I have two tables, Products and Orders, inside my Flask-SqlAlchemy setup, and they are linked so an order can have several products:
class Products(db.Model):
id = db.Column(db.Integer, primary_key=True)
....
class Orders(db.Model):
guid = db.Column(db.String(36), default=generate_uuid, primary_key=True)
products = db.relationship(
"Products", secondary=order_products_table, backref="orders")
....
linked via:
order_products_table = db.Table("order_products_table",
db.Column('orders_guid', db.String(36), db.ForeignKey('orders.guid')),
db.Column('products_id', db.Integer, db.ForeignKey('products.id'))
# db.Column('license', dbString(36))
)
For my purposes, each product in an order will receive a unique license string, which logically should be added to the order_products_table rows of each product in an order.
How do I declare this third license column on the join table order_products_table so it gets populated it as I insert an Order?
I've since found the documentation for the Association Object from the SQLAlchemy docs, which allows for exactly this expansion to the join table.
Updated setup:
# Instead of a table, provide a model for the JOIN table with additional fields
# and explicit keys and back_populates:
class OrderProducts(db.Model):
__tablename__ = 'order_products_table'
orders_guid = db.Column(db.String(36), db.ForeignKey(
'orders.guid'), primary_key=True)
products_id = db.Column(db.Integer, db.ForeignKey(
'products.id'), primary_key=True)
order = db.relationship("Orders", back_populates="products")
products = db.relationship("Products", back_populates="order")
licenses = db.Column(db.String(36), nullable=False)
class Products(db.Model):
id = db.Column(db.Integer, primary_key=True)
order = db.relationship(OrderProducts, back_populates="order")
....
class Orders(db.Model):
guid = db.Column(db.String(36), default=generate_uuid, primary_key=True)
products = db.relationship(OrderProducts, back_populates="products")
....
What is really tricky (but also shown on the documentation page), is how you insert the data. In my case it goes something like this:
o = Orders(...) # insert other data
for id in products:
# Create OrderProducts join rows with the extra data, e.g. licenses
join = OrderProducts(licenses="Foo")
# To the JOIN add the products
join.products = Products.query.get(id)
# Add the populated JOIN as the Order products
o.products.append(join)
# Finally commit to database
db.session.add(o)
db.session.commit()
I was at first trying to populate the Order.products (or o.products in the example code) directly, which will give you an error about using a Products class when it expects a OrderProducts class.
I also struggled with the whole field naming and referencing of the back_populates. Again, the example above and on the docs show this. Note the pluralization is entirely to do with how you want your fields named.
Related
In the case of many-to-many relationships, an association table can be used in the form of Association Object pattern.
I have the following setup of two classes having a M2M relationship through UserCouncil association table.
class Users(Base):
name = Column(String, nullable=False)
email = Column(String, nullable=False, unique=True)
created_at = Column(DateTime, default=datetime.utcnow)
password = Column(String, nullable=False)
salt = Column(String, nullable=False)
councils = relationship('UserCouncil', back_populates='user')
class Councils(Base):
name = Column(String, nullable=False)
created_at = Column(DateTime, default=datetime.utcnow)
users = relationship('UserCouncil', back_populates='council')
class UserCouncil(Base):
user_id = Column(UUIDType, ForeignKey(Users.id, ondelete='CASCADE'), primary_key=True)
council_id = Column(UUIDType, ForeignKey(Councils.id, ondelete='CASCADE'), primary_key=True)
role = Column(Integer, nullable=False)
user = relationship('Users', back_populates='councils')
council = relationship('Councils', back_populates='users')
However, in this situation, suppose I want to search for a council with a specific name cname for a given user user1. I can do the following:
for council in user1.councils:
if council.name == cname:
dosomething(council)
Or, alternatively, this:
session.query(UserCouncil) \
.join(Councils) \
.filter((UserCouncil.user_id == user1.id) & (Councils.name == cname)) \
.first() \
.council
While the second one is more similar to raw SQL queries and performs better, the first one is simpler. Is there any other, more idiomatic way of expressing this query which is better performing while also utilizing the relationship linkages instead of explicitly writing traditional joins?
First, I think even the SQL query you bring as an example might need to go to fetch the UserCouncil.council relationship again to the DB if it is not loaded in the memory already.
I think that given you want to search directly for the Council instance given its .name and the User instance, this is exactly what you should ask for. Below is the query for that with 2 options on how to filter on user_id (you might be more familiar with the second option, so please use it):
q = (
select(Councils)
.filter(Councils.name == councils_name)
.filter(Councils.users.any(UserCouncil.user_id == user_id)) # v1: this does not require JOIN, but produces the same result as below
# .join(UserCouncil).filter(UserCouncil.user_id == user_id) # v2: join, very similar to original SQL
)
council = session.execute(q).scalars().first()
As to making it more simple and idiomatic, I can only suggest to wrap it in a method or property on the User instance:
class Users(...):
...
def get_council_by_name(self, councils_name):
q = (
select(Councils)
.filter(Councils.name == councils_name)
.join(UserCouncil).filter(with_parent(self, Users.councils))
)
return object_session(self).execute(q).scalars().first()
so that you can later call it user.get_council_by_name('xxx')
Edit-1: added SQL queries
v1 of the first q query above will generate following SQL:
SELECT councils.id,
councils.name
FROM councils
WHERE councils.name = :name_1
AND (EXISTS
(SELECT 1
FROM user_councils
WHERE councils.id = user_councils.council_id
AND user_councils.user_id = :user_id_1
)
)
while v2 option will generate:
SELECT councils.id,
councils.name
FROM councils
JOIN user_councils ON councils.id = user_councils.council_id
WHERE councils.name = :name_1
AND user_councils.user_id = :user_id_1
I have got a not very common join and filter problem.
Here are my models;
class Order(Base):
id = Column(Integer, primary_key=True)
order_id = Column(String(19), nullable=False)
... (other fields)
class Discard(Base):
id = Column(Integer, primary_key=True)
order_id = Column(String(19), nullable=False)
I want to query all and full instances of Order but just exclude those that have a match in Discard.order_id based on Order.order_id field. As you can see there is no relationship between order_id fields.
I've tried outer left join, notin_ but ended up with no success.
With this answer I've achieved desired results.
Here is my code;
orders = (
session.query(Order)
.outerjoin(Discard, Order.order_id == Discard.order_id)
.filter(Discard.order_id == None) # noqa: E711
.all()
)
I was paying too much attention to flake8 wrong syntax message at Discard.order_id == None and was using Discard.order_id is None. It appeared out they were rendered differently by sqlalchemy.
I have a many-to-many relationship, with instances of OsmAdminUnit (polygon geometries) grouped into OsmAdminAgg instances.
The model definitions are essentially:
class OsmAdminUnit(db.Model):
__tablename__ = 'osm_admin'
id = db.Column(db.Integer, primary_key=True)
geometry = db.Column(Geometry(
geometry_type='GEOMETRY',
srid=3857), nullable=False)
agg_units = db.relationship('OsmAdminAgg',
secondary=aggregations,
backref=db.backref('osm_admin', lazy='dynamic'))
class OsmAdminAgg(db.Model):
__tablename__ = 'admin_agg'
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(100), unique=True, nullable=False)
Now what I am struggling to do is selecting OsmAdminUnits that belong to a certain OsmAdminAgg AND getting the polgyons merged by applying ST_Union from GeoAlchemy.
Selecting all admin units that belong to admin agg with id=1 works:
units = OsmAdminUnit.query.filter(OsmAdminUnit.agg_units.any(id=1)).all()
But I don't get how I can apply ST_Union on that result.
My approach so far was:
union = db.session.query(
OsmAdminUnit.geometry.ST_Union().ST_AsGeoJSON().label('agg_union')
).filter(OsmAdminUnit.agg_units.any(id=1)).subquery()
So how do I get the union of these geometries, and get it as GeoJSON?
Btw, I am building this on top of Flask, using SQLAlchemy, Flask-SQLAlchemy, Geoalchemy2.
Try this:
from sqlalchemy.sql.functions import func
union = db.session.query(func.ST_AsGeoJSON(func.ST_Union(
OsmAdminUnit.geometry)).label('agg_union')
).filter(OsmAdminUnit.agg_units.any(id=1)).subquery()
You can see a basic template for this in the GeoAlchemy 2 docs. Essentially, you need to pass func to the query, rather than the model, to select the union itself.
In your case, something like:
import sqlalchemy
union = db.session.query(
sqlalchemy.func.ST_AsGeoJSON(
sqlalchemy.func.ST_Union(OsmAdminUnit.geometry)
).label('agg_union')
).filter(
OsmAdminUnit.agg_units.any(id=1)
).all()
This grabs the union of geometry values for OsmAdminUnit records matching the filter, and returns it as stringified GeoJSON.
The accepted answer didn't work for me, I think the import may be different in the version of sqlalchemy I'm using.
I am creating a website using Flask and SQLAlchemy. This website keeps track of classes that a student has taken. I would like to find a way to search my database using SQLAlchemy to find all unique classes that have been entered. Here is code from my models.py for Class:
class Class(db.Model):
__tablename__ = 'classes'
id = db.Column(db.Integer, primary_key=True)
title = db.Column(db.String(100))
body = db.Column(db.Text)
created = db.Column(db.DateTime, default=datetime.datetime.now)
user_email = db.Column(db.String(100), db.ForeignKey(User.email))
user = db.relationship(User)
In other words, I would like to get all unique values from the title column and pass that to my views.py.
Using the model query structure you could do this
Class.query.with_entities(Class.title).distinct()
query = session.query(Class.title.distinct().label("title"))
titles = [row.title for row in query.all()]
titles = [r.title for r in session.query(Class.title).distinct()]
As #van has pointed out, what you are looking for is:
session.query(your_table.column1.distinct()).all(); #SELECT DISTINCT(column1) FROM your_table
but I will add that in most cases, you are also looking to add another filter on the results. In which case you can do
session.query(your_table.column1.distinct()).filter_by(column2 = 'some_column2_value').all();
which translates to sql
SELECT DISTINCT(column1) FROM your_table WHERE column2 = 'some_column2_value';
I have items, warehouses, and items are in warehouses.
So I have table that has information about items (sku, description, cost ...) and a table that describes warehouses(location, code, name, ...). Now I need a way to store inventory so that I know I have X items in warehouse Y. An item can be in any warehouse.
How would I go about setting up the relationship between them and storing the qty?
class Item(DeclarativeBase):
__tablename__ = 'items'
item_id = Column(Integer, primary_key=True,autoincrement=True)
item_code = Column(Unicode(35),unique=True)
item_description = Column(Unicode(100))
item_long_description = Column(Unicode())
item_cost = Column(Numeric(precision=13,scale=4))
item_list = Column(Numeric(precision=13,scale=2))
def __init__(self,code,description,cost,list):
self.item_code = code
self.item_description = description
self.item_cost = cost
self.item_list = list
class Warehouse(DeclarativeBase):
__tablename__ = 'warehouses'
warehouse_id = Column(Integer, primary_key=True, autoincrement=True)
warehouse_code = Column(Unicode(15),unique=True)
warehouse_description = Column(Unicode(55))
If I am correct I would setup the many to many using an intermediate table something like ...
item_warehouse = Table(
'item_warehouse', Base.metadata,
Column('item_id', Integer, ForeignKey('items.item_id')),
Column('warehouse_id', Integar, ForeignKey('warehouses.warehouse_id'))
)
But i would need to start the qty available on this table but since its not its own class I am not sure how that would work.
What would be the "best" practice for modeling this and having it usable in my app?
Model:
As mentioned by #Lafada, you need an Association Object. As such I would create a SA-persistent object and not only a table:
class ItemWarehouse(Base):
# version-1:
__tablename__ = 'item_warehouse'
__table_args__ = (PrimaryKeyConstraint('item_id', 'warehouse_id', name='ItemWarehouse_PK'),)
# version-2:
#__table_args__ = (UniqueConstraint('item_id', 'warehouse_id', name='ItemWarehouse_PK'),)
#id = Column(Integer, primary_key=True, autoincrement=True)
# other columns
item_id = Column(Integer, ForeignKey('items.id'), nullable=False)
warehouse_id = Column(Integer, ForeignKey('warehouses.id'), nullable=False)
quantity = Column(Integer, default=0)
This covers the model requirement with the following:
added a PrimaryKey
added a UniqueConstraint covering the (item_id, warehouse_id) pairs.
In the code above this is solved in two ways:
version-1: uses composite primary key (which must be unique)
version-2: uses simple primary key, but also adds an explicit unique constraint [I personally prefer this option]
Relationship: Association Object
Now. You can use the Association Object as is, which will look similar to this:
w = Warehouse(...)
i = Item(name="kindle", price=...)
iw = ItemWarehouse(quantity=50)
iw.item = i
w.items.append(i)
Relationship: Association Proxy extension
or, you could go one step further and use the Composite Association Proxies example, and you may configure dictionary-like access to the association object similar to this:
w = Warehouse(...)
i = Item(name="kindle", price=...)
w[i] = 50 # sets the quantity to 50 of item _i_ in warehouse _w_
i[w] = 50 # same as above, if you configure it symmetrically
Beware: the code for the relationships definition might look really not easily readable, but the usage pattern is really nice. So if this option is too much to digest, I would start with Association Object with maybe some helper functions to add/get/update the item stocks, and eventually move to the Association Proxy extesion.
You have to use "Association Object".
I try to give you hint for your problem you have to create table like you mention in your question
item_warehouse = Table( 'item_warehouse',
Base.metadata,
Column('item_id',
Integer,
ForeignKey('items.item_id')
),
Column('warehouse_id',
Integar,
ForeignKey('warehouses.warehouse_id')
),
Column('qty',
Integer,
default=0,
),
)
Now you can add warehouse, item and qty in single object and you have to write method which will take warehouse_id and item_id and get the sum of qty for those itmes.
Hope this will help you to solve your problem.