I see the document
https://developers.google.com/appengine/docs/python/ndb/keyclass#Key_integer_id
Returns the integer id in the last (kind, id) pair, or None if the key
has an string id or is incomplete.
see I think the id of a key can be a int ; so I write
r = ndb.Key(UserSession, int(id)).get()
if r:
return r.session
but the dev_server.py , will always raise
File "/home/bitcoin/down/google_appengine/google/appengine/datastore/datastore_stub_util.py", line 346, in CheckReference
raise datastore_errors.BadRequestError('missing key id/name')
BadRequestError: missing key id/name
I chanage the int(id) -> str(id)
seems right ;
so my question is , How to use ndb key with integer_id ?
the model is
class UserSession(ndb.Model):
session = ndb.BlobProperty()
The type of the id you use when reading the entity must match the type of the id you used when you wrote the entity. Normally, integer ids are assigned automatically when you write a new entity without specifying an id or key; you then get the id out of the key returned by entity.put(). It is generally not recommended to assign your own integer ids; when the app assigns the keys, the convention is that they should be strings.
There's an easier way to fetch:
UserSession.get_by_id(int(id))
https://developers.google.com/appengine/docs/python/ndb/modelclass#Model_get_by_id
If that doesn't work, I suspect that id is wrong or empty.
There must be something wrong with your variable 'id'.
Your code here should be no problem, and it's better to user long instead of int.
You can try your code on interactive console of development server with specific integer id.
It may be easier to identify your entities in the sessions with their keys instead of their ids. There really is no need to extract the ID from the key to identify the session (other than maybe saving a bit of memory. I think the way your thinking is based on a RDB. I learned that using the key actually makes entity/session identifications easier.
'id' is also a python builtin function. Maybe you are taking that by mistake.
Related
I would like to avoid costly repeated data base queries in Anylogic. I have seen the following thread in Stack Overflow What is the fastest way to look up continuous data on Anylogic (Java, SQL) where a simple three step answer is provided but I'm not sure what the second point of the three actually means:
Save all rows as instances of that class at model start-up into a map - you can use Origin/Destination as the key (use Anylogic's Pair object) and the class instance as the value
I have created a class that takes as inputs the information from each column of my database. I would now like to save each row as an instance of that class - is there an easy way to do this? I may be missing something simple as I'm new to Anylogic.
I'm also unsure of how to create a mapping, if anyone could add more detail to point 2 above I'd be very grateful!
this is effectively the best advice, you created the class, which is a great step, but now, one element of that class, will be used as the key... for example the name... for instance if your class has firstName as one variable and lastName as another variable, you will use a string that is the concatenation of firstName and lastName as your key. Of course any key is fine, assuming that it is unique for all your table. Also an integer as an id is ok too.
create a collection of type LinkedHashMap
Create a class (you did that)
Your collection will take as the key a String (first + last name) and as the value of the elment the class...
now, when you read your database you will have something like this:
for(Tuple t : yourQueryResults){
YourClass yc=new YourClass(t.get(db.var1),t.get(db.var2));
String totalName=t.get(db.first_name)+"_"+t.get(db.last_name);
yourCollection.put(totalName,yc);
}
Now every time you want to find someone with the a name, for example "John Doe", instead of making a query, you will do
yourCollection.get("John_Doe").theVarYouWant;
if you use an id instead of the name, you can set an int as the key, and then you will just do yourCollection.get(theId).theVarYouWant
I want to get an entity key knowing entity ID and an ancestor.
ID is unique within entity group defined by the ancestor.
It seems to me that it's not possible using ndb interface. As I understand datastore it may be caused by the fact that this operation requires full index scan to perform.
The workaround I used is to create a computed property in the model, which will contain the id part of the key. I'm able now to do an ancestor query and get the key
class SomeModel(ndb.Model):
ID = ndb.ComputedProperty( lambda self: self.key.id() )
#classmethod
def id_to_key(cls, identifier, ancestor):
return cls.query(cls.ID == identifier,
ancestor = ancestor.key ).get( keys_only = True)
It seems to work, but are there any better solutions to this problem?
Update
It seems that for datastore the natural solution is to use full paths instead of identifiers. Initially I thought it'd be too burdensome. After reading dragonx answer I redesigned my application. To my suprise everything looks much simpler now. Additional benefits are that my entities will use less space and I won't need additional indexes.
I ran into this problem too. I think you do have the solution.
The better solution would be to stop using IDs to reference entities, and store either the actual key or a full path.
Internally, I use keys instead of IDs.
On my rest API, I used to do http://url/kind/id (where id looked like "123") to fetch an entity. I modified that to provide the complete ancestor path to the entity: http://url/kind/ancestor-ancestor-id (789-456-123), I'd then parse that string, generate a key, and then get by key.
Since you have full information about your ancestor and you know your id, you could directly create your key and get the entity, as follows:
my_key = ndb.Key(Ancestor, ancestor.key.id(), SomeModel, id)
entity = my_key.get()
This way you avoid making a query that costs more than a get operation both in terms of money and speed.
Hope this helps.
I want to make a little addition to dargonx's answer.
In my application on front-end I use string representation of keys:
str(instance.key())
When I need to make some changes with instence even if it is a descendant I use only string representation of its key. For example I have key_str -- argument from request to delete instance':
instance = Kind.get(key_str)
instance.delete()
My solution is using urlsafe to get item without worry about parent id:
pk = ndb.Key(Product, 1234)
usafe = LocationItem.get_by_id(5678, parent=pk).key.urlsafe()
# now can get by urlsafe
item = ndb.Key(urlsafe=usafe)
print item
I'm trying to use an app engine User object's user_id (returned by the User.user_id() method) as a key_name in my own User class. The problem is that it keeps telling me that it's an invalid key_name. I've tried sha2'ing it, and using the digest() as well as the hexdigest() method to reduce the number of possible characters, but still no good result. Is this because the value is too long, or because key names can't have certain characters? And also, how can I modify a user_id in such a way that it stays unique, but is also usable as a key_name for an entity? Extra bonus if it uses a hash so that thje user_id can't be guessed.
Here is the code where the error occured:
def get_current_user():
return User.get(db.Key(hashlib.sha1(users.get_current_user().user_id()).hexdigest()))
I'm now doing some more testing, concidering suggestions from the comments and answer.
I'm not sure why it isn't working for you, the following has no issues when I run it in the dev console.
from google.appengine.ext import db
from google.appengine.api import users
user = users.get_current_user()
name = user.user_id()
print db.Key.from_path ('User', name)
However if you are hashing it (which it sounds like you may be), be aware that you may get a collision. I would avoid against using a hash and would consider some other means of anonymization if you are giving the key to clients. Such as another model whose key you can give away, that has the user's key stored in it. Another method would be to encrypt the id (using the same key for all users) rather than hash it.
If you are doing something that generates binary data (encryption / hash digest) app engine (the sdk at-least) has issues, so you need to encode it first, and use that as the key_name.
name = user.user_id()
hashed_name = hashlib.sha1(name).digest()
encoded_name = base64.b64encode (name)
db.Key.from_path ('User', encoded_name)
Let's take the Django tutorial. In the first part we can find this model:
class Poll(models.Model):
question = models.CharField(max_length=200)
pub_date = models.DateTimeField('date published')
with which Django generates the following SQL:
CREATE TABLE "polls_poll" (
"id" serial NOT NULL PRIMARY KEY,
"question" varchar(200) NOT NULL,
"pub_date" timestamp with time zone NOT NULL
);
One can note that Django automatically added an AutoField, gloriously named id, which is akin to an IntegerField in that it handles integers.
On part 3, we build a custom view, reachable through the following url pattern:
(r'^polls/(?P<poll_id>\d+)/$', 'polls.views.detail'),
The tutorial helpfully explains that a subsequent HTTP request will result in the following call:
detail(request=<HttpRequest object>, poll_id='23')
A few scrolls later, we can find this snippet:
def detail(request, poll_id):
try:
p = Poll.objects.get(pk=poll_id)
Notice how the URL tail component becomes the poll_id argument with a string value of '23', happily churned by the Manager (and therefore QuerySet) get method to produce the result of an SQL query containing a WHERE clause with an integer value of 23 certainly looking like that one:
SELECT * FROM polls_poll WHERE id=23
Certainly Django performed the conversion from the fact that the id field is an AutoField one. The question is how, and when. Specifically, I want to know which internal methods are called, and in what order (kind of like what the doc explains for form validation).
Note: I took a look at sources in django.db.models and found a few *prep* methods, but don't know neither when or where they are called, let alone if they're what I'm looking for.
PS: I know it's not casting stricto sensu, but I think you get the idea.
I think it's in django.db.models.query.get_where_clause
Does get(key) require the entity key and get_by_key_name(key_names) require the key_name?
There is a difference. An Entity in the datastore is identified by a combination of its Kind, its parent, and its identifier. (link) The "identifier" can either be a number or a string. A Key object contains both the identifier and the parent information. So when you call get(), there is just one argument - the Key object. When you call get_by_key_name, notice that there are 2 arguments - one is the key_name, one is the parent.
So a Key is an object with several parts, whereas a key name is just a string. To make things more confusing, a Key object can be encoded as a string.
Here is official explanation for get() and this one is for get_by_key_name()
I hope this helps.