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django query to get data

I am into a very confusing situation where I have one to many relation and I want to query data like I want all parent table data but want to get only data from child tables which fulfill condition of site_id = 100.
class Policy(Base):
"""table containing details for Policies"""
__tablename__ = "UmbrellaPolicy"
id = Column(Integer, primary_key=True)
policy_id = Column(Integer, nullable=False, index=True)
user_defined_name = Column(String(255), nullable=True)
and child is like this
class Site(Base):
__tablename__ = "Site"
id = Column(Integer, primary_key=True)
policy_id = Column(Integer, ForeignKey("Policy.id"))
site_id = Column(String(32), nullable=False, index=True)
policy = relationship("Policy", backref="sites")

You should be able to filter join relations like this
parents = Policy.objects.filter(site__site_id=100)
You can find more info about the Django query API here but its generally of the form where you reference the relation with classname__columnname there are many other ways to filter/query that you can reference in the docs

SQLAlchemy join only one column

I have following models defined:
class Attribute(Base):
__tablename__ = "attributes"
id = Column(BigInteger, primary_key=True, index=True)
data_id = Column(BigInteger, ForeignKey("data.art_no"))
name = Column(VARCHAR(500), index=True)
data = relationship("Data", back_populates="attributes")
class Data(Base):
__tablename__ = "data"
art_no = Column(BigInteger, primary_key=True, index=True)
multiplier = Column(Float)
attributes = relationship("Attribute", back_populates="data", cascade="all, delete, delete-orphan")
If I query for a Data object, I get this for attributes:
[<app.db.models.Attribute object at 0x10d755d30>]
But I want to get:
['attribute name X']
What I want to get is, that the attributes field should be an array of the Attribute.name fields of the join'ed attributes.
My current query is:
db.query(models.Data).all()
How do I need to modify my query so the attributes field of Data contains not Attribute objects but just the strings name of `Attributes?
I hope you understand the question well ;)

db.query(models.Data).all() returns an array of Data objects. So you can define a custom property on the Data class to extract names from attributes relationship:
class Attribute(Base):
__tablename__ = "attributes"
id = Column(BigInteger, primary_key=True, index=True)
data_id = Column(BigInteger, ForeignKey("data.art_no"))
name = Column(VARCHAR(500), index=True)
data = relationship("Data", back_populates="attributes_rel")
class Data(Base):
__tablename__ = "data"
art_no = Column(BigInteger, primary_key=True, index=True)
multiplier = Column(Float)
attributes_rel = relationship("Attribute", back_populates="data", cascade="all, delete, delete-orphan")
#property
def attributes(self):
return [attribute.name for attribute in self.attributes_rel]
Note that by default sqlalchemy will fetch attributes_rel for each Data object separately upon access. This might result in N+1 selects problem. To avoid that you should specify relationship loading technique
Also take a look at with_entities and hybrid attributes

Ordering relationship by property of child elements

I use flask-sqlalchemy on a Flask project to model my database.
I need to sort the elements of a many-to-many relationship based on properties of different child elements of one side.
I have "Work" (the parent element), "Tag" (the children), "Type" (a one-to-many relationship on Tag) and "Block" (a one-to-many relationship on Type). Tags and Works are joined with a mapping table "work_tag_mapping".
In essence, each tag has exactly one type, each type belongs to exactly one block, and many tags can be added on many works.
I now want the list of tags on a work be sorted by block first and type second (both have a "position" column for that purpose).
Here are my tables (simplified for the sake of the question):
class Work(db.Model):
__tablename__ = 'work'
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(255, collation='utf8_bin'))
tags = db.relationship('Tag', order_by="Tag.type.block.position, Tag.type.position", secondary=work_tag_mapping)
class Tag(db.Model):
__tablename__ = 'tag'
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(255, collation='utf8_bin'))
type_id = db.Column(db.Integer, db.ForeignKey('type.id'), nullable=False)
type = db.relationship('Type')
work_tag_mapping = db.Table('work_tag_mapping',
db.Column('id', db.Integer, primary_key=True),
db.Column('work_id', db.Integer, db.ForeignKey('work.id'), nullable=False),
db.Column('tag_id', db.Integer, db.ForeignKey('tag.id'), nullable=False)
)
class Type(db.Model):
__tablename__ = 'type'
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(255, collation='utf8_bin'))
position = db.Column(db.Integer)
block_id = db.Column(db.Integer, db.ForeignKey('block.id'), nullable=False)
block = db.relationship('Block')
class Block(db.Model):
__tablename__ = 'block'
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(255, collation='utf8_bin'))
position = db.Column(db.Integer)
Now, it is the "order_by" in the "tags" relationship that doesn't work as I initially hoped.
The error I get is "sqlalchemy.exc.InvalidRequestError: Property 'type' is not an instance of ColumnProperty (i.e. does not correspond directly to a Column)."
I am new to SQLalchemy, Flask and indeed Python, and none of the ressources or questions here mention a case like this.

While this appears not to be possible directly, adding a getter and performing the sorting on retrieval does the trick. Adding lazy='dynamic' ensures the collection behaves as a query, so joins can be performed.
_tags = db.relationship('Tag', lazy='dynamic')
#hybrid_property
def tags(self):
return self._tags.join(Type).join(Block).order_by(Block.position, Type.position)

SQLAlchemy column_property basics

I have two models:
class Report(Base):
__tablename__ = 'report'
id = Column(Integer, primary_key=True)
class ReportPhoto(Base):
__tablename__ = 'report_photo'
id = Column(Integer, primary_key=True)
report_id = Column(Integer, ForeignKey(Report.id), nullable=False)
report = relationship(Report, uselist=False, backref=backref('report_photo', uselist=True))
And I would like to add column to Report model which indicates is there any records within ReportPhoto. I try to use column_property this way:
class Report(Base):
__tablename__ = 'report'
id = Column(Integer, primary_key=True)
has_photo = column_property(
select(ReportPhoto.any())
)
but get an error NameError: name 'ReportPhoto' is not defined. How I can fix this issue?

something like that should work:
class ReportPhoto(Base):
__tablename__ = 'report_photo'
id = Column(Integer, primary_key=True)
report_id = Column(Integer, ForeignKey('report.id'), nullable=False)
class Report(Base):
__tablename__ = 'report'
id = Column(Integer, primary_key=True)
report_photos = relationship(ReportPhoto, backref='report')
has_photo = column_property(
exists().where(ReportPhoto.report_id==id)
)

I will add to #Vladimir lliev's response with some clarification for anyone else that might not see how to do this.
Place the table that will have the 'foreign table referencing' column_property after that which it references. In this case, it means placing Report after ReportPhoto. This will solve your NameError, however, you would be left with a new error on your ReportPhoto foreign key reference. To solve this, place your foreign key table reference in quotes. You can read more by referencing the declarative documentation (e.g., declarative.py) and looking under "Configuring Relationships" --- specifically, read the portion on quoting your foreign references.
With your code, this would look like:
class ReportPhoto(Base):
# This now goes first
__tablename__ = 'report_photo'
id = Column(Integer, primary_key=True)
# Notice the quotations around Report references here
report_id = Column(Integer, ForeignKey("Report.id"), nullable=False)
# Notice the quotations around Report references here
report = relationship("Report",
uselist=False,
backref=backref("report_photo", uselist=True))
class Report(Base):
# This is now _after_ ReportPhoto
__tablename__ = 'report'
id = Column(Integer, primary_key=True)
# ReportPhoto now exists and we will not trip a NameError exception
has_photo = column_property(
select(ReportPhoto.any())
)

How to build many-to-many relations using SQLAlchemy: a good example

I have read the SQLAlchemy documentation and tutorial about building many-to-many relation but I could not figure out how to do it properly when the association table contains more than the 2 foreign keys.
I have a table of items and every item has many details. Details can be the same on many items, so there is a many-to-many relation between items and details
I have the following:
class Item(Base):
__tablename__ = 'Item'
id = Column(Integer, primary_key=True)
name = Column(String(255))
description = Column(Text)
class Detail(Base):
__tablename__ = 'Detail'
id = Column(Integer, primary_key=True)
name = Column(String)
value = Column(String)
My association table is (It's defined before the other 2 in the code):
class ItemDetail(Base):
__tablename__ = 'ItemDetail'
id = Column(Integer, primary_key=True)
itemId = Column(Integer, ForeignKey('Item.id'))
detailId = Column(Integer, ForeignKey('Detail.id'))
endDate = Column(Date)
In the documentation, it's said that I need to use the "association object". I could not figure out how to use it properly, since it's mixed declarative with mapper forms and the examples seem not to be complete. I added the line:
details = relation(ItemDetail)
as a member of Item class and the line:
itemDetail = relation('Detail')
as a member of the association table, as described in the documentation.
when I do item = session.query(Item).first(), the item.details is not a list of Detail objects, but a list of ItemDetail objects.
How can I get details properly in Item objects, i.e., item.details should be a list of Detail objects?

From the comments I see you've found the answer. But the SQLAlchemy documentation is quite overwhelming for a 'new user' and I was struggling with the same question. So for future reference:
ItemDetail = Table('ItemDetail',
Column('id', Integer, primary_key=True),
Column('itemId', Integer, ForeignKey('Item.id')),
Column('detailId', Integer, ForeignKey('Detail.id')),
Column('endDate', Date))
class Item(Base):
__tablename__ = 'Item'
id = Column(Integer, primary_key=True)
name = Column(String(255))
description = Column(Text)
details = relationship('Detail', secondary=ItemDetail, backref='Item')
class Detail(Base):
__tablename__ = 'Detail'
id = Column(Integer, primary_key=True)
name = Column(String)
value = Column(String)
items = relationship('Item', secondary=ItemDetail, backref='Detail')

Like Miguel, I'm also using a Declarative approach for my junction table. However, I kept running into errors like
sqlalchemy.exc.ArgumentError: secondary argument <class 'main.ProjectUser'> passed to to relationship() User.projects must be a Table object or other FROM clause; can't send a mapped class directly as rows in 'secondary' are persisted independently of a class that is mapped to that same table.
With some fiddling, I was able to come up with the following. (Note my classes are different than OP's but the concept is the same.)
Example
Here's a full working example
from sqlalchemy import create_engine, Column, Integer, String, ForeignKey
from sqlalchemy.orm import declarative_base, relationship, Session
# Make the engine
engine = create_engine("sqlite+pysqlite:///:memory:", future=True, echo=False)
# Make the DeclarativeMeta
Base = declarative_base()
class User(Base):
__tablename__ = "users"
id = Column(Integer, primary_key=True)
name = Column(String)
projects = relationship('Project', secondary='project_users', back_populates='users')
class Project(Base):
__tablename__ = "projects"
id = Column(Integer, primary_key=True)
name = Column(String)
users = relationship('User', secondary='project_users', back_populates='projects')
class ProjectUser(Base):
__tablename__ = "project_users"
id = Column(Integer, primary_key=True)
notes = Column(String, nullable=True)
user_id = Column(Integer, ForeignKey('users.id'))
project_id = Column(Integer, ForeignKey('projects.id'))
# Create the tables in the database
Base.metadata.create_all(engine)
# Test it
with Session(bind=engine) as session:
# add users
usr1 = User(name="bob")
session.add(usr1)
usr2 = User(name="alice")
session.add(usr2)
session.commit()
# add projects
prj1 = Project(name="Project 1")
session.add(prj1)
prj2 = Project(name="Project 2")
session.add(prj2)
session.commit()
# map users to projects
prj1.users = [usr1, usr2]
prj2.users = [usr2]
session.commit()
with Session(bind=engine) as session:
print(session.query(User).where(User.id == 1).one().projects)
print(session.query(Project).where(Project.id == 1).one().users)
Notes
reference the table name in the secondary argument like secondary='project_users' as opposed to secondary=ProjectUser
use back_populates instead of backref
I made a detailed writeup about this here.

Previous Answer worked for me, but I used a Class base approach for the table ItemDetail. This is the Sample code:
class ItemDetail(Base):
__tablename__ = 'ItemDetail'
id = Column(Integer, primary_key=True, index=True)
itemId = Column(Integer, ForeignKey('Item.id'))
detailId = Column(Integer, ForeignKey('Detail.id'))
endDate = Column(Date)
class Item(Base):
__tablename__ = 'Item'
id = Column(Integer, primary_key=True)
name = Column(String(255))
description = Column(Text)
details = relationship('Detail', secondary=ItemDetail.__table__, backref='Item')
class Detail(Base):
__tablename__ = 'Detail'
id = Column(Integer, primary_key=True)
name = Column(String)
value = Column(String)
items = relationship('Item', secondary=ItemDetail.__table__, backref='Detail')