Golang Template - Use Template Variable As Global Variable From Within Range Loop

Question

I will try to make this as simple as possible. I have two variables in Golang which are parsed to the template file.

Here is my Golang code which declares the variables:

for _, issue := range issues {
    issueIDStr := strconv.Itoa(*issue.ID)
    parse[*issue.ID] = issueIDStr 
    parse[issueIDStr+"-label"] = "blah blah"
}

Then in my HTML file:

{{ range .issues }}
    <!-- Here I want to use the current issue's ID as a global variable which is outside the range loop -->
    <!-- According to the Golang doc which says the following:
             When execution begins, $ is set to the data argument passed to Execute, that is, to the starting value of dot.
         I can use $.Something to access a variable outside my range loop right...
         but how can I use a template variable as a global variable? I tried the following which doesn't work.
    {{ $ID := .ID }}
    <p>{{ index $.$ID "author" }}</p>
{{ end }}

After running this code, I get the error: bad character U+0024 '$' along with a panic thrown.

Is what I am attempting completely impossible currently, or is there some trick I am missing?

Thanks :)

Answer 1

You should simply make a map[string]interface{} containing the issues array and then use that as the template execution data.

Then you can loop over the issues and directly access its' members from the template.

Here's a small complete example:

const t = `
{{ range .issues }}
issue: {{ .ID }}
    author: {{ .Author }}
{{ end }}
`

type Issue struct {
    ID     int
    Author string
}

func main() {
    issues := []Issue{{1, "Pepe"}, {2, "Cholo"}}
    data := map[string]interface{}{"issues": issues}
    tpl := template.Must(template.New("bla").Parse(t))
    tpl.Execute(os.Stdout, data)
}

Which outputs:

issue: 1
        author: Pepe

issue: 2
        author: Cholo

Additionally if you want/need to add data specific to the template rendering process you should define a "rich" issue struct for this purpose and transform your model before passing it to template execution. This could be done for both statically known extra data (as simple members of RichIssue) and for dynamically loaded data (as key/values of a map).

Here's an extended example showing the above suggestions:

const t = `
{{ range .issues }}
issue: {{ .ID }}
    author: {{ .Author }}
    static1: {{ .Static1 }}
    dyn1: {{ .Data.dyn1 }}
{{ end }}
`

type Issue struct {
    ID     int
    Author string
}

type RichIssue struct {
    Issue
    Static1 string                 // some statically known additional data for rendering
    Data    map[string]interface{} // container for dynamic data (if needed)
}

func GetIssueStatic1(i Issue) string {
    return strconv.Itoa(i.ID) + i.Author // whatever
}

func GetIssueDyn1(i Issue) string {
    return strconv.Itoa(len(i.Author)) // whatever
}

func EnrichIssue(issue Issue) RichIssue {
    return RichIssue{
        Issue:   issue,
        Static1: GetIssueStatic1(issue),
        // in this contrived example I build "dynamic" members from static
        // hardcoded strings but these fields (names and data) should come from
        // some kind of configuration or query result to be actually dynamic
        // (and justify being set in a map instead of being simple static
        // members as Static1)
        Data: map[string]interface{}{
            "dyn1": GetIssueDyn1(issue),
            "dyn2": 2,
            "dyn3": "blabla",
        },
    }
}

func EnrichIssues(issues []Issue) []RichIssue {
    r := make([]RichIssue, len(issues))
    for i, issue := range issues {
        r[i] = EnrichIssue(issue)
    }
    return r
}

func main() {
    issues := []Issue{{1, "Pepe"}, {2, "Cholo"}}
    data := map[string]interface{}{"issues": EnrichIssues(issues)}
    tpl := template.Must(template.New("bla").Parse(t))
    tpl.Execute(os.Stdout, data)
}

Which produces the following output:

issue: 1
    author: Pepe
    static1: 1Pepe
    dyn1: 4

issue: 2
    author: Cholo
    static1: 2Cholo
    dyn1: 5