Appending content to Go templates within a certain section

Question

I am interested in appending content to a Go template but within a certain section of the template. Since the template has a structure defined, whenever I try to append new content on executing the template, it appends the new content to the previously executed template content:

Example template:

 type Client struct {
    Opts    *ClientOpts
    Schemas *Schemas
    Types   map[string]Schema

    Container *{{.schema.Id}}Client

  }

Actual output:

type Client struct {
        Opts    *ClientOpts
        Schemas *Schemas
        Types   map[string]Schema

        Container *abcClient

      }

type Client struct {
        Opts    *ClientOpts
        Schemas *Schemas
        Types   map[string]Schema

        Container *xyzClient

      }
}

Desired output:

type Client struct {
        Opts    *ClientOpts
        Schemas *Schemas
        Types   map[string]Schema

        Container *abcClient
        Container *xyzClient

      }

My current Go code looks like this:

func appendToFile(filename string, template *template.Template, schema client.Schema) error {
    output, err := os.OpenFile(filename, os.O_APPEND|os.O_WRONLY, 0600)
    if err != nil {
        panic(err)
    }
    defer output.Close()

    data := map[string]interface{}{
        "schema": schema,
    }
    err = template.Execute(output, data)
    return err

}

One solution I can think of is to make a seek every time to the previous appended content and write the new content there. But I am not sure how to do that in Go. Could someone provide me with a code snippet for that or suggest a better strategy?

Answer 1

Instead of appending the data, generate the output with one execution of the template:

package main

import (
    "fmt"
    "os"
    "text/template"
)

var t = template.Must(template.New("").Parse(` type Client struct {
    Opts    *ClientOpts
    Schemas *Schemas
    Types   map[string]Schema

{{range .}}
    Container *{{.schema.Id}}Client{{end}}

}
`))

type schema struct {
    Id string
}

func main() {
    data := []map[string]interface{}{
        {"schema": schema{Id: "abcClient"}},
        {"schema": schema{Id: "xyzClient"}},
    }
    if err := t.Execute(os.Stdout, data); err != nil {
        fmt.Println(err)
    }
}

Try it on the playground.