I am working with django and having a hard time grasping how to do complex queries
Here is my model
class TankJournal(models.Model):
user = models.ForeignKey(User)
tank = models.ForeignKey(TankProfile)
ts = models.DateTimeField(auto_now=True)
title = models.CharField(max_length=50)
body = models.TextField()
class Meta:
ordering = ('-ts',)
get_latest_by = 'ts'
I need to pull the username given the tank object.
The user object is the one built into django.. thanks!
EDIT:
I have tried this
print User.objects.filter(tankjournal__tank__exact=id)
It seems to not pull out just the id.. and pull out everything in tankjournal and match it to the tank object
If you already have your tank object you should be able to do:
tank.user.username
To reduce the database queries you might want to consider the use of select_related(), e.g.
tanks = TankJournal.objects.all().select_related()
for tank in tanks:
username = tank.user.username
if you have a specific tank id then:
tank = TankJournal.objects.select_related().get(id=123456)
username = tank.user.username
I may be misunderstanding your question, but a request on User.objects.filter() will return a list of User objects, not User ids. What you've written looks technically correct.
Remember, though, that the model you have sets up a one-to-many between the TankProfile object and the TankJournal. In other words, a single TankProfile can be associated with more than one TankJournal, and therefore to more than one user. Given this, you're query is doing the right thing, returning more than one User.
Related
I'm new to django, but working on an app for a volunteer sailing organization in my local area. Not sure how to ask this question since it's fairly general but I want the following to happen based on two models;
Yacht class (boat name, skipper, color, etc.)
Race_Event class (event date, time results for each boat)
Step 1: The user will need to create a Race_Event each week. I want the boats from the Yacht model to be loaded into the Race_Event.
Step 2: The user will enter race times for each boat.
Is there a way to pre-load objects from one model into another? With a ForeignKey the user has to add the boats each time. Any direction for me to research would be helpful.
Here is the simplified code so far;
class Yacht (models.Model):
yacht_classes = [('A', 'A'),('A1', 'A1'),]
yacht_type = [('J-29','J-29'),('J-24','J-24'),]
yacht_name = models.CharField(max_length=75)
yacht_type = models.CharField(max_length=25, choices=yacht_type,
default='J-29')
yacht_class = models.CharField(max_length=25, choices=yacht_classes)
skipper = models.ForeignKey(Skipper, on_delete=models.CASCADE)
def __str__(self):
return self.yacht_name
class Event (models.Model):
race_date = models.DateTimeField(default=timezone.now)
#yachts = #how to Include Yacht.objects.all() to the field?
class Results (models.Model):
pass
Thanks
Yes, u can use signals...
after objects is saved u can call post_save and add all yachts to race
more => https://docs.djangoproject.com/en/3.1/ref/signals/#post-save
but i dont think this is good way...
(not every time all the data must be present or must be saved => this save rows in database)
i recomment you to use m2M between race and ship with throught table where time is saved in table between.
then its on you how you present this problem to end-user.
with this solution you save only data which are needed.
this can be done with
https://docs.djangoproject.com/en/3.1/topics/db/models/#extra-fields-on-many-to-many-relationships
I have the following models:
class Deal(models.Model):
date = models.DateTimeField(auto_now_add=True)
retailer = models.ForeignKey(Retailer, related_name='deals')
description = models.CharField(max_length=255)
...etc
class CustomerProfile(models.Model):
saved_deals = models.ManyToManyField(Deal, related_name='saved_by_customers', null=True, blank=True)
dismissed_deals = models.ManyToManyField(Deal, related_name='dismissed_by_customers', null=True, blank=True)
What I want to do is retrieve deals for a customer, but I don't want to include deals that they have dismissed.
I'm having trouble wrapping my head around the many-to-many relationship and am having no luck figuring out how to do this query. I'm assuming I should use an exclude on Deal.objects() but all the examples I see for exclude are excluding one item, not what amounts to multiple items.
When I naively tried just:
deals = Deal.objects.exclude(customer.saved_deals).all()
I get the error: "'ManyRelatedManager' object is not iterable"
If I say:
deals = Deal.objects.exclude(customer.saved_deals.all()).all()
I get "Too many values to unpack" (though I feel I should note there are only 5 deals and 2 customers in the database right now)
We (our client) presumes that he/she will have thousands of customers and tens of thousands of deals in the future, so I'd like to stay performance oriented as best I can. If this setup is incorrect, I'd love to know a better way.
Also, I am running django 1.5 as this is deployed on App Engine (using CloudSQL)
Where am I going wrong?
Suggest you use customer.saved_deals to get the list of deal ids to exclude (use values_list to quickly convert to a flat list).
This should save you excluding by a field in a joined table.
deals = Deals.exclude( id__in=customer.saved_deals.values_list('id', flat=True) )
You'd want to change this:
deals = Deal.objects.exclude(customer.saved_deals).all()
To something like this:
deals = Deal.objects.exclude(customer__id__in=[1,2,etc..]).all()
Basically, customer is the many-to-many foreign key, so you can't use it directly with an exclude.
Deals saved and deals dismissed are two fields describing almost same thing. There is also a risk too much columns may be used in database if these two field are allowed to store Null values. It's worth to consider remove dismissed_deals at all, and use saved_deal only with True or False statement.
Another thing to think about is move saved_deals out of CustomerProfile class to Deals class. Saved_deals are about Deals so it can prefer to live in Deals class.
class Deal(models.Model):
saved = models.BooleandField()
...
A real deal would have been made by one customer / buyer rather then few. A real customer can have milions of deals, so relating deals to customer would be good way.
class Deal(models.Model):
saved = models.BooleanField()
customer = models.ForeignKey(CustomerProfile)
....
What I want to do is retrieve deals for a customer, but I don't want to include deals that they have dismissed.
deals_for_customer = Deals.objects.all().filter(customer__name = "John")
There is double underscore between customer and name (customer__name), which let to filter model_name (customer is related to CustomerProfile which is model name) and name of field in that model (assuming CutomerProfile class has name attribute)
deals_saved = deals_for_customer.filter(saved = True)
That's it. I hope I could help. Let me know if not.
I'm trying to figure out how to design my model. I've been going over the documentation, and it ultimately seems like I should be using the "through" attribute, but I just can't figure out how to get it to work how I want.
If someone could take a look and point out what I'm missing, that would be really helpful. I have pasted my model below.
This is what I am trying to do:
1) Have a list of server types
2) Each server type will need to have different parts available to that specific server type
3) The asset has a FK to the servermodel, which has a M2M to the parts specific to that server type.
My question is, how can each "Asset" store meta data for each "Part" specific to that "Asset"? For example, each "Asset" should have it's own last_used data for the part that's assigned to it.
Thanks! :)
class Part(models.Model):
part_description = models.CharField(max_length=30,unique=1)
last_used = models.CharField(max_length=30)
def __unicode__(self):
return self.part_description
class ServerModel(models.Model):
server_model = models.CharField(max_length=30,unique=1)
parts = models.ManyToManyField(Part)
def __unicode__(self):
return self.server_model
class Asset(models.Model):
server_model = models.ForeignKey(ServerModel)
serial_number = models.CharField(max_length=10,unique=1)
def __unicode__(self):
return self.server_model.server_model
EDIT:
Thank you for the help!
I may have not explained myself clearly, though. It's probably my confusing model names.
Example:
ServerModel stores the type of server being used, say "Dell Server 2000".
The "Dell Server 2000" should be assigned specific parts:
"RAM"
"HARD DISK"
"CDROM"
Then, I should be able to create 10x Assets with a FK to the ServerModel. Now, each of these assets should be able to mark when the "RAM" part was last used for this specific asset.
I'm not sure I exactly understand what you want to do, but basically you can solve that with a "through" model, as you expected:
import datetime
class Part(models.Model):
name = models.CharField(max_length=30,unique=1)
class ServerModel(models.Model):
server_model = models.CharField(max_length=30,unique=1)
parts = models.ManyToManyField(Part,through='Asset')
class Asset(models.Model):
server_model = models.ForeignKey(ServerModel)
part = models.ForeignKey(Part)
serial_number = models.CharField(max_length=10,unique=1)
used = models.DateTimeField(default=datetime.datetime.now())
First thing to notice is the relation of the parts to the servermodel using the "through"-model: that way for each Part instance assigned to the "parts"-property of a ServerModel instance a new Asset instance is created (Phew - hope that doesn't sound too complicated). At the time of creation the "used"-property of the Asset instance is set to the current date and time (thats what default=datetime.datetime.now() does).
If you do that, you can then just query the database for the last asset containing your part. That queryset can then be sorted by the "used" property of the Asset model, which is the date when the Asset instance has been created.
ServerModel.objects.filter(parts__name='ThePartYouAreLookingFor').order_by('asset__used')
I'm not absolutely sure if the queryset is correct, so if someone finds huge nonsense in it, feel free to edit ;)
edit:
The models above do not exactly that. But you do not even need a through model for what you want:
class ServerModel(models.Model):
server_model = models.CharField(max_length=30,unique=1)
parts = models.ManyToManyField(Part)
class Asset(models.Model):
server_model = models.ForeignKey(ServerModel)
parts = models.ForeignKey(Part)
serial_number = models.CharField(max_length=10,unique=1)
used = models.DateTimeField(default=datetime.datetime.now())
Basically you can just add assets and then query all assets that have a RAM in parts.
Asset.objects.filter(parts__contains='RAM').order_by('used')
Get the date of the first (or last) result of that queryset and you have the date of the last usage of your 'RAM'-part.
Previously had a go at asking a more specific version of this question, but had trouble articulating what my question was. On reflection that made me doubt if my chosen solution was correct for the problem, so this time I will explain the problem and ask if a) I am on the right track and b) if there is a way around my current brick wall.
I am currently building a web interface to enable an existing database to be interrogated by (a small number of) users. Sticking with the analogy from the docs, I have models that look something like this:
class Musician(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
dob = models.DateField()
class Album(models.Model):
artist = models.ForeignKey(Musician)
name = models.CharField(max_length=100)
class Instrument(models.Model):
artist = models.ForeignKey(Musician)
name = models.CharField(max_length=100)
Where I have one central table (Musician) and several tables of associated data that are related by either ForeignKey or OneToOneFields. Users interact with the database by creating filtering criteria to select a subset of Musicians based on data the data on the main or related tables. Likewise, the users can then select what piece of data is used to rank results that are presented to them. The results are then viewed initially as a 2 dimensional table with a single row per Musician with selected data fields (or aggregates) in each column.
To give you some idea of scale, the database has ~5,000 Musicians with around 20 fields of related data.
Up to here is fine and I have a working implementation. However, it is important that I have the ability for a given user to upload there own annotation data sets (more than one) and then filter and order on these in the same way they can with the existing data.
The way I had tried to do this was to add the models:
class UserDataSets(models.Model):
user = models.ForeignKey(User)
name = models.CharField(max_length=100)
description = models.CharField(max_length=64)
results = models.ManyToManyField(Musician, through='UserData')
class UserData(models.Model):
artist = models.ForeignKey(Musician)
dataset = models.ForeignKey(UserDataSets)
score = models.IntegerField()
class Meta:
unique_together = (("artist", "dataset"),)
I have a simple upload mechanism enabling users to upload a data set file that consists of 1 to 1 relationship between a Musician and their "score". Within a given user dataset each artist will be unique, but different datasets are independent from each other and will often contain entries for the same musician.
This worked fine for displaying the data, starting from a given artist I can do something like this:
artist = Musician.objects.get(pk=1)
dataset = UserDataSets.objects.get(pk=5)
print artist.userdata_set.get(dataset=dataset.pk)
However, this approach fell over when I came to implement the filtering and ordering of query set of musicians based on the data contained in a single user data set. For example, I could easily order the query set based on all of the data in the UserData table like this:
artists = Musician.objects.all().order_by(userdata__score)
But that does not help me order by the results of a given single user dataset. Likewise I need to be able to filter the query set based on the "scores" from different user data sets (eg find all musicians with a score > 5 in dataset1 and < 2 in dataset2).
Is there a way of doing this, or am I going about the whole thing wrong?
edit: nevermind, it's wrong. I'll keep it so you can read, but then I'll delete afterward.
Hi,
If I understand correctly, you can try something like this:
artists = Musician.objects.select_related('UserDataSets').filter( Q(userdata__score_gt=5, userdata__id=1) | Q(userdata__sorce_lt=2, userdata__id=2 )
For more info on how to use Q, check this: Complex lookups with Q objects.
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.