I come from an PHP/SQL background, so the whole data access technique in django is a challenge to get my head around. So far in the djangobook, it has taught me to use a unicode function in models.py if i want to get a value from a database.
def __unicode__(self):
return self.name
Surely this is not how its meant to be done? Firstly, it only returns one column, and also if i wanted to access the table from some other function for another purpose, is the "name" column the only thing i can ever return? Its as if i can only ever return one column from the table - in this case i have chosen "name".
currently i am doing:
Publisher.objects.filter(name='test')
but here there is no way of altering what i want to be returned, its always down to the static unicode function back in models.py that has to be manually altered?
Surely there is a way of just running a python version query looking like
"SELECT name, firstname FROM users WHERE user_id = 1"
so basically, how would i write this query in python without having to use the stupid, limited unicode function? How would a production web app do it? Am i completely missing the concept here?
Thanks, and sorry for the badly explained question.
P.S would it be better to just use SQL in django? or is this bad practice?
Fields can simply be accessed using attribute access:
publisher = Publisher.objects.get(name='test')
print publisher.pk
print publisher.name
print publisher.some_other_field
Or, regarding your example
user = User.objects.get(pk=1)
print user.first_name
print user.last_name
And, in a template:
{{ user.first_name }}
The __unicode__ method is used to display a representation of an object. It is not used for generic field access.
Regarding your question, no, you shouldn't be using SQL here.
Related
I think I have a pretty common use case and am surprised at how much trouble it's giving me.
I want to use a key-value pair for a ReferenceField in the Flask-Admin edit form generated by the following two classes:
class Communique(db.Document):
users = db.ListField(db.ReferenceField(User), default=[])
class User(db.Document):
email = db.StringField(max_length=255, required=True)
def __unicode__(self):
return '%s' % self.id
I want the select to be constructed out of the ObjectId and the an email field in my model.
By mapping the __unicode__
attribute to the id field I get nice things on the mongoengine side like using the entire object in queries:
UserInformation.objects(user=current_user)
This has the unfortunate effect of causing the Flask-Admin form to display the mongo ObjectId in the edit form like so:
The docs say I have to provide the label_attr to the ModelSelectMultipleField created by Flask-Admin. I've done so by overriding the get_form method on my ModelView:
def get_form(self):
form = super(ModelView, self).get_form()
form.users = ModelSelectMultipleField(model=User,
label_attr='email',
widget=form.users.__dict__['kwargs']['widget'])
return form
I'm reusing the the widget used by the original form.users (which may be wrong). It works fine when editing an existing item, BUT throws an exception when creating a new one (perhaps because I'm reusing the widget).
All of this seems like way more work than should be needed to simply provide a label_attr to my SelectField. Fixing up the listing view was a simple matter of adding an entry to the column_formatters dictionary. Is there no simple way to specify the label_attr when creating my ModelView class?
I know I could make this problem go away by returning the email property in the __unicode__ attribute, but I feel like I shouldn't have to do that! Am I missing something?
Oy, now I see how to do it, though it's not that obvious from the docs. form_args is a dictionary with items keyed to the form models. All I needed to do was...
form_args = dict(users=dict(label_attr='email'))
Which does seem about the right amount of effort (considering Flask-Admin isn't some sort of java framework).
Im working on a django application and I can't seem to get this one piece of code working. I know of some hacky ways to accomplish my task, but it seems like there should be a better way. I have a model class with one field in it:
class MobileUser(models.Model):
phone_id = models.AutoField(primary_key=True)
and everything is groovy. However, when I want to create an instance of the class and save it to the database like so:
mobile = MobileUser()
mobile.save()
I get a warning saying that MobileUser has no default value arguments, which makes sense. I know I can do things like get the last inserted row/the max id + 1 and pass that in as an argument but I'd prefer not to do that. Any ideas? Oh, and side note, the operation works it just throws a warning.
Why not let django handle it then, just do this:
class MobileUser(models.Model):
pass
Whenever you create an entry it would auto increment the id field.
I have the following in my model:
class info(models.Model):
add = models.CharField(max_length=255)
name = models.CharField(max_length=255)
An in the views when i say
info_l = info.objects.filter(id=1)
logging.debug(info_l.name)
i get an error saying name doesnt exist at debug statement.
'QuerySet' object has no attribute 'name'
1.How can this be resolved.
2.Also how to query for only one field instead of selecting all like select name from info.
1. Selecting Single Items
It looks like you're trying to get a single object. Using filter will return a QuerySet object (as is happening in your code), which behaves more like a list (and, as you've noticed, lacks the name attribute).
You have two options here. First, you can just grab the first element:
info_l = info.objects.filter(id=1)[0]
You could also use the objects.get method instead, which will return a single object (and raise an exception if it doesn't exist):
info_l = info.objects.get(id=1)
Django has some pretty good documentation on QuerySets, and it may be worth taking a look at it:
Docs on using filters
QuerySet reference
2. Retrieving Specific Fields
Django provides the defer and only methods, which will let you choose specific fields from the database, rather than fetching everything at once. These don't actually prevent the fields from being read; rather, it loads them lazily. defer is an "opt-in" mode, which lets you specify what fields should be lazily loaded. only is "out-out" -- you call it, and only the fields you pass will by eagerly loaded.
So in your example, you'd want to do something like this:
info_l = info.objects.filter(id=1).only('name')[0]
Though with a model as simple as the example you give, I wouldn't worry much at all about limiting fields.
In django, I'm trying to do something like this:
# if form is valid ...
article = form.save(commit=False)
article.author = req.user
product_name = form.cleaned_data['product_name']
try:
article.product = Component.objects.get(name=product_name)
except:
article.product = Component(name=product_name)
article.save()
# do some more form processing ...
But then it tells me:
null value in column "product_id" violates not-null constraint
But I don't understand why this is a problem. When article.save() is called, it should be able the create the product then (and generate an id).
I can get around this problem by using this code in the except block:
product = Component(name=product_name)
product.save()
article.product = product
But the reason this concerns me is because if article.save() fails, it will already have created a new component/product. I want them to succeed or fail together.
Is there a nice way to get around this?
The way the Django ManyToManyField works is that it creates an extra table. So say you have two models, ModelA and ModelB. If you did...
ModelA.model_b = models.ManyToManyField(ModelB)
What Django actually does behind the scenes is it creates a table... app_modela_modelb with three columns: id, model_a_id, model_b_id.
Hold that thought in your mind. Regarding the saving of ModelB, Django does not assign it an ID until it's saved. You could technically manually assign it an ID and avoid this problem. It seems you're letting django handle that which is perfectly acceptable.
Django has a problem then doing the M2M. Why? If ModelB doesn't have an id yet, what goes in the model_b_id column on the M2M table? The error for null product_id is more than likely a null constraint error on the M2M field, not the ModelB record id.
If you would like them to "succeed together" or "fail together" perhaps it's time to look into transactions. You, for example, wrap the whole thing in a transaction, and do a rollback in the case of a partial failure. I haven't done a whole lot of work personally in this area so hopefully someone else will be of assistance on that topic.
You could get around this by using :
target_product, created_flag = Component.objects.get_or_create(name=product_name)
article.product = target_product
as I'm pretty sure get_or_create() will set the id of an object, if it has to create one.
Alternatively, if you don't mind empty FK relations on the Article table, you could add null=True to the definition.
There's little value in including a code snippet on transactions, as you should read the Django documentation to gain a good understanding.
Say I have 2 models:
class Poll(models.Model):
category = models.CharField(u"Category", max_length = 64)
[...]
class Choice(models.Model):
poll = models.ForeignKey(Poll)
[...]
Given a Poll object, I can query its choices with:
poll.choice_set.all()
But, is there a utility function to query all choices from a set of Poll?
Actually, I'm looking for something like the following (which is not supported, and I don't seek how it could be):
polls = Poll.objects.filter(category = 'foo').select_related('choice_set')
for poll in polls:
print poll.choice_set.all() # this shouldn't perform a SQL query at each iteration
I made an (ugly) function to help me achieve that:
def qbind(objects, target_name, model, field_name):
objects = list(objects)
objects_dict = dict([(object.id, object) for object in objects])
for foreign in model.objects.filter(**{field_name + '__in': objects_dict.keys()}):
id = getattr(foreign, field_name + '_id')
if id in objects_dict:
object = objects_dict[id]
if hasattr(object, target_name):
getattr(object, target_name).append(foreign)
else:
setattr(object, target_name, [foreign])
return objects
which is used as follow:
polls = Poll.objects.filter(category = 'foo')
polls = qbind(polls, 'choices', Choice, 'poll')
# Now, each object in polls have a 'choices' member with the list of choices.
# This was achieved with 2 SQL queries only.
Is there something easier already provided by Django? Or at least, a snippet doing the same thing in a better way.
How do you handle this problem usually?
Time has passed and this functionality is now available in Django 1.4 with the introduction of the prefetch_related() QuerySet function. This function effectively does what is performed by the suggested qbind function. ie. Two queries are performed and the join occurs in Python land, but now this is handled by the ORM.
The original query request would now become:
polls = Poll.objects.filter(category = 'foo').prefetch_related('choice_set')
As is shown in the following code sample, the polls QuerySet can be used to obtain all Choice objects per Poll without requiring any further database hits:
for poll in polls:
for choice in poll.choice_set:
print choice
Update: Since Django 1.4, this feature is built in: see prefetch_related.
First answer: don't waste time writing something like qbind until you've already written a working application, profiled it, and demonstrated that N queries is actually a performance problem for your database and load scenarios.
But maybe you've done that. So second answer: qbind() does what you'll need to do, but it would be more idiomatic if packaged in a custom QuerySet subclass, with an accompanying Manager subclass that returns instances of the custom QuerySet. Ideally you could even make them generic and reusable for any reverse relation. Then you could do something like:
Poll.objects.filter(category='foo').fetch_reverse_relations('choices_set')
For an example of the Manager/QuerySet technique, see this snippet, which solves a similar problem but for the case of Generic Foreign Keys, not reverse relations. It wouldn't be too hard to combine the guts of your qbind() function with the structure shown there to make a really nice solution to your problem.
I think what you're saying is, "I want all Choices for a set of Polls." If so, try this:
polls = Poll.objects.filter(category='foo')
choices = Choice.objects.filter(poll__in=polls)
I think what you are trying to do is the term "eager loading" of child data - meaning you are loading the child list (choice_set) for each Poll, but all in the first query to the DB, so that you don't have to make a bunch of queries later on.
If this is correct, then what you are looking for is 'select_related' - see https://docs.djangoproject.com/en/dev/ref/models/querysets/#select-related
I noticed you tried 'select_related' but it didn't work. Can you try doing the 'select_related' and then the filter. That might fix it.
UPDATE: This doesn't work, see comments below.