Posts.objects.order_by('published')

it is repetitive and most of the times we will want to order by a particular field.

We can specify the order in the models like this

class Post(models.Model):
    title = models.CharField(max_length=140)
    content = models.TextField()
    published = models.DateTimeField()

    def __unicode__(self):
        return self.title

    class Meta:
        ordering = ['published']

To prevent this, set the function attribute alters_data on the method:

def delete(self):
# Delete the account
delete.alters_data = True

The template system won’t execute any method marked in this way. Continuing the above example, if a template includes {{ account.delete }} and the delete() method has the alters_data=True, then the delete() method will not be executed when the template is rendered. Instead, it will fail silently.

For example consider the  following url pattern

urlpatterns = patterns('',
    url(r'^hello/$', hello),
)

Without the dollar sign at the end it will match any url starting with hello like

• hello/satish
• hello/gandham/
• hello/satish/123/pqr

Note: use url(r'^$', my_homepage_view), to match root.

I don’t have any idea about what it means at this time except that it sets the terminal to display all languages. If you can explain what actually it does, please drop a comment.

Traceback doesn’t kick when there is no error.  There may be no coding errors but there is a flaw in our application logic which we want to debug. In such cases we can trigger Django traceback by adding assert False in our code.

example:

def hello_world(response):
test_var = "Something"
test_var2 = "another var"
assert false
return HttpResponse(test_var+test_var2)

Library/Python/2.7/site-packages

• Get the number of authors with at least one post.
• Get the number of distinct tags/categories

You can use the following commands to get the answers for the above questions

db.posts.distinct('author').length
db.posts.distinct('tag').length

Reference:

• Aggregation – MongoDB
• JavaScript array object

If you follow this procedure carefully,  it will prevent errors like

Exception: java.lang.NoSuchMethodError: com.mongodb.Mongo.fsyncAndLock()

{
“error” : “IndexMissingException[[testmongo] missing]”,
“status” : 404
}

Follow these guide to install MonogDB and Elastic Search

• Install MongoDB on OSX
• Installing elastic search

After you installed them, its time to install elastic search river.

Download the snapshot from here. Extract it and copy its contents to elastic_search_root/plugins/plugins/mongodb_river

Note: The initial implementation tutorial give on the git page points to a older version of the snapshot and it doesn’t work with the latest versions of elastic search and mongodb.

Installing the mongodb river

Run the following two commands to install the mongodb river. If you are on a slow connection, the first command can take more than 15 minutes.

ES_HOME/bin/plugin -install elasticsearch/elasticsearch-mapper-attachments/1.4.0 
ES_HOME/bin/plugin -install richardwilly98/elasticsearch-river-mongodb/1.4.0 

After you install both of them, restart elasticsearch.

ES_HOME/bin/service/elasticsearch restart

Enable replica sets in mongodb by following this tutorial

Tell elastic search to index the “person” colletion in testmongo database by issuing the following command in your terminal

curl -XPUT 'http://localhost:9200/_river/mongodb/_meta' -d '{ 
    "type": "mongodb", 
    "mongodb": { 
        "db": "testmongo", 
        "collection": "person"
    }, 
    "index": {
        "name": "mongoindex", 
        "type": "person" 
    }
}'

add some data to the mongodb through mongo terminal

use testmongo
var p = {firstName: "John", lastName: "Doe"}
db.person.save(p)

Use this command to search the data

curl -XGET 'http://localhost:9200/mongoindex/_search?q=firstName:John'

File "/Library/Python/2.7/site-packages/django/contrib/auth/management/__init__.py", line 85, in get_system_username
    return getpass.getuser().decode(locale.getdefaultlocale()[1])
TypeError: decode() argument 1 must be string, not None

This problem was rectified by running this command in terminal before adding the user
export LANG="en_US.UTF-8"

I’m guessing this character encoding problem while running syncdb is the reason for the following error while accessing admin section of django1.4

DoesNotExist at /admin/
Site matching query does not exist.
Request Method: GET
Request URL: http://127.0.0.1:8000/admin/
Django Version: 1.4
Exception Type: DoesNotExist
Exception Value:
Site matching query does not exist.
Exception Location: /Library/Python/2.7/site-
packages/Django-1.4-py2.7.egg/django/db/models/query.py in get, line
366
Python Executable: /usr/bin/python
Python Version: 2.7.1
Python Path:
['/Users/mohammedelsebaey/a/mysite1',
'/Library/Python/2.7/site-packages/Django-1.4-py2.7.egg',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/
python27.zip',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/
python2.7',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/
python2.7
/plat-darwin',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/
python2.7
/plat-mac',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/
python2.7
/plat-mac/lib-scriptpackages',
'/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/
python',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/
python2.7
/lib-tk',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/
python2.7
/lib-old',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/
python2.7
/lib-dynload',
'/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/
python/PyObjC',
'/Library/Python/2.7/site-packages']

If i’m wrong you can solve the above problem by running this code i django shell

from django.contrib.sites.models import Site
Site.objects.create(pk=1)

If the above commands don’t work, try these

/etc/init.d/httpd start
/etc/init.d/httpd stop
/etc/init.d/httpd restart

You can also start the web server from the system preferences. Open system preferences, go to sharing and then enable web sharing

starting webserver in mac osx