Let say I have two tables
Order table
class Order(db.Model):
id = db.Column(db.Integer, primary_key=True)
deliveries = db.relationship("Delivery", back_populates="order")
Delivery table
class Delivery(db.Model):
id = db.Column(db.Integer, primary_key=True)
status= db.Column(db.String(20))
order_id = db.Column(db.Integer, db.ForeignKey('order.id'), nullable=False)
order = db.relationship("Order", back_populates="deliveries")
One order may have many deliveries. The status field of delivery table has following possible values ["Planned", "Failed", "Delivered"]
Question:
How can make sure that one order does not have two planned deliveries ?
Tips:
I know I can implemt unique constraint on multiple tables like adding below line to delivery schema
__table_args__ = (db.UniqueConstraint('order_id', 'status', name='_order_status_uc'))
But by doing so I will prevent one order from having two failed deliveries.
Any help will be greatly appreciate as I want to enforce this constraint directly in the db.
Update (given it is PostgreSQL)
Given you use PostgreSQL, the most straightforward solution is to use Partial Unique Index:
CREATE UNIQUE INDEX _order_status_uc ON delivery (order_id) WHERE status = 'Planned';
If you use SQLAlchemy to create the index, the below should be equivalent:
class Delivery(Base):
# ...
__table_args__ = (
Index(
'_order_status_uc',
'order_id',
unique=True,
postgresql_where=(status == 'Planned'),
),
)
Original answer
Assuming the RDBMS you are using supports this, one implementation idea would be to do as follows:
create a computed column on delivery table which would have value of order_id for "Planned" status and NULL otherwise
create a UNIQUE Constraint on this computed column.
This assumes the following regarding used RDBMS:
Computed columns are not only supported but also can be part of the index/constraint.
the implementation of UNIQUE index is such that multiple NULL values are allowed.
Please see PostgreSQL unique constraint null: Allowing only one Null article for more background.
You could query your delivery model with the current order id and status planned, if there is a result, raise validation.
planned_delivery_exist_boolean = db.session.query(Delivery).filter(Delivery.order_id==order_id).filter(Delivery.status=="Planned").first()
if planned_delivery_exist_boolean:
# trigger validation error
Related
I am currently trying to build a query which
give me for a one-to-many sqlalchemy query in flask both my result filters grouped and then says how many individual entries there are for it
Following is my database model to illustrate the question:
class cellphone(db.Model):
__tablename__ = 'cellphone'
id = db.Column(db.Integer, primary_key = True)
number = db.Column(db.String(30), unique=True)
sms = db.relationship('sms_accounting', backref="cellphone", lazy='dynamic')
class sms_accounting(db.Model):
__tablename__ = 'sms_accounting'
id = db.Column(db.Integer, primary_key = True)
timestamp = db.Column(db.DateTime, index=True, default=datetime.utcnow)
cellphone_id = db.Column(db.Integer, db.ForeignKey('cellphone.id'))
What I want to do now is find out how many SMS were sent within X days per number.
Filtering and grouping I managed to do, but to calculate the sum per device correctly is not possible.
def sms_count():
search_peroid='90' #time to fetch events in days
period_start = datetime.utcnow() - timedelta(hours=int(search_peroid))
phone_count = sms_accounting.query.filter(sms_accounting.timestamp.between(period_start, \
datetime.utcnow() )).group_by(sms_accounting.cellphone_id).all()
I found some examples for func.count, but unfortunately none of them works. This already starts with the usage,
AttributeError: BaseQuery object has no attribute 'func'
even though it was imported especially.
from sqlalchemy.sql.functions import func
Forgive me if I am wrong.
As an option, you could try executing an SQL Query through Flask.
db.session.execute('select number, count(sms_accounting.id) from cellphone join sms_accounting on sms_accounting.cellphone_id = cellphone.id');
You can easily add the time based filter using where.
Regarding the AttributeError, are you sure you are using the 'func' method properly? The correct usage can be found on this unrelated answer at https://stackoverflow.com/a/4086229/4854064. It might be that you accidentally called func as a method of the query object.
In my application, a 'set' can have a number of 'products' associated with it. Products listed against a set must have quantities defined. For this many-to-many relationship I have followed the SQLAlchemy documentation to use an association table with an additional column (quantity).
I am trying to create a form where the user can assign products and quantities against a given set. Both the sets and products already exist in the database. The data from the form are:
set.id
product.id
quantity
This works to create a new association (e.g. set 1 is 'linked' to product 3 with quantity=XYZ) but I get an integrity error when I try to update an existing record.
I can manually add a relationship/record (dummy data) or within the Flask view function as follows:
s = Set.query.get(2)
p = Product.query.get(3)
a = Set_Product_Association(set=s, product=p, quantity=23)
db.session.add(a)
db.session.commit()
Updating the record (different quantity) manually as follows works:
s.products[0].quantity = 43
db.session.add(s)
db.session.commit()
However when I use the code from the first block instead (with the aim to update the quantity field for a given, existing set and product ID), i.e.:
a = Set_Product_Association(set=s, product=p, quantity=43)
I get an integrity error
sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) UNIQUE constraint failed: set_product_association.set_id, set_product_association.product_id [SQL: 'INSERT INTO set_product_association (set_id, product_id, quantity) VALUES (?, ?, ?)'] [parameters: (2, 3, 43)]
I assume this is to tell me that I'm trying to append a new record rather than updating the existing one.
How should I approach this? The 'manual' method works but relies on working out the correct index in the list (i.e. for the correct product.id).
Curiously, if I use form.popluate_obj(set) in my Flask view function to process the form data as described in my question here, I can update fields but not create new 'associations'. Unfortunately, I don't know what goes on behind the scenes there....
My models are defined like so:
class Set_Product_Association(db.Model):
__tablename__ = 'set_product_association'
set_id = db.Column(db.Integer, db.ForeignKey('sets.id'), primary_key=True)
product_id = db.Column(db.Integer, db.ForeignKey('products.id'), primary_key=True)
quantity = db.Column(db.Integer)
product = db.relationship("Product", back_populates="sets")
set = db.relationship("Set", back_populates="products")
class Set(db.Model):
__tablename__ = 'sets'
id = db.Column(db.Integer, primary_key=True)
products = db.relationship("Set_Product_Association",
back_populates="set")
class Product(db.Model):
__tablename__= 'products'
id = db.Column(db.Integer, primary_key=True)
part_number = db.Column(db.String(100), unique=True, nullable=False)
sets = db.relationship("Set_Product_Association",
back_populates="product")
Edit:
I've also tried reversing the operation as suggested here:
s = Set.query.get(2)
a = Set_Product_Association()
a.quantity = 43
a.product = Product.query.get(3)
a.set = s
db.session.commit()
But I still get an error:
sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) UNIQUE constraint failed: set_product_association.set_id, set_product_association.product_id [SQL: 'INSERT INTO set_product_association (set_id, product_id, quantity) VALUES (?, ?, ?)'] [parameters: (2, 3, 43)]
You get an integrity error because you are trying to create a new object with the same primary keys.
This:
a = Set_Product_Association(set=s, product=p, quantity=43)
Does not update, but create.
If you want to update the actual row in the table, you need to update the existing one:
assoc = Set_Product_Association.query.filter_by(set=s, product=p).first()
assoc.quantity = 43
db.session.commit()
Also, from the documentation it is advised to not use a model but an actual table.
I've read the documentation and looked at other questions posted here, but I can't find or figure out whether this is possible in Django.
I have a model relating actors and movies:
class Role(models.Model):
title_id = models.CharField('Title ID', max_length=20, db_index=True)
name_id = models.CharField('Name ID', max_length=20, db_index=True)
role = models.CharField('Role', max_length=300, default='?')
This is a single table that has pairs of actors and movies, so given a movie (title_id), there's a row for each actor in that movie. Similarly, given an actor (name_id), there's a row for every movie that actor was in.
I need to execute a query to return the list of all title_id's that are related to a given title_id by a common actor. The SQL for this query looks like this:
SELECT DISTINCT r2.title_id
FROM role as r1, role as r2
WHERE r1.name_id = r2.name_id
AND r1.title_id != r2.title_id
AND r1.title_id = <given title_id>
Can something like this be expressed in a single Django ORM query, or am I forced to use two queries with some intervening code? (Or raw SQL?)
Normally I would break this into Actor and Movie table to make it easier to query, but your requirement is there so I will give it a go
def get_related_titles(title_id)
all_actors = Role.objects.filter(title_id=title_id).values_list('pk', flat=True)
return Role.objects.filter(pk__in=all_actors).exclude(title_id=title_id) # maybe u need .distinct() here
this should give you one query, validate it this way:
print(get_related_titles(some_title_id).query)
Is there a way in Django to write a query using the ORM, not raw SQL that allows you to JOIN on another table without there being a foreign key? Looking through the documentation it appears in order for the One to One relationship to work there must be a foreign key present?
In the models below I want to run a query with a JOIN on UserActivity.request_url to UserActivityLink.url.
class UserActivity(models.Model):
id = models.IntegerField(primary_key=True)
last_activity_ip = models.CharField(max_length=45L, blank=True)
last_activity_browser = models.CharField(max_length=255L, blank=True)
last_activity_date = models.DateTimeField(auto_now_add=True)
request_url = models.CharField(max_length=255L, blank=True)
session_id = models.CharField(max_length=255L)
users_id = models.IntegerField()
class Meta:
db_table = 'user_activity'
class UserActivityLink(models.Model):
id = models.IntegerField(primary_key=True)
url = models.CharField(max_length=255L, blank=True)
url_description = models.CharField(max_length=255L, blank=True)
type = models.CharField(max_length=45L, blank=True)
class Meta:
db_table = 'user_activity_link'
The link table has a more descriptive translation of given URLs in the system, this is needed for some reporting the system will generate.
I've tried creating the foreign key from UserActivity.request_url to UserActivityLink.url but it fails with the following error: ERROR 1452: Cannot add or update a child row: a foreign key constraint fails
No, there isn't an effective way unfortunately.
The .raw() is there for this exact thing. Even if it could it probably would be a lot slower than raw SQL.
There is a blogpost here detailing how to do it with query.join() but as they themselves point out. It's not best practice.
Just reposting some related answer, so everyone could see it.
Taken from here: Most efficient way to use the django ORM when comparing elements from two lists
First problem: joining unrelated models
I'm assuming that your Model1 and Model2 are not related,
otherwise you'd be able to use Django's related objects
interface. Here are two approaches you could take:
Use extra and a SQL subquery:
Model1.objects.extra(where = ['field in (SELECT field from myapp_model2 WHERE ...)'])
Subqueries are not handled very efficiently in some databases
(notably MySQL) so this is probably not as good as #2 below.
Use a raw SQL query:
Model1.objects.raw('''SELECT * from myapp_model1
INNER JOIN myapp_model2
ON myapp_model1.field = myapp_model2.field
AND ...''')
Second problem: enumerating the result
Two approaches:
You can enumerate a query set in Python using the built-in enumerate function:
enumerate(Model1.objects.all())
You can use the technique described in this answer to do the enumeration in MySQL. Something like this:
Model1.objects.raw('''SELECT *, #row := #row + 1 AS row
FROM myapp_model1
JOIN (SELECT #row := 0) rowtable
INNER JOIN myapp_model2
ON myapp_model1.field = myapp_model2.field
AND ...''')
The Django ForeignKey is different from SQL ForeignKey. Django ForeignKey just represent a relation, it can specify whether to use database constraints.
Try this:
request_url = models.ForeignKey(UserActivityLink, to_field='url_description', null=True, on_delete=models.SET_NULL, db_constraint=False)
Note that the db_constraint=False is required, without it Django will build a SQL like:
ALTER TABLE `user_activity` ADD CONSTRAINT `xxx` FOREIGN KEY (`request_url`) REFERENCES `user_activity_link` (`url_description`);"
I met the same problem, after a lot of research, I found the above method.
Hope it helps.
We are making a synchronization between one master database and many slave databases over tight link (7kb).
To minimize amount of data send we tag each record that was sent successfully.
To do so we created following models:
class SynchronizationTag(models.Model):
STATUSES = ((0, "Invalid"),
(1, "Pending"),
(2, "Synchronized"),
)
status = models.IntegerField(choices=STATUSES, default = 1)
storage = models.ForeignKey("Storage")
_content_type = models.ForeignKey(ContentType)
_object_id = models.PositiveIntegerField()
content_object = generic.GenericForeignKey('_content_type', '_object_id')
class Storage(models.Model):
id = models.AutoField(primary_key=True)
To select records that need synchronization we wrote a general query that would filter out records that were tagged as synchronized from a query set.
We came up with following suboptimal solution:
delta = queryset.extra(
select={
"status" : ("SELECT status FROM rv3adapter_synchronizationtag "
"WHERE `_content_type_id` = %d and `_object_id`= %s.id and `storage_id` = %d"
% (content_type.pk, table, storage.pk))
},
where=["`status` <> 2 or `status` is NULL")
The query above is now broken as we have few models that don't have primary keys named id.
Do you know how to improve/correct above query?
Note: The query need to return django objects.
Get the name of the primary key field with queryset.model._meta.pk.name.
You might also find ._meta.db_table useful.
Granted, this is using an unofficial API that isn't guaranteed to be compatible in future versions of django, but if you have a few unit tests you should be fine.