Go struct Converter

A library for converting between Go structs.

chain := converter.NewChain(V1{}, V2{}, V3{})

chain.Convert(myV1struct, &myV3struct)

Details

At its core, this library provides a Convert function, which automatically handles converting fields with the same name, and “convertable” types. Some examples are:

• string -> string
• string -> *string
• int -> string
• string -> []string

The automatic conversions are implemented when there is an obvious way to convert between the types. A lot more automatic conversions happen -- see the converter tests for a more comprehensive list of what is currently supported.

Not everything can be handled automatically, however, so there is also a ConvertFrom interface any struct in the graph can implement to perform custom conversion, similar to how the stdlib MarshalJSON and UnmarshalJSON would be implemented.

Additionally, and maybe most importantly, there is a converter.Chain available, which orchestrates conversions between multiple versions of structs. This could be thought of similar to database migrations: given a starting struct and a target struct, the chain.Convert function iterates through every intermediary migration in order to arrive at the target struct.

Basic Usage

To illustrate usage we'll start with a few basic structs, some of which implement the ConvertFrom interface due to breaking changes:

// --------- V1 struct definition below ---------

type V1 struct {
  Name     string
  OldField string
}

// --------- V2 struct definition below ---------

type V2 struct {
  Name     string
  NewField string // this was a renamed field
}

func (to *V2) ConvertFrom(from interface{}) error {
  if from, ok := from.(V1); ok { // forward migration
    to.NewField = from.OldField
  }
  return nil
}

// --------- V3 struct definition below ---------

type V3 struct {
  Name       []string
  FinalField []string // this field was renamed and the type was changed
}

func (to *V3) ConvertFrom(from interface{}) error {
  if from, ok := from.(V2); ok { // forward migration
    to.FinalField = []string{from.NewField}
  }
  return nil
}

Given these type definitions, we can easily set up a conversion chain like this:

chain := converter.NewChain(V1{}, V2{}, V3{})

This chain can then be used to convert from an older version to a newer version. This is because our ConvertFrom definitions are only handling forward migrations.

This chain can be used to convert from a V1 struct to a V3 struct easily, like this:

v1 := // somehow get a populated v1 struct
v3 := V3{}
chain.Convert(v1, &v3)

Since we‘ve defined our chain as V1 → V2 → V3, the chain will execute conversions to all intermediary structs (V2, in this case) and ultimately end when we’ve populated the v3 instance.

Note we haven't needed to define any conversions on the Name field of any structs since this one is convertible between structs: string → string → []string.

Backwards Migrations

If we wanted to also provide backwards migrations, we could also easily add a case to the ConvertFrom methods. The whole set of structs would look something like this:

// --------- V1 struct definition below ---------

type V1 struct {
  Name     string
  OldField string
}

func (to *V1) ConvertFrom(from interface{}) error {
  if from, ok := from.(V2); ok { // backward migration
    to.OldField = from.NewField
  }
  return nil
}

// --------- V2 struct definition below ---------

type V2 struct {
  Name     string
  NewField string
}

func (to *V2) ConvertFrom(from interface{}) error {
  if from, ok := from.(V1); ok { // forward migration
    to.NewField = from.OldField
  }
  if from, ok := from.(V3); ok { // backward migration
    to.NewField = from.FinalField[0]
  }
  return nil
}

// --------- V3 struct definition below ---------

type V3 struct {
  Name       []string
  FinalField []string
}

func (to *V3) ConvertFrom(from interface{}) error {
  if from, ok := from.(V2); ok { // forward migration
    to.FinalField = []string{from.NewField}
  }
  return nil
}

At this point we could convert in either direction, for example a V3 struct could convert to a V1 struct, with the caveat that there may be data loss, as might need to happen due to changes in the data shapes.

Contributing

If you would like to contribute to this repository, please see the CONTRIBUTING.md.