Appengine > Go > Mapping datastore results

I'm trying to map the results retreived by query.GetAll()

I need to map the results since the Template will need the datastore 'Key' associated with each entity.

At the moment I'm doing the following:

// Query
q := datastore.NewQuery("Article").Limit(10)
// Define array where the entities will be retreived
var a[] Article;
// Retreive entities
key, _ := q.GetAll(c, &a)
// Create an empty map
article := map[string] Article{}
// Build the map
for k := range a {
	article[key[k].Encode()] = a[k];
}
template.Execute(w, map[string]interface{} { "Articles" : article})

Is there a more efficient way to build the map directly using query.GetAll(), since creating an array, a map and loop over the array to build the map doesn't seem wise?

Or a more efficient way to get the datastore Key (Encoded) associated with each entity?

Maybe best is to zip the slices into a single slice. Along these lines,

package main

import (
    "os"
    "text/template"
)

type pair struct {
    Key     string
    Article string
}

var tmpt = `Here's the list:{{range $p := .}}
    {{$p.Key}}: {{$p.Article}}{{end}}
`

func main() {
    list := template.Must(template.New("list").Parse(tmpt))
    // query
    key := []string{"banana", "donkey", "curious"}
    a := []string{"long fruit", "who needs one?", "human nature"}
    // zip
    pairs := make([]pair, len(key))
    for i, k := range key {
        pairs[i].Key = k
        pairs[i].Article = a[i]
    }
    // execute on zipped list
    if err := list.Execute(os.Stdout, pairs); err != nil {
        os.Stdout.WriteString(err.Error())
    }
}

Output:
<pre>
Here's the list:
banana: long fruit
donkey: who needs one?
curious: human nature
</pre>

I don't think you need the map. If I understand, after GetAll, you have two parallel slices, key and a. (I don't know GAE, but the _ raises my eyebrow. Is it something you should check?) Templates can handle parallel arrays. See in the doc how the range action can return two results, a value and and index. You should be able to range over one slice and use the index to get corresponding values from the other. It will be a more complex template, but should let you avoid the map.

Edit: Sorry, I thought I knew how to do this, but failed when I tried to code an example. I'll leave it here in case someone else knows how to do this, otherwise, downvote...

Maybe you could embedding the Key in the Article. You would still have to loop over the articles and keys, but at least you wouldn't have to create a separate map.

type Article struct {
    // Add a key to the struct, but prevent it from being saved to the datastore.
    Key datastore.Key `datastore:"-"`
}

// Query
q := datastore.NewQuery("Article").Limit(10)
// Define array where the entities will be retreived
var a[] Article
// Retreive entities
key, _ := q.GetAll(c, &a)

// Loop through the articles adding the key as you go.
for i := range a {
    a[i].Key = key[i]
}
template.Execute(w, map[string]interface{} { "Articles" : a})

Then in your template you would call article.Key.Encode