Protobuf Field Mask utils for Go

Features:

• Copy from any Go struct to any compatible Go struct with a field mask applied
• Copy from any Go struct to a map[string]interface{} with a field mask applied
• Extensible masks (e.g. inverse mask: copy all except those mentioned, etc.)
• Supports Protobuf Any message types.

If you're looking for a simple FieldMask library to work with protobuf messages only (not arbitrary structs) consider this tiny repo: https://github.com/mennanov/fmutils

Examples

Copy from a protobuf message to a protobuf message:

// testproto/test.proto

message UpdateUserRequest {
    User user = 1;
    google.protobuf.FieldMask field_mask = 2;
}

package main

import fieldmask_utils "github.com/mennanov/fieldmask-utils"

// A function that maps field mask field names to the names used in Go structs.
// It has to be implemented according to your needs.
// Scroll down for a reference on how to apply field masks to your gRPC services.
func naming(s string) string {
    if s == "foo" {
        return "Foo"
    }
    return s
}

func main () {
    var request UpdateUserRequest
    userDst := &testproto.User{} // a struct to copy to
    mask, _ := fieldmask_utils.MaskFromPaths(request.FieldMask.Paths, naming)
    fieldmask_utils.StructToStruct(mask, request.User, userDst)
    // Only the fields mentioned in the field mask will be copied to userDst, other fields are left intact
}

Copy from a protobuf message to a map[string]interface{}:

package main

import fieldmask_utils "github.com/mennanov/fieldmask-utils"

func main() {
    var request UpdateUserRequest
    userDst := make(map[string]interface{}) // a map to copy to
    mask, _ := fieldmask_utils.MaskFromProtoFieldMask(request.FieldMask, naming)
    err := fieldmask_utils.StructToMap(mask, request.User, userDst)
    // Only the fields mentioned in the field mask will be copied to userDst, other fields are left intact
}

Copy with an inverse mask:

package main

import fieldmask_utils "github.com/mennanov/fieldmask-utils"

func main() {
    var request UpdateUserRequest
    userDst := &testproto.User{} // a struct to copy to
    mask := fieldmask_utils.MaskInverse{"Id": nil, "Friends": fieldmask_utils.MaskInverse{"Username": nil}}
    fieldmask_utils.StructToStruct(mask, request.User, userDst)
    // Only the fields that are not mentioned in the field mask will be copied to userDst, other fields are left intact.
}

Naming function

For developers that are looking for a mechanism to apply a mask field in their update endpoints using gRPC services, there are multiple options for the naming function described above:

• Using the CamelCase function provided in the original protobuf repository. This repository has been deprecated and it will potentially trigger lint errors.
  • You can copy-paste the CamelCase function to your own project or,
  • You can use an Open Source alternative that provides the same functionality, already took care of copying the code, and also added tests.

func main() {
    mask := &fieldmaskpb.FieldMask{Paths: []string{"username"}}
    mask.Normalize()
    req := &UpdateUserRequest{
        User: &User{
            Id:       1234,
            Username: "Test",
        },
    }
    if !mask.IsValid(req) {
        return
    }
    protoMask, err := fieldmask_utils.MaskFromProtoFieldMask(mask, strings.PascalCase)
    if err != nil {
        return
    }
    m := make(map[string]any)
    err = fieldmask_utils.StructToMap(protoMask, req, m)
    if err != nil {
        return
    }
    fmt.Println("Resulting map:", m)
}

This will result in a map that contains the fields that need to be updated with their respective values.

Limitations

1. Larger scope field masks have no effect and are not considered invalid:

   field mask strings "a", "a.b", "a.b.c" will result in a mask a{b{c}}, which is the same as "a.b.c".

2. Masks inside a protobuf Map are not supported.

3. When copying from a struct to struct the destination struct must have the same fields (or a subset) as the source struct. Either of source or destination fields can be a pointer as long as it is a pointer to the type of the corresponding field.

4. oneof fields are represented differently in fieldmaskpb.FieldMask compared to fieldmask_util.Mask. In FieldMask the fields are represented using their property name, in this library they are prefixed with the oneof name matching how Go generated code is laid out. This can lead to issues when converting between the two, for example when using MaskFromPaths or MaskFromProtoFieldMask.