How to access struct fields in a template

Question

I'd like to compare two variables of string type inside a range loop as follows:

<select name="category" id="category">            
    {{range $c := .cats}}
      <option value="{{$c.Title}}"  {{ if eq $c.Title .category}}active{{end}}>{{$c.Title}}</option>                                       
    {{end}}    
</select>

both $c.Title and category are strings dispatched by the controller.

However, instead on drop down menue being in the rendered template, I get :

can't evaluate field category in type model.category

$c is of struct type category:

type Category struct {
    ID        int       `db:"id"`
    Title     string    `db:"title"`
    Slug      string    `db:"slug"`
    CreatedAt time.Time `db:"created_at"`
}

When I put directly the string value of category instead of .category in the code above, there is no issues.
I'm using gowebapp MVC framework, if it does matter.

How can I fix this?

Answer 1

Your .category value you want to compare to is not part of your model, but the template engine will attempt to resolve .category as category being a field or method of your model value.

This is because the {{range}} action sets the dot . to the successive elements in each iteration.

To refer to the top-level category, you may use the $ sign like this:

<select name="category" id="category">            
    {{range $c := .cats}}
      <option value="{{$c.Title}}"  {{ if eq $c.Title $.category}}active{{end}}>{{$c.Title}}</option>                                       
    {{end}}    
</select>

See this runnable example:

func main() {
    t := template.Must(template.New("").Parse(src))
    params := map[string]interface{}{
        "category": "Cars",
        "cats": []struct{ Title string }{
            {"Animals"}, {"Cars"}, {"Houses"},
        },
    }
    if err := t.Execute(os.Stdout, params); err != nil {
        panic(err)
    }
}

const src = `<select name="category" id="category">            
    {{range $c := .cats}}
      <option value="{{$c.Title}}"  {{ if eq $c.Title $.category}}active{{end}}>{{$c.Title}}</option>                                       
    {{end}}    
</select>`

Output (try it on the Go Playground):

<select name="category" id="category">            

      <option value="Animals"  >Animals</option>

      <option value="Cars"  active>Cars</option>

      <option value="Houses"  >Houses</option>

</select>