I need to build an MySQL query and I want to try with django ORM first and then use raw as last resort.
I found documentation on single JOIN or JOINs between two tables but there is no examples or at least a simple (beginner wise) explanation of JOINs between three tables
Content of models.py is
from django.db import models
# Create your models here.
class Threads(models.Model):
name = models.CharField(max_length=100)
author = models.CharField(max_length=100)
date = models.DateTimeField("date published")
slug = models.SlugField()
def __unicode__(self):
return self.name
class Posts(models.Model):
name = models.CharField(max_length=100)
text = models.TextField()
author = models.CharField(max_length=100)
date = models.DateTimeField("date published")
slug = models.SlugField()
def __unicode__(self):
return self.name
class Relations(models.Model):
thread = models.ForeignKey(Threads, related_name = "%(app_label)s_%(class)s_related")
post = models.ForeignKey(Posts, related_name = "%(app_label)s_%(class)s_related")
and this is SQL query in raw that I am trying to build
SELECT forum_threads.id AS t_id, forum_threads.name AS t_name, forum_threads.slug AS t_slug, forum_posts.*
FROM forum_threads
LEFT JOIN forum_relations ON forum_threads.id=forum_relations.thread_id
LEFT JOIN forum_posts ON forum_relations.post_id=forum_posts.id
WHERE forum_threads.slug="<slug_name>"
GROUP BY forum_threads.id
"forum" is my app name
Now I don't know if I need to tweak/change my Models and if, how. Note that I can change my models no important data whatsoever.
EDIT
Thank you for all your answers!
Ok I played a bit with various examples until i managed to produce someting. I got it like this:
thread = Threads.objects.filter(slug = slug)
posts = Posts.objects.filter(forum_relations_related__thread = thread[0].id)
first query is to retrieve id of thread from slug and second one returns all post related to thread on that thread id.
I'll try and play around with a M2M part since I have at least one working example.
Why not just use a M2M relation, you can use through if need be.
You could then get a thread by slug
thread = Threads.objects.get(slug=slug_name)
then you can access the posts related to a thread via
thread.posts_set.all()
Related
I really don't understand all the ways to build the right query.
I have the following models in the code i'm working on. I can't change models.
models/FollowUp:
class FollowUp(BaseModel):
name = models.CharField(max_length=256)
questions = models.ManyToManyField(Question, blank=True, )
models/Survey:
class Survey(BaseModel):
name = models.CharField(max_length=256)
followup = models.ManyToManyField(
FollowUp, blank=True, help_text='questionnaires')
user = models.ManyToManyField(User, blank=True, through='SurveyStatus')
models/SurveyStatus:
class SurveyStatus(models.Model):
user = models.ForeignKey(User, on_delete=models.CASCADE)
survey = models.ForeignKey(Survey, on_delete=models.CASCADE)
survey_status = models.CharField(max_length=10,
blank=True,
null=True,
choices=STATUS_SURVEY_CHOICES,
)
models/UserSurvey:
class UserSurvey(BaseModel):
user = models.ForeignKey(User, null=True, blank=True,
on_delete=models.DO_NOTHING)
followups = models.ManyToManyField(FollowUp, blank=True)
surveys = models.ManyToManyField(Survey, blank=True)
questions = models.ManyToManyField(Question, blank=True)
#classmethod
def create(cls, user_id):
user = User.objects.filter(pk=user_id).first()
cu_quest = cls(user=user)
cu_quest.save()
cu_quest._get_all_active_surveys
cu_quest._get_all_followups()
cu_quest._get_all_questions()
return cu_quest
def _get_all_questions(self):
[[self.questions.add(ques) for ques in qstnr.questions.all()]
for qstnr in self.followups.all()]
return
def _get_all_followups(self):
queryset = FollowUp.objects.filter(survey__user=self.user).filter(survey__user__surveystatus_survey_status='active')
# queryset = self._get_all_active_surveys()
[self.followups.add(quest) for quest in queryset]
return
#property
def _get_all_active_surveys(self):
queryset = Survey.objects.filter(user=self.user,
surveystatus__survey_status='active')
[self.surveys.add(quest) for quest in queryset]
return
Now my questions:
my view sends to the create of the UserSurvey model in order to create a questionary.
I need to get all the questions of the followup of the surveys with a survey_status = 'active' for the user (the one who clicks on a button)...
I tried several things:
I wrote the _get_all_active_surveys() function and there I get all the surveys that are with a survey_status = 'active' and then the _get_all_followups() function needs to call it to use the result to build its own one. I have an issue telling me that
a list is not a callable object.
I tried to write directly the right query in _get_all_followups() with
queryset = FollowUp.objects.filter(survey__user=self.user).filter(survey__user__surveystatus_survey_status='active')
but I don't succeed to manage all the M2M relationships. I wrote the query above but issue also
Related Field got invalid lookup: surveystatus_survey_status
i read that a related_name can help to build reverse query but i don't understand why?
it's the first time i see return empty and what it needs to return above. Why this notation?
If you have clear explanations (more than the doc) I will very appreciate.
thanks
Quite a few things to answer here, I've put them into a list:
Your _get_all_active_surveys has the #property decorator but neither of the other two methods do? It isn't actually a property so I would remove it.
You are using a list comprehension to add your queryset objects to the m2m field, this is unnecessary as you don't actually want a list object and can be rewritten as e.g. self.surveys.add(*queryset)
You can comma-separate filter expressions as .filter(expression1, expression2) rather than .filter(expression1).filter(expression2).
You are missing an underscore in surveystatus_survey_status it should be surveystatus__survey_status.
Related name is just another way of reverse-accessing relationships, it doesn't actually change how the relationship exists - by default Django will do something like ModelA.modelb_set.all() - you can do reverse_name="my_model_bs" and then ModelA.my_model_bs.all()
For the below sample schema
# schema sameple
class A(models.Model):
n = models.ForeignKey(N, on_delete=models.CASCADE)
d = models.ForeignKey(D, on_delete=models.PROTECT)
class N(models.Model):
id = models.AutoField(primary_key=True, editable=False)
d = models.ForeignKey(D, on_delete=models.PROTECT)
class D(models.Model):
dsid = models.CharField(max_length=255, primary_key=True)
class P(models.Model):
id = models.AutoField(primary_key=True, editable=False)
name = models.CharField(max_length=255)
n = models.ForeignKey(N, on_delete=models.CASCADE)
# raw query for the result I want
# SELECT P.name
# FROM P, N, A
# WHERE (P.n_id = N.id
# AND A.n_id = N.id
# AND A.d_id = \'MY_DSID\'
# AND P.name = \'MY_NAME\')
What am I trying to achieve?
Well, I’m trying to find a way somehow be able to write a single queryset which does the same as what the above raw query does. So far I was able to do it by writing two queryset, and use the result from one queryset and then using that queryset I wrote the second one, to get the final DB records. However that’s 2 hits to the DB, and I want to optimize it by just doing everything in one DB hit.
What will be the queryset for this kinda raw query ? or is there a better way to do it ?
Above code is here https://dpaste.org/DZg2
You can archive it using related_name attribute and functions like select_related and prefetch_related.
Assuming the related name for each model will be the model's name and _items, but it is better to have proper model names and then provided meaningful related names. Related name is how you access the model in backward.
This way, you can use this query to get all models in a single DB hit:
A.objects.all().select_related("n", "d", "n__d").prefetch_related("n__p_items")
I edited the code in the pasted site, however, it will expire soon.
I'm trying to sort model Game by each title and most recent update(post) without returning duplicates.
views.py
'recent_games': Game.objects.all().order_by('title', '-update__date_published').distinct('title')[:5],
The distinct method on the query works perfectly however the update__date_published doesn't seem to be working.
models.py
Model - Game
class Game(models.Model):
title = models.CharField(max_length=100)
slug = models.SlugField(unique=True)
description = models.TextField()
date_published = models.DateTimeField(default=timezone.now)
cover = models.ImageField(upload_to='game_covers')
cover_display = models.ImageField(default='default.png', upload_to='game_displays')
developer = models.CharField(max_length=100)
twitter = models.CharField(max_length=50, default='')
reddit = models.CharField(max_length=50, default='')
platform = models.ManyToManyField(Platform)
def __str__(self):
return self.title
Model - Update
class Update(models.Model):
author = models.ForeignKey(User, models.SET_NULL, blank=True, null=True,) # If user is deleted keep all updates by said user
article_title = models.CharField(max_length=100, help_text="Use format: Release Notes for MM/DD/YYYY")
content = models.TextField(help_text="Try to stick with a central theme for your game. Bullet points is the preferred method of posting updates.")
date_published = models.DateTimeField(db_index=True, default=timezone.now, help_text="Use date of update not current time")
game = models.ForeignKey(Game, on_delete=models.CASCADE)
article_image = models.ImageField(default='/media/default.png', upload_to='article_pics', help_text="")
platform = ChainedManyToManyField(
Platform,
horizontal=True,
chained_field="game",
chained_model_field="game",
help_text="You must select a game first to autopopulate this field. You can select multiple platforms using Ctrl & Select (PC) or ⌘ & Select (Mac).")
See this for distinct reference Examples (those after the first will only work on PostgreSQL)
See this one for Reverse Query - See this one for - update__date_published
Example -
Entry.objects.order_by('blog__name', 'mod_date').distinct('blog__name', 'mod_date')
Your Query-
Game.objects.order_by('title', '-update__date_published').distinct('title')[:5]
You said:
The -update__date_published does not seem to be working as the Games are only returning in alphabetical order.
The reason is that the first order_by field is title; the secondary order field -update__date_published would only kick in if you had several identical titles, which you don't because of distinct().
If you want the Game objects to be ordered by latest update rather their title, omitting title from the ordering seems the obvious solution until you get a ProgrammingError that DISTINCT ON field requires field at the start of the ORDER BY clause.
The real solution to sorting games by latest update is:
games = (Game.objects
.annotate(max_date=Max('update__date_published'))
.order_by('-update__date_published'))[:5]
The most probable misunderstanding here is the join in your orm query. They ussually lazy-loading, so the date_published field is not yet available, yet you are trying to sort against it. You need the select_related method to load the fk relation as a join.
'recent_games': Game.objects.select_related('update').all().order_by('title', '-update__date_published').distinct('title')[:5]
What is the best way to create a checkout system with Django that keeps track of the checkout / checkin history?
My models for inventory/models.py
from django.db import models
class Groups(models.Model):
name = models.CharField(max_length=200)
def __unicode__(self):
return self.name
class Inventory(models.Model):
name = models.CharField(max_length=200)
serial = models.CharField(max_length=200)
barcode = models.CharField(max_length=200)
active = models.BooleanField(verbose_name="Active (can be checked out if not out for repair)",blank=True,default=True)
repair = models.BooleanField(verbose_name="Out for repair?",blank=True)
group = models.ForeignKey(Groups)
def __unicode__(self):
return self.name
I am thinking I will need another model that will store the checkout / in information? I am guessing I will need to only obtain the last one so that I know if it is checked in or out? I want to keep a history of the items so that I can create a report with it.
How would I go about making it so I have a history of the items and if the items can be checked in or out?
Yes, it isn't totally clear from your question what a checkout/checkin is, but my guess is you want something like
class Checkout(models.Model)
item = models.ForeignKey(Inventory)
user = models.ForeignKey(User)
checked_out = models.DateTimeField()
checked_in = models.DateTimeField(null=True)
...
You would then create one of these objects each time an item was checked out, and then update it to set the checkin date when it was checked back in.
To find the current checkout (or determine if something is not checked out) you could do a query like:
try:
checkout = Checkout.objects.get(item=my_item, checked_in=None)
except Checkout.DoesNotExist:
#item isn't checked out
checkout = None
I'm learning Django and trying to get the hang of querying foreign keys across a bridging table. Apologies if this is a duplicate, I haven't been able to find the answer by searching. I've got models defined as follows
class Place(models.Model):
id = models.IntegerField(primary_key=True)
name = models.CharField(max_length=100)
class PlaceRef(models.Model):
place = models.ForeignKey(Place) # many-to-one field
entry = models.ForeignKey(Entry) # many-to-one field
class Entry(models.Model):
id = models.IntegerField(primary_key=True)
name = models.CharField(max_length=10)
If I want to retrieve all the Entries associated with a particular Place, how do I do it?
place = get_object_or_404(Place, id=id)
placerefs = PlaceRef.objects.filter(place=place)
entries = Entry.objects.filter(id.....)
Also, if there is a more sensible way for me to define (or get rid of) PlaceRefs in Django, please feel free to suggest alternatives.
Thanks for helping out a beginner!
First, I'd suggest rewriting the models to:
class Place(models.Model):
id = models.IntegerField(primary_key=True)
name = models.CharField(max_length=100)
class Entry(models.Model):
id = models.IntegerField(primary_key=True)
name = models.CharField(max_length=10)
places = models.ManyToManyField(Place, related_name='places')
So you can query:
Entry.objects.filter(places__id=id)
In your current model:
Entry.objects.filter(placeref_set__place__id=id)
Note that the double underscore __ is used to jump from one model to the next. Also, django creates some fields on the model that help you navigate to related objects. In this example: Entry.placeref_set. You can read more about it here:
http://docs.djangoproject.com/en/dev/topics/db/queries/#following-relationships-backward