Here is a screen shot of the entities I'm trying to read.
Entities
Here is my go code:
package readfromgcd
import (
"net/http"
"appengine"
"appengine/datastore"
"fmt"
)
type person struct {
firstname string
lastname string
}
func init () {
http.HandleFunc("/", readpeople)
}
func readpeople (w http.ResponseWriter, r *http.Request) {
c := appengine.NewContext(r)
q := datastore.NewQuery("person")
people := make([]person, 0, 20)
if _, err := q.GetAll(c, &people); err !=nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
}
fmt.Fprint(w, "Hello world!")
}
I get the following result:
datastore: cannot load field "firstName" into a "readpeople.person": no such struct field
Here is a screenshot.
result
This code does not show doing anything with this data. I wanted to limit this post to the retrieval. I must be missing something simple. Where have I gone wrong? Thanks in advance for any help.
Property names in your datastore do not match field names in your Go struct person. For example in your datastore person has property firstName but your struct has field firstname.
So first thing is to make them match; or if you want to use different names in your Go struct, use struct tags to define the mappings.
Another important thing: you have to export your type and its fields in order so that the datastore package will be able to load data into it using reflection. So you have to start your type name and its fields with uppercase letters:
type Person struct {
FirstName string `datastore:"firstName"`
LastName string `datastore:"lastName"`
}
Related
A continuation from this question:
Doing a "IN Array" query on google app engine datastore with golang
Right now, I am following the suggestion from the previous question on querying with an array of keys/ids ids []int64. These IDs may or may not actually exist (they have been deleted, but the reference on other instances have not been removed).
My method of trying to obtain these instances looks like so:
var keys []*datastore.Key
for _, id := range ids {
keys = append(keys, datastore.NewKey(c, "Category", "", id, nil))
}
categories := make([]Category, len(keys))
err := datastore.GetMulti(c, keys, categories)
if err != nil {
return nil, err
}
for i := 0; i < len(categories); i++ {
categories[i].Id = keys[i].IntID()
}
However, it errors out throwing me:
datastore: no such entity
I could on the other hand grab each one individually, but is there a more efficient way to approach this?
You need to type assert the error to an appengine.MultiError. This way you can get access to the errors for an individual entity.
if me, ok := err.(appengine.MultiError); ok {
for i, e := range me {
// e != nil if entity i failed
}
} else {
// something else went wrong (timeout, etc).
}
See the docs for MultiError here
I've created a series of structs based on the way I would create tables in mySQL:
type User struct {
UserID int
Email string
Password string
DateCreated time.Time
}
type Device struct {
DeviceID int
Udid string
DateCreated time.Time
DateUpdated time.Time
IntLoginTotal int
}
type DeviceInfo struct {
DeviceID int
DeviceName string
Model string
LocalizedModel string
SystemName string
SystemVersion string
Locale string
Language string
DateCreated time.Time
}
However, I have the impression that I will not be able to make requests like this, and instead I need to create a single struct that can contain an array of multiple devices (each containing an array of multiple device info records).
What is the best way to go about doing this?
In this case, just set the "Kind" as the name of the struct.
From the docs:
func NewKey(c appengine.Context, kind, stringID string, intID int64, parent *Key) *Key
NewKey creates a new key. kind cannot be empty. Either one or both of
stringID and intID must be zero. If both are zero, the key returned is
incomplete. parent must either be a complete key or nil.
For example to save a User.
c := appengine.NewContext(r)
u := &User{UserID: userid, Email: email, Password: password, DateCreated: datecreated}
k := datastore.NewKey(c, "User", u.UserID, 0, nil)
e := p
_, err := datastore.Put(c, k, e)
Follow the same logic to save a different struct type.
To load a user:
c := appengine.NewContext(r)
k := datastore.NewKey(c, "User", userid, 0, nil)
e := new(User)
_, err := datastore.Get(c, k, e)
I'm trying to find an effective example in how to perform updates on appengine datastore with Go.
All the examples I've found on the web are very vague and mostly explains concepts and not the "real life".
The appengine documentation for go says:
..."Updating an existing entity is a matter of performing another Put() using the same key."
My problem here is being in how to retrieve the key. So I have the code below to store and retrieve data:
func subscribe(w http.ResponseWriter, r *http.Request) {
user := User {
Name: r.FormValue("username"),
Email: r.FormValue("useremail"),
Flag: 0,
}
c := appengine.NewContext(r)
//datastore.Put(c, datastore.NewIncompleteKey(c, "User", nil), &user)
datastore.Put(c, datastore.NewKey(c, "User", "stringID", 0, nil), &user)
template.Must(template.ParseFiles("confirmation.html")).Execute(w, nil)
}
func checkusers(w http.ResponseWriter, r *http.Request) {
c := appengine.NewContext(r)
qUsers := datastore.NewQuery("User")
var users []User
qUsers.GetAll(c, &users)
template.Must(template.ParseFiles("users.html")).Execute(w, users)
}
How do I do an update on the flag property changing its value tom 1?
I'm a bit confused on this thing as I couldn't fully understand how the "key" is stored for each entity.
Any help would be very appreciated.
todo an update you first need to identify if your object is a new or an old one.
this can be simple done by adding the following method to your User struct:
type User struct {
Name string
Email string
Flag int64 `datastore:"-"`
}
func (u *User) IsNew() bool {
return u.Flag == 0
}
this tells data-store to ignore the Flag field when storing and retrieving an object
and because initial value of int64 is zero, a newly created object can be identified if Flag is zero
so creating a new object just needs to set UserName and Email:
user := User {
Name: r.FormValue("username"),
Email: r.FormValue("useremail")
}
then next step is to either use a IncompleteKey or a Key, for the put statement
could look like this:
var k *datastore.Key
if user.IsNew() {
k = datastore.NewIncompleteKey(c, "Users", nil)
} else {
k = datastore.NewKey(c, "Users", "", user.Flag, nil)
}
k, err := datastore.Put(c, k, user)
if err != nil {
return k, err
}
with an incomplete key, app-engine will generate a new key for you.
after put you can assign the new key to your object:
user.Flag = k.IntID
now if you do a query later you need to assign the Id to your query result objects,
the query will return the keys of query result in the same order so you can change your code like this:
keys, err := q.GetAll(c, &users)
if err != nil {
return
}
l := len(users)
for i := 0; i < l; i++ {
users[i].Flag = keys[i].IntID()
}
thats all, for more information, just have a look a the reference document there is explained with methods return which values.
https://developers.google.com/appengine/docs/go/datastore/reference
I'm playing with Google App Engine using the Go APIs and despite everything I've tried I can't get the Queries to return data that is in the datastore. I can see that Put() works as expected, and the entities are listable/accessible from inspection of the Admin Console available from dev_appserver.py
The struct I'm storing is defined this way:
type TweetData struct {
Id int64 `datastore:",noindex" json:"id"`
Text string `datastore:",noindex" json:"text"`
Screen_name string `json:"screen_name"`
}
And my calls to query it are such:
func getDatastoreTweets(c appengine.Context, twitterUser string) []*TweetData {
q := datastore.NewQuery("TweetData").Filter("Screen_name =", twitterUser).Order("-Id").Limit(10)
var oldTweets []*TweetData
if _, err := q.GetAll(c, &oldTweets); err != nil {
fmt.Printf("Getall had non-nil error! %v\n", err)
}
return oldTweets
}
For this query, err is never non-nil, but always returns 0 results, even when the Console tells me there are many. I've tried it without the Filter call as well, after the guestbook example the SDK provides, to no avail.
In case it's an issue with keys (unlikely if I understand correctly, because I'm querying on a property), the call to Put is as follows:
// tweetData passed in via parameter...
key := datastore.NewIncompleteKey(c, "TweetData", nil)
_, err := datastore.Put(c, key, &tweetData)
Any help would be appreciated, thanks! ^_^
The query ask for ordering by Id desc, while the index field is un-indexed, you should either:
rewrite the TweetData annotation to index Id field:
Id int64 `json:"id"`
Remove the Order clause of your query:
q := datastore.NewQuery("TweetData").Filter("Screen_name =", twitterUser).Limit(10)
Here is an Example of the app. The essential code is in: golang-code/handler/handler.go (After the subject should appear an ID!)
Im trying to build a little blog system in Golang on Google Appengine and use Mustache as template engine.
So, i have a struct:
type Blogposts struct {
PostTitle string
PostPreview string
Content string
Creator string
Date time.Time
}
The data is passed to GAE via
datastore.Put(c, datastore.NewIncompleteKey(c, "Blogposts", nil), &blogposts)
So, GAE assigns automatically a intID (int64).
Now I tried to get the latest blogposts
// Get the latest blogposts
c := appengine.NewContext(r)
q := datastore.NewQuery("Blogposts").Order("-Date").Limit(10)
var blogposts []Blogposts
_, err := q.GetAll(c, &blogposts)
Until there all things works fine, but when I try to access intID (or stringID, whatever) I dont have access to this :-(
<h3>{{{PostTitle}}}</h3>
(PostTitle works, intID not, i've tried thousand of things, nothing worked :-( )
Anyone an idea? This would be great!
Edit:
I use mustache.
http://mustache.github.com/
In the code I use:
x["Blogposts"] = blogposts
data := mustache.RenderFile("templates/about.mustache", x)
sendData(w, data) // Equivalent to fmt.Fprintf
And then the data can be accessed in the .mustache template with {{{Content}}} or {{{PostTitle}}} etc.
As hyperslug pointed out, the id field of an entity is on the key, not the struct it gets read into.
Another way around this is to add an id field to your struct and tell datastore to ignore it, eg:
type Blogposts struct {
PostTitle string
PostPreview string
Content string
Creator string
Date time.Time
Id int64 `datastore:"-"`
}
You can then populate the Id field manually after a call to GetAll() like so
keys, err := q.GetAll(c, &blogposts)
if err != nil {
// handle the error
return
}
for i, key := range keys {
blogposts[i].Id = key.IntID()
}
This has the benefit of not introducing an extra type.
intID is an internal property of a Key not the struct, and is accessible through a getter:
id := key.IntID()
GetAll returns []*Key, which you're not using:
_, err := q.GetAll(c, &blogposts)
One way to get around this is to create a viewmodel struct that has both your post and key info (untested, but this is the gist of it):
//... handler code ...
keys, err := q.GetAll(c, &blogposts)
if err != nil {
http.Error(w, "Problem fetching posts.", http.StatusInternalServerError)
return
}
models := make([]BlogPostVM, len(blogposts))
for i := 0; i < len(blogposts); i++ {
models[i].Id = keys[i].IntID()
models[i].Title = blogposts[i].Title
models[i].Content = blogposts[i].Content
}
//... render with mustache ...
}
type BlogPostVM struct {
Id int
Title string
Content string
}
I know this question is a couple years old, but the following article was very helpful to me in this regard: Golang basics with Google Datastore.
In the article, the author provides a nice example of how you can run a query that gets an entity by its ID...
func GetCategory(c appengine.Context, id int64) (*Category, error) {
var category Category
category.Id = id
k := category.key(c)
err := datastore.Get(c, k, &category)
if err != nil {
return nil, err
}
category.Id = k.IntID()
return &category, nil
}
...as well as getting a list/collection of entities with their associated ID:
func GetCategories(c appengine.Context) ([]Category, error) {
q := datastore.NewQuery("Category").Order("Name")
var categories []Category
keys, err := q.GetAll(c, &categories)
if err != nil {
return nil, err
}
// you'll see this a lot because instances
// do not have this by default
for i := 0; i < len(categories); i++ {
categories[i].Id = keys[i].IntID()
}
return categories, nil
}
The snippet above is very close to the helpful answer by #koz.
AFAICS, the Blogposts struct has no field intID, but it has a field PostTitle. I guess that could be the reason why the former doesn't and the later does get rendered, though I've never used Mustache...