == nil
+		}
+		if !setSucceeded {
+			return assignToErr
+		}
+	}
+
+	return nil
+}
+
+func (src CompositeType) assignToPtrStruct(dst interface{}) (bool, error) {
+	dstValue := reflect.ValueOf(dst)
+	if dstValue.Kind() != reflect.Ptr {
+		return false, nil
+	}
+
+	if dstValue.IsNil() {
+		return false, nil
+	}
+
+	dstElemValue := dstValue.Elem()
+	dstElemType := dstElemValue.Type()
+
+	if dstElemType.Kind() != reflect.Struct {
+		return false, nil
+	}
+
+	exportedFields := make([]int, 0, dstElemType.NumField())
+	for i := 0; i < dstElemType.NumField(); i++ {
+		sf := dstElemType.Field(i)
+		if sf.PkgPath == "" {
+			exportedFields = append(exportedFields, i)
+		}
+	}
+
+	if len(exportedFields) != len(src.fields) {
+		return false, nil
+	}
+
+	for i := range exportedFields {
+		err := assignToOrSet(src.fields[i], dstElemValue.Field(exportedFields[i]).Addr().Interface())
+		if err != nil {
+			return true, errors.Errorf("unable to assign to field %s: %v", dstElemType.Field(exportedFields[i]).Name, err)
+		}
+	}
+
+	return true, nil
+}
+
 func (src CompositeType) EncodeBinary(ci *ConnInfo, buf []byte) (newBuf []byte, err error) {
 	switch src.status {
 	case Null:
diff --git a/composite_type_test.go b/composite_type_test.go
index 17d34251..0225e443 100644
--- a/composite_type_test.go
+++ b/composite_type_test.go
@@ -130,6 +130,23 @@ func TestCompositeTypeAssignTo(t *testing.T) {
 		assert.Equal(t, pgtype.Text{String: "foo", Status: pgtype.Present}, a)
 		assert.Equal(t, pgtype.Int4{Int: 42, Status: pgtype.Present}, b)
 	}
+
+	// Struct fields positionally via reflection
+	{
+		err := ct.Set([]interface{}{"foo", int32(42)})
+		assert.NoError(t, err)
+
+		s := struct {
+			A string
+			B int32
+		}{}
+
+		err = ct.AssignTo(&s)
+		if assert.NoError(t, err) {
+			assert.Equal(t, "foo", s.A)
+			assert.Equal(t, int32(42), s.B)
+		}
+	}
 }
 
 func TestCompositeTypeTranscode(t *testing.T) {