go/appengine: faster datastore tests

Recently I've been playing around with golang and google's app engine.

I've been experiencing really slow unit test executions when implementing repositories.

The thing I do is to call ctx,_ := aetest.NewContext(nil) in each test in order to obtain a clean database. This starts a new server in each test and thus leads in slow testing.

Lately I've been trying to work this around by starting it in TestMain.

var ctx aetest.Context

func TestMain(m *testing.M) {
	ctx,_ = aetest.NewContext(nil)
    code := m.Run()
    ctx.Close()
    os.Exit(code)
}

func TestMyRepository(t *testing.T){
    cleanDatastore()
    repo := &MyRepository{ctx}
    repo.DoSomething()
}

In the function [cleanDatastore](https://gist.github.com/NewestUser/9468b0a7b763c84a8625 "gist") I've been executing a bash script that basically runs a SQLite command to clean the local database saved in /tmp.

#!/usr/bin/env bash

PATH=$1
cd $PATH

echo "Cleaning datastore..."

/usr/bin/sqlite3 datastore "delete from \"dev~testapp!!EntitiesByProperty\";"
/usr/bin/sqlite3 datastore "delete from \"dev~testapp!!Entities\";"

echo "Datastore is clean."

Is what I am trying to do making any sense or is there a simpler way to achieve better testing time.

A kindless, keys-only query (getting the keys of all entities, regardless of kind) should help:

   q := datastore.NewQuery("").KeysOnly()

and then, you loop over all such keys and delete each of them, e.g:

   for t := q.Run(ctx); ; {
        key, err := t.Next(nil)
        if err == datastore.Done {
            break
        }
        if err != nil {
            serveError(c, w, err)
            return
        }
        datastore.Delete(ctx, key)
    }

It may be slightly faster to get the keys into an array and use DeleteMulti, but, on a local datastore, I suspect that would not make a measurable difference.