Go Template : Use nested struct's field and {{range}} tag together

Question

I have the following nested struct and I would like to iterate them in a template, in a {{range .Foos}} tag.

type Foo struct {
    Field1, Field2 string
}

type NestedStruct struct {
    NestedStructID string
    Foos []Foo
}

I'm trying with the following html/template but it can't access the NestedStructID from NestedStruct.

{{range .Foos}} { source: '{{.Field1}}', target: '{{.NestedStructID}}' }{{end}}

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

Answer 1

You can't reach the NestedStructID field like that because the {{range}} action sets the pipeline (the dot .) in each iteration to the current element.

You may use the $ which is set to the data argument passed to Template.Execute(); so if you pass a value of NestedStruct, you can use $.NestedStructID.

For example:

func main() {
    t := template.Must(template.New("").Parse(x))

    ns := NestedStruct{
        NestedStructID: "nsid",
        Foos: []Foo{
            {"f1-1", "f2-1"},
            {"f1-2", "f2-2"},
        },
    }
    fmt.Println(t.Execute(os.Stdout, ns))
}

const x = `{{range .Foos}}{ source: '{{.Field1}}', target: '{{$.NestedStructID}}' }
{{end}}`

Output (try it on the Go Playground):

{ source: 'f1-1', target: 'nsid' }
{ source: 'f1-2', target: 'nsid' }
<nil>

This is documented in text/template:

When execution begins, $ is set to the data argument passed to Execute, that is, to the starting value of dot.