sesame

An object-to-object mapper generator for Go that can 'scale'

Why the name 'sesame'?

sesame is a go mapper. Japanese often abbreviate this kind of terms as 'goma'. Goma means the sesame in Japanese.

Motivation

Multitier architectures like good-old 3tier architecture, Clean architecture, Hexagonal architecture etc have similar objects in each layers.

It is a hard work that you must write so many bolierplate code to map these objects. There are some kind of libraries(an object-to-object mapper) that simplify this job using reflections.

Object-to-object mappers that use reflections are very easy to use, but these are difficult to 'scale' .

• Hard to debug: Objects in the real world, rather than examples, are often very large. In reflection-based libraries, it can be quite hard to debug if some fields are not mapped correctly.
• Performance: Go's reflection is fast enough for most usecases. But, yes, large applications that selects multitier architectures often have very large objects. Many a little makes a mickle.

sesame generates object-to-object mappers source codes that DO NOT use reflections.

Status

This project is in very early stage. Any kind of feedbacks are wellcome.

Features

• Fast : sesame generates object-to-object mappers source codes that DO NOT use reflections.
• Easy to debug : If some fields are not mapped correctly, you just look a generated mapper source codes.
• Flexible : sesame provides various way to map objects.
  • By name
    • Simple field to field mapping
    • Field to nesting field mapping like TodoModel.UserID -> TodoEntity.User.ID .
    • Embedded struct mapping
  • By type
  • By helper function that is written in Go
• Zero 3rd-party dependencies at runtime : sesame generates codes that depend only standard libraries.
• Scalable :
  • Fast, Easy to debug and flexible.
  • Mapping configurations can be separated into multiple files.
    • You do not have to edit over 10000 lines of a single YAML file that has 1000 mapping definitions.

Current limitations

• Your project must be a Go module.

Installation

Binary installation

Get a binary from releases .

go get

sesame requires Go 1.20+.

$ go install github.com/yuin/sesame/cmd/sesame@latest

Usage

Outline

1. Create a configuration file(s).
2. Run the sesame command.
3. Create a Mappers object in your code.
4. (Optional) Add helpers and custom mappers.
5. Get a mapper from Mappers.
6. Map objects by the mapper.

See tests for examples.

Mapping configuration file

sesame uses mapping configuration files written in YAML.

${YOUR_GO_MODULE_ROOT}/sesame.yml:

mappers:                                         # configurations for a mapper collection
  package: mapper                                # package name for generated mapper collection
  destination: ./mapper/mappers_gen.go           # destination for generation
  nil-map: nil                                   # how are nil collections mapped
  nil-slice: nil                                 #   a value should be one of 'nil', 'empty' (default: nil)
mappings:                                        # configurations for object-to-object mappings
  - name: TodoMapper                             # name of the mapper. This must be unique within all mappers
    package: mapper                              # package name for generated mapper
    destination: ./mapper/todo_mapper_gen.go     # definition for generation
    bidirectional: true                          # generates a-to-b and b-to-a mapping if true(default: false)
    a-to-b: ModelToEntity                        # mapping function name(default: `{AName}To{BName}`)
    b-to-a: EntityToModel                        # mapping function name(default: `{BName}To{AName}`)
    a:                                           # mapping operand A
      package: ./model                           # package path for this operand
      name: TodoModel                            # struct name of this operand
    b:                                           # mapping operand B
      package: ./domain
      name: Todo
    explicit-only: false                         # sesame maps same names automatically if false(default: false)
    allow-unmapped:false                         # sesame fails with unmapped fields if false(default: false)
                                                 #   This value is ignored if `explicit-only' is set true.
    ignore-case:   false                         # sesame ignores field name cases if true(default: false)
    nil-map: nil                                 # how nil collections are mapped
    nil-slice: nil                               #   a default value is inherited from mappers
    fields:                                      # relationships between A fields and B fields
      - a: Done                                  #   you can define nested mappings like UserID
        b: Finished                              #   you can define mappings for embedded structs by '*'
      - a: UserID                                # 
        b: User.ID                               #
    ignores:                                     # ignores fields in operand X
      - a: ValidateOnly
      - b: User
_includes:                                       # includes separated configuration files
  - ./*/**/*_sesame.yml

And now, you can generate source codes just run sesame command in ${YOUR_GO_MODULE_ROOT}.

This configuration will generate the codes like the following:

./mapper/todo_mapper_gen.go :

package mapper

import (
	pkg00000 "context"
	pkg00003 "time"

	pkg00002 "example.com/testmod/domain"
	pkg00001 "example.com/testmod/model"
)

type TodoMapperHelper interface {
	TodoModelToTodo(pkg00000.Context, *pkg00001.TodoModel, *pkg00002.Todo) error
	TodoToTodoModel(pkg00000.Context, *pkg00002.Todo, *pkg00001.TodoModel) error
}

type TodoMapper interface {
	TodoModelToTodo(pkg00000.Context, *pkg00001.TodoModel, *pkg00002.Todo) error
	TodoToTodoModel(pkg00000.Context, *pkg00002.Todo, *pkg00001.TodoModel) error
}

// ... (TodoMapper default implementation)

Mapping in your code

sesame generates a mapper collection into the mappers.destination . Mapping codes look like the following:

1. Create new Mappers object as a singleton object. The Mappers object is a groutine-safe.

   mappers := mapper.NewMappers()           // Creates new Mappers object
   mapper.AddTimeToStringMapper(mappers)    // Add custom mappers
   mappers.Add("TodoMapperHelper", &todoMapperHelper{}) // Add helpers
   ```

2. Get a mapper and call it for mapping.

   obj, err := mappers.Get("TodoMapper")    // Get mapper by its name
   if err != nil {
       t.Fatal(err)
   }
   todoMapper, _ := obj.(TodoMapper)
   var entity Todo
   err := todoMapper.ModelToEntity(ctx, model, &entity) 

Custom mappers

By default, sesame can map following types:

• Same types
• Castable types(i.e. int -> int64, type MyType int <-> int)
• map, slice and array

For others, you can write and register custom mappers.

Example: string <-> time.Time mapper

type TimeStringMapper struct {
}

func (m *TimeStringMapper) StringToTime(ctx context.Context, source string, dest *time.Time) error {
	t, err := time.Parse(time.RFC3339, source)
	if err != nil {
		return err
	}
	*dest = t
	return nil
}

func (m *TimeStringMapper) TimeToString(ctx context.Context, source *time.Time, dest *string) error {
	*dest = source.Format(time.RFC3339)
	return nil
}

type Mappers interface {
    AddFactory(string, func(MapperGetter) (any, error))
    AddMapperFuncFactory(string, string, func(MapperGetter) (any, error))
}

func AddTimeToStringMapper(mappers Mappers) {
    mappers.AddFactory("TimeStringMapper", func(m MapperGetter) (any, error) {
        return &TimeStringMapper{}, nil
    })
    mappers.AddMapperFuncFactory("string", "time#Time", func(m MapperGetter) (any, error) {
        obj, _ := m.Get("TimeStringMapper")
        stringTime := obj.(*TimeStringMapper)
        return stringTime.StringToTime, nil
    })
    mappers.AddMapperFuncFactory("time#Time", "string", func(m MapperGetter) (any, error) {
        obj, _ := m.Get("TimeStringMapper")
        stringTime := obj.(*TimeStringMapper)
        return stringTime.TimeToString, nil
    })
}

or if a mapper does not require other mappers, you can do it just

func AddTimeToStringMapper(mappers Mappers) {
    mappers.Add("TimeStringMapper", &TimeStringMapper{})
}

Mappers.AddMapperFuncFactory takes qualified type names as arguments. A qualified type name is FULL_PACKAGE_PATH#TYPENAME(i.e. time#Time, example.com/testmod/domain#Todo).

Source argument types in custom mapping functions must be a

• Raw value: primitive types(i.e. string, int, slice ...)
• Pointer: others

Destination arguments are pointers.

So func (m *TimeStringMapper) TimeToString(ctx context.Context, source *time.Time, dest *string) error defines source type as a pointer(*time.Time).

Mappers.Add finds given mapper methods name like 'XxxToYyy' and calls AddMapperFuncFactory.

Helpers

You can define helper functions for more complex mappings.

type todoMapperHelper struct {
}

var _ TodoMapperHelper = &todoMapperHelper{} // TodoMapperHelper interface is generated by sesame

func (h *todoMapperHelper) ModelToEntity(ctx context.Context, source *model.TodoModel, dest *domain.Todo) error {
    if source.ValidateOnly {
        dest.Attributes["ValidateOnly"] = []string{"true"}
    }
    return nil
}

func (h *todoMapperHelper) EntityToModel(ctx context.Context, source *domain.Todo, dest *model.TodoModel) error {
    if _, ok := source.Attributes["ValidateOnly"]; ok {
        dest.ValidateOnly = true
    }
    return nil
}

and register it as {MAPPER_NAME}Helper:

mappers.Add("TodoMapperHelper", &todoMapperHelper{})

or

mappers.AddFactory("TodoMapperHelper", func(ms MapperGetter) (any, error) {
    // you can get other mappers or helpers from MapperGetter here
    return &todoMapperHelper{}, nil
})

Helpers will be called at the end of the generated mapping implementations.

Hierarchized mappers

Large applications often consist of multiple go modules.

/
|
+--- domain        : core business logics
|      |
|      +--- go.mod
|
+--- grpc          : gRPC service
|      |
|      +--- go.mod
|      +--- sesame.yml
|
+--- lib           : libraries
       |
       +--- go.mod
       +--- sesame.yml

• lib defines common mappers like 'StringTimeMapper' .
• gRPC defines gRPC spcific mappers that maps protoc generated models to domain entities

You can hierarchize mappers by a delegation like the following:

func NewDefaultMappers(parent Mappers) Mappers {
	m := NewMappers(parent)
    // Add gRPC specific mappers and helpers
    return m
}

// mappers := grpc_mappers.NewDefaultMappers(lib_mappers.NewMappers())

Donation

BTC: 1NEDSyUmo4SMTDP83JJQSWi1MvQUGGNMZB

License

MIT

Author

Yusuke Inuzuka