SQLAlchemy newbie here.
I'm trying to define a model subclass that represents a subset of table data. Specifically, I want the subclass to map the most recent row for a given ID.
For example, suppose I have the following model:
class AddressHistory(Base):
__table__ = 'address_table'
date = Column(Date, index=True, nullable=False)
id = Column(BigInteger, primary_key=True)
street = Column(String(2000))
city = Column(String(2000))
state = Column(String(2000))
zip = Column(Integer)
What I want to do is define a subclass of this model which represents the most recent address record for a given id:
class MostRecentAddress(Address):
“””
Represents a row in AddressHistory with the most recent date for a given id.
”””
Is there some sort of subquery I can pass to the mapper_args ? Or is there a way to define the table as a select statement?
You're looking for single table inheritance.
https://docs.sqlalchemy.org/en/13/orm/inheritance.html#single-table-inheritance
Your code sample is very nearly exactly how to go about doing this. You just need to add the mapper.
class Person(Base):
__tablename__ = 'people'
id = Column(Integer, primary_key=True)
discriminator = Column('type', String(50))
__mapper_args__ = {'polymorphic_on': discriminator}
class Engineer(Person):
__mapper_args__ = {'polymorphic_identity': 'engineer'}
primary_language = Column(String(50))
Related
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
I'm implementing an Adjacency List in SQL Alchemy which I have working. It's the basic example here of Node. I have it working.
class Node(Base):
__tablename__ = 'node'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('node.id'))
data = Column(String(50))
children = relationship("Node")
But, I want to enforce a constraint where the parent_id != id. That is, a row can not be its own parent. I am not sure how to enforce this. Do I need to use a #validates or is there a DB constraint I can set up on the column(s).
You could use either #validates or a db constraint. The constraint would look like this:
import sqlalchemy as sa
class Node(Base):
__tablename__ = 'node'
id = sa.Column(sa.Integer, primary_key=True)
parent_id = sa.Column(sa.Integer, sa.ForeignKey('node.id'))
data = sa.Column(sa.String(50))
children = orm.relationship("Node")
__table_args__ = (sa.CheckConstraint('parent_id != id'),)
I have a problem with SQL Alchemy, while trying to think about an SQL schema I encountered the following problem.
My schema is based on 2 classes, Flight and Trip.
A Trip includes 2 fields: flights_to and flights_from.
Any of the fields is basically a list of flights, it could be made of one flight, or many flights (Connection flights).
class Trip(Base):
__tablename__ = "Trip"
__table_args__ = {'sqlite_autoincrement': True}
id = Column(Integer, primary_key = True)
flights_to = relationship("Flight", backref="Trip")
flights_from = relationship("Flight", backref="Trip")
class Flight(Base):
__tablename__ = "Flight"
__table_args__ = {'sqlite_autoincrement': True}
id = Column(Integer, primary_key = True)
arrival_airport = Column(String(20))
departure_airport = Column(String(20))
flight_number = Column(Integer)
trip_id = Column(Integer, ForeignKey('Trip.id'))
The problem happens when I create 2 fields in the same type:
sqlalchemy.exc.ArgumentError: Error creating backref 'Trip' on relationship 'Trip.flights_from': property of that name exists on mapper 'Mapper|Flight|Flight'
I have thought about using 2 inheriting classes of types FlightTo and FlightFrom and saving them at two different tables, but what if I want to use a FlightFrom as a FlightTo? will the flight be duplicated in 2 tables?
I would appreciate your help.
backref is used to define a new property on the other class you are using relationship with. So you can't have two property which have the same name
You should rename your backref for the flights_from to any other name than Trip.
It will work then.
For Example:
class Person(Model):
id = Column(Integer, primary_key=True)
name = Column(String)
address = relationship("Address",backref="address")
class Address(Model):
id = Column(Integer, primary_key=True)
house_no = Column(Integer)
person_id = Column(Integer, ForeignKey('person.id'))
So you can access the person name with house_no 100 by:
query_address = Address.query.filter_by(house_no=100).first()
person = query_address.address
This returns you the person object.
Thus if you have multiple such names , it will give you an error
I have two models, related with many-to-many, one of them is hierarchical model:
#hierarchical model
class Tag(Base):
__tablename__ = "tags"
id = Column(Integer, primary_key=True)
name = Column(String)
Tag.parent_id = Column(Integer, ForeignKey(Tag.id, ondelete='CASCADE'))
Tag.childs = relationship(Tag, backref=backref('parent', remote_side=[Tag.id]),
cascade="all, delete")
class Subject(Base):
__tablename__ = "subjects"
id = Column(Integer, primary_key=True, doc="ID")
name = Column(String)
tags = relationship(Tag, secondary="tags_subjects", backref="subjects")
#many-to-many relations model
class TagsSubjects(Base):
__tablename__ = "tags_subjects"
id = Column(Integer, primary_key=True)
tag_id = Column(Integer, ForeignKey("tags.id"))
subject_id = Column(Integer, ForeignKey("subjects.id"))
So, I'll try to explain what I want to do... I want to make one (or several) query, for search all Subject's objects,
that have 'name' field value like 'foo' OR that has related tags having names with values like 'foo'
OR that has related tags, that has one or more parents (or above by hierarchy) tag with 'name' value like 'foo'
I've tried to do somethis like this:
>>> subjects = session.query(Subject).filter(or_(
Subject.name.ilike('%{0}%'.format('foo')),
Subject.tags.any(
Tag.name.ilike('%{0}%'.format('foo')))
)).order_by(Subject.name).all()
But it isn't correct and "flat" query, without hierarchical feature :(
How to do this by SQLAlchemy's API?
Thanks!
P.S. I'm using SQLite backend
I have a model Zone, a model Entity and a model Transit. Transit is minimally defined as:
class Entity(db.Model):
__tablename__ = 'entities'
id = db.Column(db.Integer, primary_key=True)
contained_by = db.Column(db.Integer, db.ForeignKey('entities.id'))
contains = db.relationship('Entity', backref='container')
discriminator = db.Column('type', db.String(50))
__mapper_args__ = {'polymorphic_on': discriminator}
class Zone(Entity):
__mapper_args__ = {'polymorphic_identity': 'zones'}
routes = db.relationship('Transit')
(stuff goes here)
class Transit(db.Model):
__tablename__ = "transits"
start = db.Column(db.Integer, db.ForeignKey('zones.id'))
end = db.Column(db.Integer, db.ForeignKey('zones.id'))
Zone also has a couple of bits about distance and how defensible it is, but that is irrelevant for this.
First off, due to the fact that Zone is subclassed from Entity using single-table inheritance can I reference zones.id?
Secondly, will the Zone.routes property merge Transit.start and Transit.end?
no, you need to use the table name, which in your case (Single-table inheritance) is entities
no, these will not be merged. You can create two relationships, and have a (hybrid) property which would combine both, but this will only be for reading purposes, as when you would like to modify this property (for example, add Transits), you would still need to specify both sides (start and end).
I am not sure I understand the question here
update-1: as requested in comment, Concrete-Table inheritance code below:
class Zone(Entity):
__mapper_args__ = {'polymorphic_identity': 'zones'}
__tablename__ = "zones"
id = Column(Integer, ForeignKey('entities.id'), primary_key=True)
#property
def transits(self):
return self.transits_from_here + self.transits_to_here
class Transit(Base):
__tablename__ = "transits"
id = Column(Integer, primary_key=True)
start = Column(Integer, ForeignKey('zones.id'))
end = Column(Integer, ForeignKey('zones.id'))
s_zone = relationship(Zone, primaryjoin=(start==Zone.id), backref="transits_from_here")
e_zone = relationship(Zone, primaryjoin=(end==Zone.id), backref="transits_to_here")