i have two tables like categories and items in models items has foreign key of categories,here we have 4 categories and 12 items where each category has 3 items how to write a query set to get query set to get items with same category
i know how to write Sql query (select * from category where category_id =1;). how to write it in d'jango query set.
You can achieve it by using Django Queries:
Item.objects.filter(category__id=1)
As mentioned in the Kamil's answer, you could use filters or if you want to use SQL query as is, you could also use raw queries. An example (taken from official docs) -
class Person(models.Model):
first_name = models.CharField(...)
last_name = models.CharField(...)
birth_date = models.DateField(...)
And querying it would be -
# querying with SQL raw query
for p in Person.objects.raw('SELECT * FROM myapp_person'):
And in your case -
# assuming your query is correct
Item.objects.raw('select * from category where myapp_category_id = 1')
Related
I have three models
class A(models.Model):
field1 = models.IntegerField()
class B(models.Model):
id_a = models.ForeignKey(A,on_delete=models.CASCADE)
field1 = models.IntegerField()
field2 = models.IntegerField()
class C(models.Model):
id_a = models.ForeignKey(A,on_delete=models.CASCADE)
field1 = models.IntegerField()
field2 = models.IntegerField()
I want to write a request that looks like this: SELECT * FROM B,C,A WHERE B.id_a=C.id_a WHERE A.id_a=2 and display all the attributes of the two tablesHere is what I tried to do:
a_id_att = 1
data = B.objects.filter(id_a=C.objects.filter(id_a=a_id_att)[0])
It does not work. How to write the join and make to display all the attributes of the tables?
The SQL statement that you wrote seems strange.
SELECT * FROM B, C, A
WHERE B.id_a = C.id_a
AND A.id_a = 2
It seems that you want a single row from A and then all related rows from B and C, which your SQL query does NOT achieve.
Did you mean something like this:
SELECT * FROM B, C, A
WHERE A.id = 2
AND B.id_a = A.id
AND C.id_a = A.id
You can achieve something like that in Django using prefetch_related(), which builds a query so that the related rows are also loaded into memory in the first query and not in subsequent queries.
# this will return a queryset with a single element, or empty
qs = A.objects.prefetch_related('b_set', 'c_set').filter(id=2)
for elem in qs: # here the single DB query is made
print(elem.field1) # A.field1
for det in elem.b_set.all():
print(det.field1) # B.field1, does NOT make another DB query
print(det.field2) # B.field2, does NOT make another DB query
for det in elem.c_set.all():
print(det.field1) # C.field1, does NOT make another DB query
print(det.field2) # C.field2, does NOT make another DB query
Note: I use b_set here because that is the default for the ForeignKey field; this changes if the field would specify a different related_name.
Does this address and solve your issue?
I need to use raw sql in my django project. I'm using count command and then I associated it with as command like "the_count" but i got an error. The error like this, the_count does not exist.
And my my code here,
# First Model
class AModel(models.Model):
user = models.ForeignKey(User, on_delete=models.CASCADE)
name = models.CharField(max_lenth=100)
# Second Model
class BModel(models.Model):
a_model = models.ForeignKey(AModel, on_delete=models.CASCADE)
user = models.ForeignKey(User, on_delete=models.CASCADE)
query = 'SELECT "app_AModel"."id",
(SELECT COUNT(*) FROM "app_BModel" INNER JOIN ON ("app_BModel"."user_id" = "app_AModel"."user_id") WHERE ("app_BModel"."a_model_id" = "app_AModel"."id")) AS "the_count"
FROM "app_AModel"
WHERE ("the_count" = 0)'
BModel.objects.raw(query)
Thank you for yours helps...
From postgreSQL documentation
An output column's name can be used to refer to the column's value in
ORDER BY and GROUP BY clauses, but not in the WHERE or HAVING clauses;
there you must write out the expression instead.
Also you probably want to use GROUP BY and HAVING on my_app table instead so something in a line of following:
SELECT a_model_id, COUNT(*) from app_BModel GROUP BY a_model_id HAVING count(*) > 0
I'm using Django, Python 3.7, and PostgreSQL 9.5. I have these models:
class Article(models.Model):
...
label = models.TextField(default='', null=True)
class Label(models.Model):
name = models.CharField(max_length=200)
I want to write a Django query that retrieves all the articles whose label contains a name from the Labels table. In PostGres, I can structure my query like so:
select a.* from myapp_article a join myapp_label l on a.label ilike '%' || l.name || '%';
but I have no idea how to pull this off in Django on account of the "on" clause and "ilike". How do I pull this off?
If you've to do a case insensitive search on Article's label for matching names, then you can use regex and pass it a flat list of all the label names like so:
Article.objects.filter(label__iregex=r'(' + '|'.join(Label.objects.all().values_list('name', flat=True)) + ')')
What the above query does is, it makes a flat list of labels:
['label1' , 'label2', 'label3']
and then the string is joined like this:
'(label1|label2|label3)'
and a similar SQL query is used:
SELECT * from FROM "app_article" WHERE "app_article"."label" ~* (label1|label2|label3)
Otherwise, for case sensitive approach, you can use this:
names_list = Label.objects.all().values_list('name', flat=True)
Article.objects.filter(label__in=names_list)
This wouldn't translate into same SQL query, but would yield the same results, using an inner query.
inner_query = Label.objects.annotate(article_label=OuterRef('label')).filter(article_label__icontains=F('name'))
articles = Article.objects.annotate(labels=Subquery(inner_query.values('name')[:1])).filter(labels__isnull=False)
This should roughly should translate to this:
select a.* from myapp_article a where exists (select l.* from myapp_label l where a.label ilike '%' || l.name || '%')
But due to a current issue in Django regarding using OuterRef's in annotations, this approach doesn't work. We need to use a workaround suggested here until the issue is fixed to make this query work, like this:
Define a custom expression first
class RawCol(Expression):
def __init__(self, model, field_name):
field = model._meta.get_field(field_name)
self.table = model._meta.db_table
self.column = field.column
super().__init__(output_field=CharField())
def as_sql(self, compiler, connection):
sql = f'"{self.table}"."{self.column}"'
return sql, []
Then build your query using this expression
articles = Article.objects.all().annotate(
labels=Subquery(
Label.objects.all().annotate(
article_label=RawCol(Article, 'label')
).filter(article_label__icontains=F('name')).values('name')[:1]
)
).filter(labels__isnull=False)
This should return instances of Article model whose label field contain a value from the name field of Label model
In your class Article you will have to declare label as foreignkey to class Label
class Article(models.Model):
...
label = models.ForeignKey(Label, default='', on_delete=models.CASCADE)
And then you can access it.
there are two tables:
class TBLUserProfile(models.Model):
userid = models.IntegerField(primary_key=True)
relmusicuid = models.IntegerField()
fansnum = models.IntegerField()
class TSinger(models.Model):
fsinger_id = models.IntegerField()
ftbl_user_profile = models.ForeignKey(TBLUserProfile, db_column='Fsinger_id')
I want to get Tsinger info and then order by TBLUserProfile.fansnum, I know how to write sql query: select * from t_singer INNER JOIN tbl_user_profile ON (tbl_user_profile.relmusicuid=t_singer.Fsinger_id) order by tbl_user_profile.fansnum, but I don't want to use model raw function. relmusicuid is not primary key otherwise I can use ForeignKey to let it work. How can I use django model to achieve this？
You can do like this :
Tsinger.objects.all().order_by('ftbl_user_profile__fansnum')
For information about Django JOIN :
https://docs.djangoproject.com/en/1.8/topics/db/examples/many_to_one/
Sorry for probably simple question but I'm a newby in Django and really confused.
I have an ugly legacy tables that I can not change.
It has 2 tables:
class Salespersons(models.Model):
id = models.IntegerField(unique=True, primary_key=True)
xsin = models.IntegerField()
name = models.CharField(max_length=200)
surname = models.CharField(max_length=200)
class Store(models.Model):
id = models.IntegerField(unique=True, primary_key=True)
xsin = models.IntegerField()
brand = models.CharField(max_length=200)
So I suppose I can not add Foreign keys in class definitions because they change the tables.
I need to execute such sql request:
SELECT * FROM Salespersons, Store INNER JOIN Store ON (Salespersons.xsin = Store.xsin);
How can I achieve it using Django ORM?
Or I'm allowed to get Salespersons and Store separately i.e.
stores = Store.objects.filter(xsin = 1000)
salespersons = Salespersons.objects.filter(xsin = 1000)
Given your example query, are your tables actually named Salespersons/Store?
Anyway, something like this should work:
results = Salespersons.objects.extra(tables=["Store"],
where=["""Salespersons.xsin = Store.xsin"""])
However, given the names of the tables/models it doesn't seem to me that an inner join would be logically correct. Unless you always have just 1 salesperson per store with same xsin.
If you can make one of the xsin fields unique, you can use a ForeignKey with to_field to generate the inner join like this:
class Salespersons(models.Model):
xsin = models.IntegerField(unique=True)
class Store(models.Model):
xsin = models.ForeignKey(Salespersons, db_column='xsin', to_field='xsin')
>>> Store.objects.selected_related('xsin')
I don't see why you can't use the models.ForeignKey fields even if the database lacks the constraints -- if you don't explicitly execute the SQL to change the database then the tables won't change. If you use a ForeignKey then you can use Salespersons.objects.select_related('xsin') to request that the related objects are fetched at the same time.