schema/field: support validator for Bytes fields

Fixed #1714

schema/field/field.go

@@ -575,6 +575,21 @@ func (b *bytesBuilder) MaxLen(i int) *bytesBuilder {
 	return b
 }
 
+// Validate adds a validator for this field. Operation fails if the validation fails.
+//
+//	field.Bytes("blob").
+//		Validate(func(b []byte) error {
+//			if len(b) % 2 == 0 {
+//				return fmt.Errorf("ent/schema: blob length is even: %d", len(b))
+//			}
+//			return nil
+//		})
+//
+func (b *bytesBuilder) Validate(fn func([]byte) error) *bytesBuilder {
+	b.desc.Validators = append(b.desc.Validators, fn)
+	return b
+}
+
 // StorageKey sets the storage key of the field.
 // In SQL dialects is the column name and Gremlin is the property.
 func (b *bytesBuilder) StorageKey(key string) *bytesBuilder {

schema/field/field_test.go

@@ -187,12 +187,19 @@ func (*Pair) Scan(interface{}) error      { return nil }
 func (Pair) Value() (driver.Value, error) { return nil, nil }
 
 func TestBytes(t *testing.T) {
-	fd := field.Bytes("active").Default([]byte("{}")).Comment("comment").Descriptor()
+	fd := field.Bytes("active").
+		Default([]byte("{}")).
+		Comment("comment").
+		Validate(func(bytes []byte) error {
+			return nil
+		}).
+		Descriptor()
 	assert.Equal(t, "active", fd.Name)
 	assert.Equal(t, field.TypeBytes, fd.Info.Type)
 	assert.NotNil(t, fd.Default)
 	assert.Equal(t, []byte("{}"), fd.Default)
 	assert.Equal(t, "comment", fd.Comment)
+	assert.Len(t, fd.Validators, 1)
 
 	fd = field.Bytes("ip").GoType(net.IP("127.0.0.1")).Descriptor()
 	assert.NoError(t, fd.Err)