I have the following model.
class Car(models.Model):
owner = models.ForeignKey('Driver')
class Country(models.Model)
name = models.CharField(max_length=255)
class Driver(models.Model):
name = models.CharField(max_length=255)
age = models.IntegerField()
country = models.ForeignKey('Country')
I want to select the name of drivers owning a car.
Car.objects.all().values('owner__name')
Do I need to use select_related() method, to avoid a join for each object, or it is redundant because implicit with values() method?
Car.objects.all().select_related('owner').values('owner__name')
In the same way, I want, this time, the name of countries with drivers owning a car. Which one is the best?
Car.objects.all().values('owner__country__name')
Car.objects.all().select_related('owner', 'country').values('owner__country__name')
Car.objects.all().select_related('owner__country').values('owner__country__name')
First, all the occurrences of .all() in your examples can be deleted; the manager (.objects) already has almost all methods of the QuerySet, except for .delete().
.select_related is only helpful when your eventual query returns model instances; then, all the foreign key of each instance will be pre-loaded.
But if you are using .values you are getting dictionaries, and there are no foreign key attributes to pre-load. So it should not be used in that case.
When you do .values('owner__name') Django already sees that it needs to join owners and cars, no extra queries are done.
In the last one you want Countries, so use Country.objects:
Country.objects.filter(driver__car__isnull=False).values('name')
Related
i have few models namely
class Alpha(models.Model):
name = models.CharField()
class XXX(models.Model):
owner = models.ForeignKey(Alpha)
class YYY(models.Model):
name = models.OneToOneField(Alpha)
Now while doing select_related like this
test = Alpha.objects.filter(id=pk).select_related('XXX')
It gives me Invalid field name(s) given in select_related, choices are YYY
I understand that YYY is at OneToOne, so its showing up - but is there a way to fetch XXX also ? or should i use "prefetch_related". But i dont wanna use prefetch as its just making slow queries and meanwhile i have 7 models which needs to be select_related :(
You can use prefetch_related in order to achieve this. Also, add related_name in foreign key fields for referencing.
class Alpha(models.Model):
name = models.CharField()
class XXX(models.Model):
owner = models.ForeignKey(Alpha, related_name='xxx')
class YYY(models.Model):
name = models.OneToOneField(Alpha, related_name='yyy')
Alpha.objects.filter(id=pk).prefetch_related('xxx', 'yyy')
You can use select_related when going in the direction of the 1 model.
For example XXX.objects.all().select_related('alpha')
But when you go from 1 towards many you have to use prefetch_related.
so Alpha.objects.all().prefetch_related('xxx')
Check out this article
I have these 3 models:
class Person(models.Model):
name
age
class Person_Address(models.Model):
address
number
city
state
person = models.ForeignKey(Person)
class State_Schedules(models.Model):
state
hour
I also have a models.QuerySet for the State_Schedules for filtering based on current time (basically the contactables() below).
How can I get the Person objects comparing it's state in django?
I tried this but it seems cumbersome having to fetch the Persons using their Person_Address.
persons = Person_Address.objects.select_related('person').filter(
state__in=(
_.state for _ in State_Schedules.objects.contactables()
)
)
If I could access the Person_Address association as a keyword to the Person's filter, this should be pretty easy, but I haven't found how to do so.
Is there a better approach or I'd have to resort to raw sql?
There is a foreign key from Person_Address to Person, so you can filter on the reverse relationship. Use double underscore notation to filter on fields in the Person_Address model.
states = [_.state for _ in State_Schedules.objects.contactables()]
people = Person.objects.filter(person_address__state__in=states)
My data model consists of three main entities:
class User(models.Model):
...
class Source(models.Model):
user = models.ForeignKey(User, related_name='iuser')
country = models.ForeignKey(Country, on_delete=models.DO_NOTHING)
description = models.CharField(max_length=100)
class Destination(models.Model):
user = models.ForeignKey(User, related_name='wuser')
country = models.ForeignKey(Country)
I am trying to create a queryset which is join all sources with destinations by user (many to many). In such a way I would have a table with all possible source/destination combinations for every user.
In SQL I would simple JOIN the three tables and select the appropriate information from each table.
My question is how to perform the query? How to access the query data?
In django queries are done on the model object, its well documented. The queries or querysets are lazy and when they execute they generally return a list of dict, each dict in the list contains the field followed by the value eg: [{'user':'albert','country':'US and A :) ','description':'my description'},....].
All possible source,destination combinations for every user?
I think you will have to use a reverse relation ship to get this done eg:
my_joined_query = User.objects.values('user','source__country','source__description','destination__country')
notice that i'm using the smaller case name of the models Source and Destination which have ForeignKey relationship with User this will join all the three tabels go through the documentation its rich.
Edit:
To make an inner join you will have to tell the query, this can be simply achieved by using __isnull=False on the reverse model name:
my_innerjoined_query = User.objects.filter(source__isnull=False,destination__isnull=False)
This should do a inner join on all the tables.
Then you can select what you want to display by using values as earlier.
hope that helps. :)
I have a base LoggedEvent model and a number of subclass models like follows:
class LoggedEvent(models.Model):
user = models.ForeignKey(User, blank=True, null=True)
timestamp = models.DateTimeField(auto_now_add=True)
class AuthEvent(LoggedEvent):
good = models.BooleanField()
username = models.CharField(max_length=12)
class LDAPSearchEvent(LoggedEvent):
type = models.CharField(max_length=12)
query = models.CharField(max_length=24)
class PRISearchEvent(LoggedEvent):
type = models.CharField(max_length=12)
query = models.CharField(max_length=24)
Users generate these events as they do the related actions. I am attempting to generate a usage-report of how many of each event-type each user has caused in the last month. I am struggling with Django's ORM and while I am close I am running into a problem. Here is the query code:
def usage(request):
# Calculate date range
today = datetime.date.today()
month_start = datetime.date(year=today.year, month=today.month - 1, day=1)
month_end = datetime.date(year=today.year, month=today.month, day=1) - datetime.timedelta(days=1)
# Search for how many LDAP events were generated per user, last month
baseusage = User.objects.filter(loggedevent__timestamp__gte=month_start, loggedevent__timestamp__lte=month_end)
ldapusage = baseusage.exclude(loggedevent__ldapsearchevent__id__lt=1).annotate(count=Count('loggedevent__pk'))
authusage = baseusage.exclude(loggedevent__authevent__id__lt=1).annotate(count=Count('loggedevent__pk'))
return render_to_response('usage.html', {
'ldapusage' : ldapusage,
'authusage' : authusage,
}, context_instance=RequestContext(request))
Both ldapusage and authusage are both a list of users, each user annotated with a .count attribute which is supposed to represent how many particular events that user generated. However in both lists, the .count attributes are the same value. Infact the annotated 'count' is equal to how many events that user generated, regardless of type. So it would seem that my specific
authusage = baseusage.exclude(loggedevent__authevent__id__lt=1)
isn't excluding by subclass. I have tried id__lt=1, id__isnull=True, and others. Halp.
The key to Django model inheritance is remembering that with a non-abstract base class everything is really an instance of the base class which might happen to have some extra data strapped on the side from a separate table. This means that when you do searches on the base table you get back instances of the base class and there's no way to tell which subclass it is without doing repeated database queries on the subclass tables to see if they contain a record with a matching key ("I have an event. Does it have a record in AuthEvent? No. What about LDAP Event?…"). Among other things this means that you can't easily filter on them in normal queries on the base class without doing a join on every subclass table.
You have a couple of choices: one would simply be to do your queries on the subclass and tally the results (ldap_event_count = LDAPEvent.objects.filter(user=foo).count(), …), which might be sufficient for a single report. I usually recommend adding a content type field to the base class so you can efficiently tell which particular subclass an instance is without having to do another query:
content_type = models.ForeignKey("contenttypes.ContentType")
That allows two major improvements: the most common one is that you can deal with many Events generically without having to do something like hit the subclass-specific accessors (e.g. event.authevent or event.ldapevent) and handling DoesNotExist. In this case it would also make it trivial to rewrite your query since you could just do something like Event.objects.aggregate(Count("content_type")) to get the report values, which becomes particularly handy if your logic gets more complicated ("Event is Auth or LDAP and …").
this is a model of the view table.
class QryDescChar(models.Model):
iid_id = models.IntegerField()
cid_id = models.IntegerField()
cs = models.CharField(max_length=10)
cid = models.IntegerField()
charname = models.CharField(max_length=50)
class Meta:
db_table = u'qry_desc_char'
this is the SQL i use to create the table
CREATE VIEW qry_desc_char as
SELECT
tbl_desc.iid_id,
tbl_desc.cid_id,
tbl_desc.cs,
tbl_char.cid,
tbl_char.charname
FROM tbl_desC,tbl_char
WHERE tbl_desc.cid_id = tbl_char.cid;
i dont know if i need a function in models or views or both. i want to get a list of objects from that database to display it. This might be easy but im new at Django and python so i having some problems
Django 1.1 brought in a new feature that you might find useful. You should be able to do something like:
class QryDescChar(models.Model):
iid_id = models.IntegerField()
cid_id = models.IntegerField()
cs = models.CharField(max_length=10)
cid = models.IntegerField()
charname = models.CharField(max_length=50)
class Meta:
db_table = u'qry_desc_char'
managed = False
The documentation for the managed Meta class option is here. A relevant quote:
If False, no database table creation
or deletion operations will be
performed for this model. This is
useful if the model represents an
existing table or a database view that
has been created by some other means.
This is the only difference when
managed is False. All other aspects of
model handling are exactly the same as
normal.
Once that is done, you should be able to use your model normally. To get a list of objects you'd do something like:
qry_desc_char_list = QryDescChar.objects.all()
To actually get the list into your template you might want to look at generic views, specifically the object_list view.
If your RDBMS lets you create writable views and the view you create has the exact structure than the table Django would create I guess that should work directly.
(This is an old question, but is an area that still trips people up and is still highly relevant to anyone using Django with a pre-existing, normalized schema.)
In your SELECT statement you will need to add a numeric "id" because Django expects one, even on an unmanaged model. You can use the row_number() window function to accomplish this if there isn't a guaranteed unique integer value on the row somewhere (and with views this is often the case).
In this case I'm using an ORDER BY clause with the window function, but you can do anything that's valid, and while you're at it you may as well use a clause that's useful to you in some way. Just make sure you do not try to use Django ORM dot references to relations because they look for the "id" column by default, and yours are fake.
Additionally I would consider renaming my output columns to something more meaningful if you're going to use it within an object. With those changes in place the query would look more like (of course, substitute your own terms for the "AS" clauses):
CREATE VIEW qry_desc_char as
SELECT
row_number() OVER (ORDER BY tbl_char.cid) AS id,
tbl_desc.iid_id AS iid_id,
tbl_desc.cid_id AS cid_id,
tbl_desc.cs AS a_better_name,
tbl_char.cid AS something_descriptive,
tbl_char.charname AS name
FROM tbl_desc,tbl_char
WHERE tbl_desc.cid_id = tbl_char.cid;
Once that is done, in Django your model could look like this:
class QryDescChar(models.Model):
iid_id = models.ForeignKey('WhateverIidIs', related_name='+',
db_column='iid_id', on_delete=models.DO_NOTHING)
cid_id = models.ForeignKey('WhateverCidIs', related_name='+',
db_column='cid_id', on_delete=models.DO_NOTHING)
a_better_name = models.CharField(max_length=10)
something_descriptive = models.IntegerField()
name = models.CharField(max_length=50)
class Meta:
managed = False
db_table = 'qry_desc_char'
You don't need the "_id" part on the end of the id column names, because you can declare the column name on the Django model with something more descriptive using the "db_column" argument as I did above (but here I only it to prevent Django from adding another "_id" to the end of cid_id and iid_id -- which added zero semantic value to your code). Also, note the "on_delete" argument. Django does its own thing when it comes to cascading deletes, and on an interesting data model you don't want this -- and when it comes to views you'll just get an error and an aborted transaction. Prior to Django 1.5 you have to patch it to make DO_NOTHING actually mean "do nothing" -- otherwise it will still try to (needlessly) query and collect all related objects before going through its delete cycle, and the query will fail, halting the entire operation.
Incidentally, I wrote an in-depth explanation of how to do this just the other day.
You are trying to fetch records from a view. This is not correct as a view does not map to a model, a table maps to a model.
You should use Django ORM to fetch QryDescChar objects. Please note that Django ORM will fetch them directly from the table. You can consult Django docs for extra() and select_related() methods which will allow you to fetch related data (data you want to get from the other table) in different ways.