How Can I Pass Multiple Values Between Go Templates?

Passing Multiple Values Between Templates

When working with complex data structures involving multiple nested types, passing multiple values from one template to another becomes a challenge. Let's explore a scenario where we have these structs:

• City struct: Contains properties like ID, name, and a slice of Region structures.
• Region struct: Contains ID, name, and various other slices representing different data types.

In our main function, we attempt to execute a template with CityWithSomeData, which is an array of City structures:

tpl.ExecuteTemplate(resWriter, "cities.gohtml", CityWithSomeData)

Passing Multiple Values in Templates

Unfortunately, it's not possible to pass multiple values directly in templates using the {{.}} syntax. According to the text/template documentation, the syntax for {{template}} action allows for only one optional data value to be passed.

{{template "name"}}
    The template with the specified name is executed with nil data.

{{template "name" pipeline}}
    The template with the specified name is executed with dot set
    to the value of the pipeline.

Wrapping Data in a Single Value

To overcome this limitation, we need to wrap our multiple data values into a single value that can be passed as input to the template. However, since we can't write Go code in templates, we'll create a custom wrapper function:

func Wrap(shops []Destination, cityName, regionName string) map[string]interface{} {
    return map[string]interface{}{
        "Shops":      shops,
        "CityName":   cityName,
        "RegionName": regionName,
    }
}

This Wrap() function takes an array of Destination values, along with city and region names, and returns a map that combines all these data. Now, we can use this function in our template:

const src = `
{{define "data"}}
    City: {{.CityName}}, Region: {{.RegionName}}, Shops: {{.Shops}}
{{end}}
{{- range . -}}
        {{$city:=.Name}}
        {{- range .Regions -}}
              {{$region:=.Name}}
              {{- template "data" (Wrap .Shops $city $region) -}}
        {{end}}
{{- end}}`

Updated Example

Here's an updated example that demonstrates how the wrapper function is used:

t := template.Must(template.New("cities.gohtml").Funcs(template.FuncMap{
    "Wrap": Wrap,
}).Parse(src))
CityWithSomeData := []City{
    {
        Name: "CityA",
        Regions: []Region{
            {Name: "CA-RA", Shops: []Destination{{"CA-RA-SA"}, {"CA-RA-SB"}}},
            {Name: "CA-RB", Shops: []Destination{{"CA-RB-SA"}, {"CA-RB-SB"}}},
        },
    },
    {
        Name: "CityB",
        Regions: []Region{
            {Name: "CB-RA", Shops: []Destination{{"CB-RA-SA"}, {"CB-RA-SB"}}},
            {Name: "CB-RB", Shops: []Destination{{"CB-RB-SA"}, {"CB-RB-SB"}}},
        },
    },
}
if err := t.ExecuteTemplate(os.Stdout, "cities.gohtml", CityWithSomeData); err != nil {
    panic(err)
}

Output

City: CityA, Region: CA-RA, Shops: [{CA-RA-SA} {CA-RA-SB}]

City: CityA, Region: CA-RB, Shops: [{CA-RB-SA} {CA-RB-SB}]

City: CityB, Region: CB-RA, Shops: [{CB-RA-SA} {CB-RA-SB}]

City: CityB, Region: CB-RB, Shops: [{CB-RB-SA} {CB-RB-SB}]

By wrapping multiple data values into a single map using the Wrap() function, we successfully passed them between templates in our Go application.

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