How can I pass multiple values from a Go template to a nested template?

Passing Multiple Values from Template to Template

In Go templating, it's possible to pass a data value to a nested template using the {{template}} action. However, this action accepts only a single data value as input. When multiple data entities need to be passed to a nested template, a solution is required.

Custom Functions for Data Packaging

Go's templating system allows registering custom functions using the Template.Funcs() method. These functions can perform operations on the data before it's passed to the template, allowing for data manipulation and packaging.

In the case where multiple values need to be passed to a template, a custom function can be created to wrap these values into a single entity, such as a map or struct. This wrapped entity can then be passed to the {{template}} action.

Template Modification

Once the custom function is defined, the template can be modified to call this function and pass the wrapped data entity to the nested template. The syntax for this is:

{{template "templateName" (customFunctionName data1 data2)}}

Example

Consider the following City and Region structs:

type City struct {
    ID      int
    Name    string
    Regions []Region
}  

type Region struct {
    ID               int
    Name             string
    Shops            []Destination
    Masters          []Master
    EducationCenters []Destination
}

To pass multiple data entities to a nested template, a custom function Wrap() can be defined as follows:

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

This function wraps the shops array and city and region names into a map. The modified template would now look like this:

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}}`

Runnable Example

The following code demonstrates how this solution can be used in practice:

package main

import (
    "os"
    "text/template"
)

type City struct {
    ID      int
    Name    string
    Regions []Region
}  

type Region struct {
    ID               int
    Name             string
    Shops            []Destination
    Masters          []Master
    EducationCenters []Destination
}

type Destination struct {
    Name string
}

func main() {
    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)
    }
}

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}}`

This code demonstrates the successful passing of multiple data entities to a nested template using a custom function.

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