Go模板：是否可以嵌套范围？

Question

This one is seemingly simple but it's driving me insane.

How does one go about referencing a struct element higher in the scope within a nested range in golang templates?

Example:

type Foo struct {
  Id string
  Name string
}

type Bar struct {
  Id string
  Name string
}

var foos []Foo
var bars []Bar

// logic to populate both foos and bars

In the template:

{{range .foos}}
  <div>Foo {{.Name}}</div>
  <div>
    {{range ..bars}}
      <div>Bar {{.Name}} <input type="text" name="ids_{{..Id}}_{{.Id}}" /></div>
    {{end}}
  </div>
{{end}}

Obviously ..bars and ..Id don't work, but hopefully my intent is clear. I'd like to iterate through all combinations of Foo and Bar and generate a form element with a name build by both the Foo's Id and the Bar's Id.

The problem is that it seems it is impossible to:

1. Access bars from inside the scope of the foos range scope
2. Access Foo's Id from inside the bar's range scope

I have a temporary workaround to this by putting a bunch of redundant fields in both structs, but this seems very ugly to me, violates DRY, and in general feels very wrong.

Is there any way with golang templates to do what I'd like to do?

Answer 1

Yes. I feel as if not finding a solution comes from not reading the text/template package closely enough. If you are using html/template, the syntax is the same (and they tell you to read text/template ;)). Here is a complete working solution for what you might want to do.

Go file:

package main

import (
    "bytes"
    "io/ioutil"
    "os"
    "strconv"
    "text/template"
)

type Foo struct {
    Id   string
    Name string
}

type Bar struct {
    Id   string
    Name string
}

var foos []Foo
var bars []Bar

func main() {
    foos = make([]Foo, 10)
    bars = make([]Bar, 10)

    for i := 0; i < 10; i++ {
        foos[i] = Foo{strconv.Itoa(i), strconv.Itoa(i)} // just random strings
        bars[i] = Bar{strconv.Itoa(10 * i), strconv.Itoa(10 * i)}
    }

    tmpl, err := ioutil.ReadFile("so.tmpl")
    if err != nil {
        panic(err)
    }

    buffer := bytes.NewBuffer(make([]byte, 0, len(tmpl)))

    output := template.Must(template.New("FUBAR").Parse(string(tmpl)))
    output.Execute(buffer, struct {
        FooSlice []Foo
        BarSlice []Bar
    }{
        FooSlice: foos,
        BarSlice: bars,
    })

    outfile, err := os.Create("output.html")
    if err != nil {
        panic(err)
    }
    defer outfile.Close()
    outfile.Write(buffer.Bytes())
}

Note: You can probably do something to not load the file into an intermediate buffer (use ParseFiles), I just copied and pasted some code that I had written for one of my projects.

Template file:

{{ $foos := .FooSlice }}
{{ $bars := .BarSlice }}

{{range $foo := $foos }}
  <div>Foo {{$foo.Name}}</div>
  <div>
    {{range $bar := $bars}}
      <div>Bar {{$bar.Name}} <input type="text" name="ids_{{$foo.Id}}_{{$bar.Id}}" /></div>
    {{end}}
  </div>
{{end}}

The two morals of this story are
a) use variables in templates judiciously, they are beneficial
b) range in templates also can set variables, you do not need to rely solely on $ or .