I recently found out about sqlalchemy in Python. I'd like to use it for data science rather than website applications.
I've been reading about it and I like that you can translate the sql queries into Python.
The main thing that I am confused about that I'm doing is:
Since I'm reading data from an already well established schema, I wish I didn't have to create the corresponding models myself.
I am able to get around that reading the metadata for the table and then just querying the tables and columns.
The problem is when I want to join to other tables, this metadata reading is taking too long each time, so I'm wondering if it makes sense to pickle cache it in an object, or if there's another built in method for that.
Edit: Include code.
Noticed that the waiting time was due to an error in the loading function, rather than how to use the engine. Still leaving the code in case people comment something useful. Cheers.
The code I'm using is the following:
def reflect_engine(engine, update):
store = f'cache/meta_{engine.logging_name}.pkl'
if update or not os.path.isfile(store):
meta = alq.MetaData()
meta.reflect(bind=engine)
with open(store, "wb") as opened:
pkl.dump(meta, opened)
else:
with open(store, "r") as opened:
meta = pkl.load(opened)
return meta
def begin_session(engine):
session = alq.orm.sessionmaker(bind=engine)
return session()
Then I use the metadata object to get my queries...
def get_some_cars(engine, metadata):
session = begin_session(engine)
Cars = metadata.tables['Cars']
Makes = metadata.tables['CarManufacturers']
cars_cols = [ getattr(Cars.c, each_one) for each_one in [
'car_id',
'car_selling_status',
'car_purchased_date',
'car_purchase_price_car']] + [
Makes.c.car_manufacturer_name]
statuses = {
'selling' : ['AVAILABLE','RESERVED'],
'physical' : ['ATOURLOCATION'] }
inventory_conditions = alq.and_(
Cars.c.purchase_channel == "Inspection",
Cars.c.car_selling_status.in_( statuses['selling' ]),
Cars.c.car_physical_status.in_(statuses['physical']),)
the_query = ( session.query(*cars_cols).
join(Makes, Cars.c.car_manufacturer_id == Makes.c.car_manufacturer_id).
filter(inventory_conditions).
statement )
the_inventory = pd.read_sql(the_query, engine)
return the_inventory
I'm using the Google Cloud Datastore in a very simple way, and I try to retrieve an entity by its id. I've read this (it's in Java but seems to follow the same logic)
Def of my entity is here:
class Logs(ndb.Model):
startDate = ndb.DateTimeProperty()
endDate = ndb.DateTimeProperty()
requestedDate = ndb.DateProperty()
taskName = ndb.StringProperty()
status = ndb.StringProperty()
Then when I insert a new one I do
logs = Logs(startDate=datetime.utcnow(),
taskName=taskName,
requestedDate=requestedDate,
status=u'IN_PROGRESS')
key = logs.put()
id = key.id() # I use this variable later
And when I want to retrieve it
logs = Logs.get_by_id(id)
But it never returns any entity...
What's wrong with this ?
Thanks for helping
According to the documentation, you should be able to call get() directly from the Key object to retrieve the entity from Datastore:
logs_entity = Logs(startDate=datetime.utcnow(),
taskName=taskName,
requestedDate=requestedDate,
status=u'IN_PROGRESS')
# Saves entity to Datastore and returns Key
entity_key = logs_entity.put()
# Retrieves entity from Datastore using the previous Key
result = entity_key.get()
Edit:
In the case where you need to pass around the key as a string to rebuild the Key object later you might try using the urlsafe() method, which allows embedding it in a URL:
urlsafe_string = entity_key.urlsafe()
[...]
entity_key= ndb.Key(urlsafe=urlsafe_string)
logs_entity = entity_key.get()
i have the following model in google app engine:
class CustomUsers(db.Model):
cuid = db.StringProperty()
name = db.StringProperty()
email = db.StringProperty()
bday = db.StringProperty()
now given a cuid i just want to check if he is present in the high replication data store.
in my views i write the following, but eventually get an 500 error:
usr_prev = CustomUsers.all().filter('cuid =', request.POST['id'])
if(not usr_prev):
logging.info("this user does not exist")
else:
logging.info("user exists")
but this gives me an error. How shall i do it?
(Do not consider any importing issues.)
"500 errors" don't exist in isolation. They actually report useful information, either on the debug page or in the logs. I'd guess that your logs show that request.POST has no element id, but that is just a guess.
I noticed that you have assigned a query object to usr_prev. You need to execute the query. I think Query.count(1) function should do it.
However, this may not be related to the 500 error.
you can go like this to see:
query = Users.all()
cnt = query.filter('id =', user_id).count()
logging.info(cnt)
return int(cnt)
once this is done, seems you are using django with app engine, as per the error you are not returning string response. you might want to return
return HttpResponse("1") instead of return HttpResponse(1)
I have a list of about 20 objects and for each object I return a list of 10 dictionaries.
I am trying to store the list of 10 dictionaries for each object in the list on GAE; I do not think I am writing the code correctly to store this information to GAE.
Here is what I have:
Before my main request handler I have this class:
class Tw(db.Model):
tags = db.ListProperty()
ip = db.StringProperty()
In my main request handler I have the following:
for city in lst_of_cities: # this is the list of 20 objects
dict_info = hw12.twitter(city) # this is the function to get the list of 10 dictionaries for each object in the list
datastore = Tw() # this is the class defined for db.model
datastore.tags.append(dict_info) #
datastore.ip = self.request.remote_addr
datastore.put()
data = Data.gql("") #data entities we need to fetch
I am not sure if this code is write at all. If anyone could please help it would be much appreciated.
Welcome to Stack Overflow!
I see a few issues:
Dictionaries are not supported value types for App Engine properties.
You're only storing the last entity; the rest are discarded.
You're using a ListProperty, but instead of appending each element of dict_info, you're doing a single append of the entire list.
Since you can't store a raw dictionary inside a property, you need to serialize it to some other format, like JSON or pickle. Here's a revised example using pickle:
from google.appengine.ext import db
import pickle
class Tw(db.Model):
tags = db.BlobProperty()
ip = db.StringProperty()
entities = []
for city in lst_of_cities:
dict_info = hw12.twitter(city)
entity = Tw()
entity.tags = db.Blob(pickle.dumps(dict_info))
entity.ip = self.request.remote_addr
entities.append(entity)
db.put(entities)
When you fetch the entity later, you can retrieve your list of dictionaries with pickle.loads(entity.tags).
When I deal with data types that are not directly supported by Google App Engine like dictionaries or custom data type, I usually adopt the handy PickleProperty.
from google.appengine.ext import db
import pickle
class PickleProperty(db.Property):
def get_value_for_datastore(self, model_instance):
value = getattr(model_instance, self.name, None)
return pickle.dumps(value)
def make_value_from_datastore(self, value):
return pickle.loads(value)
Once declared the PickleProperty class in your commons.py module, you can use it to store your custom data with something like this:
from google.appengine.ext import db
from commons import PickleProperty
class Tw(db.Model):
tags = PickleProperty()
ip = db.StringProperty()
entities = []
for city in lst_of_cities:
dict_info = hw12.twitter(city)
entity = Tw()
entity.tags = dict_info
entity.ip = self.request.remote_addr
entities.append(entity)
db.put(entities)
To retrieve the data back go with:
entity.tags
Since this was written, the App Engine has pushed out their experimental "ndb" Python database model, which contains in particular the JsonProperty, something that pretty well directly implements what you want.
Now, you need to be running the Python 2.7 version of the App Engine, which is still not quite ready for production, but it all seems pretty stable these days, GvR himself seems to be writing a lot of the code which bodes well for the code quality, and I'm intending to use this in production sometime this year...
I would like to wipe out all data for a specific kind in Google App Engine. What is the
best way to do this?
I wrote a delete script (hack), but since there is so much data is
timeout's out after a few hundred records.
I am currently deleting the entities by their key, and it seems to be faster.
from google.appengine.ext import db
class bulkdelete(webapp.RequestHandler):
def get(self):
self.response.headers['Content-Type'] = 'text/plain'
try:
while True:
q = db.GqlQuery("SELECT __key__ FROM MyModel")
assert q.count()
db.delete(q.fetch(200))
time.sleep(0.5)
except Exception, e:
self.response.out.write(repr(e)+'\n')
pass
from the terminal, I run curl -N http://...
You can now use the Datastore Admin for that: https://developers.google.com/appengine/docs/adminconsole/datastoreadmin#Deleting_Entities_in_Bulk
If I were a paranoid person, I would say Google App Engine (GAE) has not made it easy for us to remove data if we want to. I am going to skip discussion on index sizes and how they translate a 6 GB of data to 35 GB of storage (being billed for). That's another story, but they do have ways to work around that - limit number of properties to create index on (automatically generated indexes) et cetera.
The reason I decided to write this post is that I need to "nuke" all my Kinds in a sandbox. I read about it and finally came up with this code:
package com.intillium.formshnuker;
import java.io.IOException;
import java.util.ArrayList;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.google.appengine.api.datastore.Key;
import com.google.appengine.api.datastore.Query;
import com.google.appengine.api.datastore.Entity;
import com.google.appengine.api.datastore.FetchOptions;
import com.google.appengine.api.datastore.DatastoreService;
import com.google.appengine.api.datastore.DatastoreServiceFactory;
import com.google.appengine.api.labs.taskqueue.QueueFactory;
import com.google.appengine.api.labs.taskqueue.TaskOptions.Method;
import static com.google.appengine.api.labs.taskqueue.TaskOptions.Builder.url;
#SuppressWarnings("serial")
public class FormsnukerServlet extends HttpServlet {
public void doGet(final HttpServletRequest request, final HttpServletResponse response) throws IOException {
response.setContentType("text/plain");
final String kind = request.getParameter("kind");
final String passcode = request.getParameter("passcode");
if (kind == null) {
throw new NullPointerException();
}
if (passcode == null) {
throw new NullPointerException();
}
if (!passcode.equals("LONGSECRETCODE")) {
response.getWriter().println("BAD PASSCODE!");
return;
}
System.err.println("*** deleting entities form " + kind);
final long start = System.currentTimeMillis();
int deleted_count = 0;
boolean is_finished = false;
final DatastoreService dss = DatastoreServiceFactory.getDatastoreService();
while (System.currentTimeMillis() - start < 16384) {
final Query query = new Query(kind);
query.setKeysOnly();
final ArrayList<Key> keys = new ArrayList<Key>();
for (final Entity entity: dss.prepare(query).asIterable(FetchOptions.Builder.withLimit(128))) {
keys.add(entity.getKey());
}
keys.trimToSize();
if (keys.size() == 0) {
is_finished = true;
break;
}
while (System.currentTimeMillis() - start < 16384) {
try {
dss.delete(keys);
deleted_count += keys.size();
break;
} catch (Throwable ignore) {
continue;
}
}
}
System.err.println("*** deleted " + deleted_count + " entities form " + kind);
if (is_finished) {
System.err.println("*** deletion job for " + kind + " is completed.");
} else {
final int taskcount;
final String tcs = request.getParameter("taskcount");
if (tcs == null) {
taskcount = 0;
} else {
taskcount = Integer.parseInt(tcs) + 1;
}
QueueFactory.getDefaultQueue().add(
url("/formsnuker?kind=" + kind + "&passcode=LONGSECRETCODE&taskcount=" + taskcount).method(Method.GET));
System.err.println("*** deletion task # " + taskcount + " for " + kind + " is queued.");
}
response.getWriter().println("OK");
}
}
I have over 6 million records. That's a lot. I have no idea what the cost will be to delete the records (maybe more economical not to delete them). Another alternative would be to request a deletion for the entire application (sandbox). But that's not realistic in most cases.
I decided to go with smaller groups of records (in easy query). I know I could go for 500 entities, but then I started receiving very high rates of failure (re delete function).
My request from GAE team: please add a feature to delete all entities of a kind in a single transaction.
Presumably your hack was something like this:
# Deleting all messages older than "earliest_date"
q = db.GqlQuery("SELECT * FROM Message WHERE create_date < :1", earliest_date)
results = q.fetch(1000)
while results:
db.delete(results)
results = q.fetch(1000, len(results))
As you say, if there's sufficient data, you're going to hit the request timeout before it gets through all the records. You'd have to re-invoke this request multiple times from outside to ensure all the data was erased; easy enough to do, but hardly ideal.
The admin console doesn't seem to offer any help, as (from my own experience with it), it seems to only allow entities of a given type to be listed and then deleted on a page-by-page basis.
When testing, I've had to purge my database on startup to get rid of existing data.
I would infer from this that Google operates on the principle that disk is cheap, and so data is typically orphaned (indexes to redundant data replaced), rather than deleted. Given there's a fixed amount of data available to each app at the moment (0.5 GB), that's not much help for non-Google App Engine users.
Try using App Engine Console then you dont even have to deploy any special code
I've tried db.delete(results) and App Engine Console, and none of them seems to be working for me. Manually removing entries from Data Viewer (increased limit up to 200) didn't work either since I have uploaded more than 10000 entries. I ended writing this script
from google.appengine.ext import db
from google.appengine.ext import webapp
from google.appengine.ext.webapp.util import run_wsgi_app
import wsgiref.handlers
from mainPage import YourData #replace this with your data
class CleanTable(webapp.RequestHandler):
def get(self, param):
txt = self.request.get('table')
q = db.GqlQuery("SELECT * FROM "+txt)
results = q.fetch(10)
self.response.headers['Content-Type'] = 'text/plain'
#replace yourapp and YouData your app info below.
self.response.out.write("""
<html>
<meta HTTP-EQUIV="REFRESH" content="5; url=http://yourapp.appspot.com/cleanTable?table=YourData">
<body>""")
try:
for i in range(10):
db.delete(results)
results = q.fetch(10, len(results))
self.response.out.write("<p>10 removed</p>")
self.response.out.write("""
</body>
</html>""")
except Exception, ints:
self.response.out.write(str(inst))
def main():
application = webapp.WSGIApplication([
('/cleanTable(.*)', CleanTable),
])
wsgiref.handlers.CGIHandler().run(application)
The trick was to include redirect in html instead of using self.redirect. I'm ready to wait overnight to get rid of all the data in my table. Hopefully, GAE team will make it easier to drop tables in the future.
The official answer from Google is that you have to delete in chunks spread over multiple requests. You can use AJAX, meta refresh, or request your URL from a script until there are no entities left.
The fastest and efficient way to handle bulk delete on Datastore is by using the new mapper API announced on the latest Google I/O.
If your language of choice is Python, you just have to register your mapper in a mapreduce.yaml file and define a function like this:
from mapreduce import operation as op
def process(entity):
yield op.db.Delete(entity)
On Java you should have a look to this article that suggests a function like this:
#Override
public void map(Key key, Entity value, Context context) {
log.info("Adding key to deletion pool: " + key);
DatastoreMutationPool mutationPool = this.getAppEngineContext(context)
.getMutationPool();
mutationPool.delete(value.getKey());
}
One tip. I suggest you get to know the remote_api for these types of uses (bulk deleting, modifying, etc.). But, even with the remote api, batch size can be limited to a few hundred at a time.
Unfortunately, there's no way to easily do a bulk delete. Your best bet is to write a script that deletes a reasonable number of entries per invocation, and then call it repeatedly - for example, by having your delete script return a 302 redirect whenever there's more data to delete, then fetching it with "wget --max-redirect=10000" (or some other large number).
With django, setup url:
url(r'^Model/bdelete/$', v.bulk_delete_models, {'model':'ModelKind'}),
Setup view
def bulk_delete_models(request, model):
import time
limit = request.GET['limit'] or 200
start = time.clock()
set = db.GqlQuery("SELECT __key__ FROM %s" % model).fetch(int(limit))
count = len(set)
db.delete(set)
return HttpResponse("Deleted %s %s in %s" % (count,model,(time.clock() - start)))
Then run in powershell:
$client = new-object System.Net.WebClient
$client.DownloadString("http://your-app.com/Model/bdelete/?limit=400")
If you are using Java/JPA you can do something like this:
em = EntityManagerFactoryUtils.getTransactionalEntityManager(entityManagerFactory)
Query q = em.createQuery("delete from Table t");
int number = q.executeUpdate();
Java/JDO info can be found here: http://code.google.com/appengine/docs/java/datastore/queriesandindexes.html#Delete_By_Query
Yes you can:
Go to Datastore Admin, and then select the Entitiy type you want to delete and click Delete.
Mapreduce will take care of deleting!
On a dev server, one can cd to his app's directory then run it like this:
dev_appserver.py --clear_datastore=yes .
Doing so will start the app and clear the datastore. If you already have another instance running, the app won't be able to bind to the needed IP and therefore fail to start...and to clear your datastore.
You can use the task queues to delete chunks of say 100 objects.
Deleting objects in GAE shows how limited the Admin capabilities are in GAE. You have to work with batches on 1000 entities or less. You can use the bulkloader tool that works with csv's but the documentation does not cover java.
I am using GAE Java and my strategy for deletions involves having 2 servlets, one for doing the actually delete and another to load the task queues. When i want to do a delete, I run the queue loading servlet, it loads the queues and then GAE goes to work executing all the tasks in the queue.
How to do it:
Create a servlet that deletes a small number of objects.
Add the servlet to your task queues.
Go home or work on something else ;)
Check the datastore every so often ...
I have a datastore with about 5000 objects that i purge every week and it takes about 6 hours to clean out, so i run the task on Friday night.
I use the same technique to bulk load my data which happens to be about 5000 objects, with about a dozen properties.
This worked for me:
class ClearHandler(webapp.RequestHandler):
def get(self):
self.response.headers['Content-Type'] = 'text/plain'
q = db.GqlQuery("SELECT * FROM SomeModel")
self.response.out.write("deleting...")
db.delete(q)
Thank you all guys, I got what I need. :D
This may be useful if you have lots db models to delete, you can dispatch it in your terminal. And also, you can manage the delete list in DB_MODEL_LIST yourself.
Delete DB_1:
python bulkdel.py 10 DB_1
Delete All DB:
python bulkdel.py 11
Here is the bulkdel.py file:
import sys, os
URL = 'http://localhost:8080'
DB_MODEL_LIST = ['DB_1', 'DB_2', 'DB_3']
# Delete Model
if sys.argv[1] == '10' :
command = 'curl %s/clear_db?model=%s' % ( URL, sys.argv[2] )
os.system( command )
# Delete All DB Models
if sys.argv[1] == '11' :
for model in DB_MODEL_LIST :
command = 'curl %s/clear_db?model=%s' % ( URL, model )
os.system( command )
And here is the modified version of alexandre fiori's code.
from google.appengine.ext import db
class DBDelete( webapp.RequestHandler ):
def get( self ):
self.response.headers['Content-Type'] = 'text/plain'
db_model = self.request.get('model')
sql = 'SELECT __key__ FROM %s' % db_model
try:
while True:
q = db.GqlQuery( sql )
assert q.count()
db.delete( q.fetch(200) )
time.sleep(0.5)
except Exception, e:
self.response.out.write( repr(e)+'\n' )
pass
And of course, you should map the link to model in a file(like main.py in GAE), ;)
In case some guys like me need it in detail, here is part of main.py:
from google.appengine.ext import webapp
import utility # DBDelete was defined in utility.py
application = webapp.WSGIApplication([('/clear_db',utility.DBDelete ),('/',views.MainPage )],debug = True)
To delete all entities in a given kind in Google App Engine you only need to do as follows:
from google.cloud import datastore
query = datastore.Client().query(kind = <KIND>)
results = query.fetch()
for result in results:
datastore.Client().delete(result.key)
In javascript, the following will delete all the entries for on page:
document.getElementById("allkeys").checked=true;
checkAllEntities();
document.getElementById("delete_button").setAttribute("onclick","");
document.getElementById("delete_button").click();
given that you are on the admin-page (.../_ah/admin) with the entities you want to delete.