Q: What is the shortest way to simply sort an array of structs by (arbitrary) field names? I just had a problem where I had an array of structs, e.g.
package main

import "log"

type Planet struct {
    Name       string  `json:"name"`
    Aphelion   float64 `json:"aphelion"`   // in million km
    Perihelion float64 `json:"perihelion"` // in million km
    Axis       int64   `json:"Axis"`       // in km
    Radius     float64 `json:"radius"`
}

func main() {
    var mars = new(Planet)
    mars.Name = "Mars"
    mars.Aphelion = 249.2
    mars.Perihelion = 206.7
    mars.Axis = 227939100
    mars.Radius = 3389.5

    var earth = new(Planet)
    earth.Name = "Earth"
    earth.Aphelion = 151.930
    earth.Perihelion = 147.095
    earth.Axis = 149598261
    earth.Radius = 6371.0

    var venus = new(Planet)
    venus.Name = "Venus"
    venus.Aphelion = 108.939
    venus.Perihelion = 107.477
    venus.Axis = 108208000
    venus.Radius = 6051.8

    planets := [...]Planet{*mars, *venus, *earth}
    log.Println(planets)
}

Lets say you want to sort it by Axis. How do you do that?
(Note: I have seen http://golang.org/pkg/sort/ and it seems to work, but I have to add about 20 lines just for simple sorting by a very simple key. I have a python background where it is as simple as sorted(planets, key=lambda n: n.Axis) - is there something similar simple in Go?)

A: UPDATE: This answer relates to older versions of go. For Go 1.8 and newer, see the AndreKR's answer above.

If you want something a bit less verbose than the standard library sort package, you could use the third party github.com/bradfitz/slice package.  It uses some tricks to generate the Len and Swap methods needed to sort your slice, so you only need to provide a Less method.
With this package, you can perform the sort with:
slice.Sort(planets[:], func(i, j int) bool {
    return planets[i].Axis < planets[j].Axis
})

The planets[:] part is necessary to produce a slice covering your array.  If you make planets a slice instead of an array you could skip that part.

A: You can, instead of implementing the Sort interface on []Planet you implement on a type that contains the collection and a closure that will do the comparison. You have to provide the implementation for the comparison closure for each property.
This method I feel is better than implementing a Sort type for each property of the struct.
This answer is almost ripped right from the sort docs so I can't take to much credit for it
package main

import (
    "log"
    "sort"
)

type Planet struct {
    Name       string  `json:"name"`
    Aphelion   float64 `json:"aphelion"`   // in million km
    Perihelion float64 `json:"perihelion"` // in million km
    Axis       int64   `json:"Axis"`       // in km
    Radius     float64 `json:"radius"`
}

type By func(p1, p2 *Planet) bool

func (by By) Sort(planets []Planet) {
    ps := &planetSorter{
        planets: planets,
        by:      by, 
    }
    sort.Sort(ps)
}

type planetSorter struct {
    planets []Planet
    by      func(p1, p2 *Planet) bool 
}

func (s *planetSorter) Len() int {
    return len(s.planets)
}

func (s *planetSorter) Swap(i, j int) {
    s.planets[i], s.planets[j] = s.planets[j], s.planets[i]
}

func (s *planetSorter) Less(i, j int) bool {
    return s.by(&s.planets[i], &s.planets[j])
}

How to call it.
func main() {
    /* Same code as in the question */

    planets := []Planet{*mars, *venus, *earth}

    By(func(p1, p2 *Planet) bool {
        return p1.Name < p2.Name
    }).Sort(planets)

    log.Println(planets)

    By(func(p1, p2 *Planet) bool {
        return p1.Axis < p2.Axis
    }).Sort(planets)

    log.Println(planets)
}

Here is a Demo

A: As of Go 1.8 you can now use sort.Slice to sort a slice:
sort.Slice(planets, func(i, j int) bool {
  return planets[i].Axis < planets[j].Axis
})


There is normally no reason to use an array instead of a slice, but in your example you are using an array, so you have to overlay it with a slice (add [:]) to make it work with sort.Slice:
sort.Slice(planets[:], func(i, j int) bool {
  return planets[i].Axis < planets[j].Axis
})

The sorting changes the array, so if you really want you can continue to use the array instead of the slice after the sorting.

A: As of Go 1.8, @AndreKR's answer is the better solution.

You can implement a collection type which implements the sort interface.
Here's an example of two such types which allow you to sort either by Axis or Name:
package main

import "log"
import "sort"

// AxisSorter sorts planets by axis.
type AxisSorter []Planet

func (a AxisSorter) Len() int           { return len(a) }
func (a AxisSorter) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
func (a AxisSorter) Less(i, j int) bool { return a[i].Axis < a[j].Axis }

// NameSorter sorts planets by name.
type NameSorter []Planet

func (a NameSorter) Len() int           { return len(a) }
func (a NameSorter) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
func (a NameSorter) Less(i, j int) bool { return a[i].Name < a[j].Name }

type Planet struct {
    Name       string  `json:"name"`
    Aphelion   float64 `json:"aphelion"`   // in million km
    Perihelion float64 `json:"perihelion"` // in million km
    Axis       int64   `json:"Axis"`       // in km
    Radius     float64 `json:"radius"`
}

func main() {
    var mars Planet
    mars.Name = "Mars"
    mars.Aphelion = 249.2
    mars.Perihelion = 206.7
    mars.Axis = 227939100
    mars.Radius = 3389.5

    var earth Planet
    earth.Name = "Earth"
    earth.Aphelion = 151.930
    earth.Perihelion = 147.095
    earth.Axis = 149598261
    earth.Radius = 6371.0

    var venus Planet
    venus.Name = "Venus"
    venus.Aphelion = 108.939
    venus.Perihelion = 107.477
    venus.Axis = 108208000
    venus.Radius = 6051.8

    planets := []Planet{mars, venus, earth}
    log.Println("unsorted:", planets)

    sort.Sort(AxisSorter(planets))
    log.Println("by axis:", planets)

    sort.Sort(NameSorter(planets))
    log.Println("by name:", planets)
}


A: Here is another way to reduce some of the boiler plate. Disclaimer, it uses reflection and losses type safety.
Here is a Demo
All the magic happens in the Prop function. It takes the struct property to sort on and the order it which you want to sort (ascending, descending) and returns a function that will perform the comparisons. 
package main

import (
    "log"
    "reflect"
    "sort"
)

func test(planets []Planet) {
    log.Println("Sort Name")
    By(Prop("Name", true)).Sort(planets)
    log.Println(planets)

    log.Println("Sort Aphelion")
    By(Prop("Aphelion", true)).Sort(planets)
    log.Println(planets)

    log.Println("Sort Perihelion")
    By(Prop("Perihelion", true)).Sort(planets)
    log.Println(planets)

    log.Println("Sort Axis")
    By(Prop("Axis", true)).Sort(planets)
    log.Println(planets)

    log.Println("Sort Radius")
    By(Prop("Radius", true)).Sort(planets)
    log.Println(planets)
}

func Prop(field string, asc bool) func(p1, p2 *Planet) bool {
    return func(p1, p2 *Planet) bool {

        v1 := reflect.Indirect(reflect.ValueOf(p1)).FieldByName(field)
        v2 := reflect.Indirect(reflect.ValueOf(p2)).FieldByName(field)

        ret := false

        switch v1.Kind() {
        case reflect.Int64:
            ret = int64(v1.Int()) < int64(v2.Int())
        case reflect.Float64:
            ret = float64(v1.Float()) < float64(v2.Float())
        case reflect.String:
            ret = string(v1.String()) < string(v2.String())
        }

        if asc {
            return ret
        }
        return !ret
    }
}

type Planet struct {
    Name       string  `json:"name"`
    Aphelion   float64 `json:"aphelion"`   // in million km
    Perihelion float64 `json:"perihelion"` // in million km
    Axis       int64   `json:"Axis"`       // in km
    Radius     float64 `json:"radius"`
}

type By func(p1, p2 *Planet) bool

func (by By) Sort(planets []Planet) {
    ps := &planetSorter{
        planets: planets,
        by:      by, // The Sort method's receiver is the function (closure) that defines the sort order.
    }
    sort.Sort(ps)
}

type planetSorter struct {
    planets []Planet
    by      func(p1, p2 *Planet) bool // Closure used in the Less method.
}

// Len is part of sort.Interface.
func (s *planetSorter) Len() int { return len(s.planets) }

// Swap is part of sort.Interface.
func (s *planetSorter) Swap(i, j int) {
    s.planets[i], s.planets[j] = s.planets[j], s.planets[i]
}

// Less is part of sort.Interface. It is implemented by calling the "by" closure in the sorter.
func (s *planetSorter) Less(i, j int) bool {
    return s.by(&s.planets[i], &s.planets[j])
}

func main() {
    test(dataSet())
}

func dataSet() []Planet {

    var mars = new(Planet)
    mars.Name = "Mars"
    mars.Aphelion = 249.2
    mars.Perihelion = 206.7
    mars.Axis = 227939100
    mars.Radius = 3389.5

    var earth = new(Planet)
    earth.Name = "Earth"
    earth.Aphelion = 151.930
    earth.Perihelion = 147.095
    earth.Axis = 149598261
    earth.Radius = 6371.0

    var venus = new(Planet)
    venus.Name = "Venus"
    venus.Aphelion = 108.939
    venus.Perihelion = 107.477
    venus.Axis = 108208000
    venus.Radius = 6051.8

    return []Planet{*mars, *venus, *earth}
}


A: You can implement using quick sort as well and inside the partition func, you choose which field to sort by, I choose Name for example.
package main

import (
    "fmt"
)

type Planet struct {
    Name       string  `json:"name"`
    Aphelion   float64 `json:"aphelion"`   // in million km
    Perihelion float64 `json:"perihelion"` // in million km
    Axis       int64   `json:"Axis"`       // in km
    Radius     float64 `json:"radius"`
}

func main() {
    var mars Planet
    mars.Name = "Mars"
    mars.Aphelion = 249.2
    mars.Perihelion = 206.7
    mars.Axis = 227939100
    mars.Radius = 3389.5

    var earth Planet
    earth.Name = "Earth"
    earth.Aphelion = 151.930
    earth.Perihelion = 147.095
    earth.Axis = 149598261
    earth.Radius = 6371.0

    var venus Planet
    venus.Name = "Venus"
    venus.Aphelion = 108.939
    venus.Perihelion = 107.477
    venus.Axis = 108208000
    venus.Radius = 6051.8

    planets := []Planet{mars, venus, earth}
    fmt.Println(quickSort(&planets,0,len(planets)-1))

}

func quickSort(arr *[]Planet, start, end int)[]Planet{
    if start < end{
        partitionIndex := partition(*arr,start,end)
        quickSort(arr,start,partitionIndex-1)
        quickSort(arr,partitionIndex+1, end)
    }
    return *arr
}

func partition(arr []Planet, start, end int) int{
    pivot := arr[end].Name
    pIndex := start
    for i:= start; i<end; i++{
        if arr[i].Name <= pivot{
            //  swap
            arr[i],arr[pIndex] = arr[pIndex],arr[i]
            pIndex++
        }
    }
    arr[pIndex],arr[end] = arr[end],arr[pIndex]
    return pIndex
}


A: The other answers are already pretty good.
I want to add this example for sorting alphabetically by a structs Value that is a string. Pretty much the same in Go, might be helpful to someone with other programming language background :)
type My struct {
    Val string
}

func main() {
    m1 := My{Val: "B"}
    m2 := My{Val: "C"}
    m3 := My{Val: "A"}
    m4 := My{Val: "D"}
    mList := []My{m1, m2, m3, m4}
    sort.Slice(mList, func(i, j int) bool {
        return mList[i].Val < mList[j].Val
    })
    fmt.Println("Sorted:", mList[0], mList[1], mList[2], mList[3])
}

