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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

Sqlalchemy: Querying on m2m relations for polymorphic classes

I've got two classes that are connected through a many-to-many relationship: Parent and Tag.
Base = declarative_base()
association_table = Table('associations', Base.metadata,
Column('parent_id', Integer, ForeignKey('parent.id')),
Column('tag_id', Integer, ForeignKey('tag.id')),
)
class Tag(Base):
__tablename__ = 'tags'
id = Column(Integer, Sequence('tag_id_seq'), primary_key=True)
name = Column(String)
class Parent(Base):
__tablename__ = 'parents'
id = Column(Integer, Sequence('parent_id_seq'), primary_key=True)
tags = relationship('Tag', secondary=association_table, backref='parents')
If I want to query for all the Tag objects that have one or more relationships to a Parent, I'd do:
session.query(Tag).filter(Tag.parents.any()).all()
However, this Parent class is parent to child classes, Alice and Bob:
class Alice(Parent):
__tablename__ = 'alices'
__mapper_args__ = {'polymorphic_identity': 'alice'}
alice_id = Column(Integer, ForeignKey('parents.id'), primary_key=True)
class Bob(Parent):
__tablename__ = 'bobs'
__mapper_args__ = {'polymorphic_identity': 'bob'}
bob_id = Column(Integer, ForeignKey('parents.id'), primary_key=True)
Now I'd like to be able to retrieve all the Tag objects that have one or more relations to an Alice object. The previous query, session.query(Tag).filter(Tag.parents.any()).all(), would not do as it doesn't discriminate between Alice or Bob objects - it doesn't even know about their existence.
I've messed around with Query's for a while with no success so I assume that if it can be done, it must have something to do with some extra lines of code in the Table classes like those shown above. While the documentation holds info about polymorphic classes and many-to-many relations and Mike Bayer himself offered someone this answer to a seemingly related question which looks interesting but which I'm far from understanding, I'm kind of stuck.
The code samples may disgust the Python interpreter, but does hopefully get my point across. Candy for those who can help me on my way!

While writing a little MWE I actually found a solution, which is actually almost the same as what I had tried already. budder gave me new hope for the approach though, thanks :)
from sqlalchemy import create_engine, ForeignKey, Column, String, Integer, Sequence, Table
from sqlalchemy.orm import sessionmaker, relationship, backref
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
association_table = Table('associations', Base.metadata,
Column('parent_id', Integer, ForeignKey('parents.id')),
Column('tag_id', Integer, ForeignKey('tags.id')),
)
class Tag(Base):
__tablename__ = 'tags'
id = Column(Integer, Sequence('tag_id_seq'), primary_key=True)
name = Column(String)
class Parent(Base):
__tablename__ = 'parents'
id = Column(Integer, Sequence('parent_id_seq'), primary_key=True)
tags = relationship('Tag', secondary=association_table, backref='parents')
class Alice(Parent):
__tablename__ = 'alices'
__mapper_args__ = {'polymorphic_identity': 'alice'}
alice_id = Column(Integer, ForeignKey('parents.id'), primary_key=True)
class Bob(Parent):
__tablename__ = 'bobs'
__mapper_args__ = {'polymorphic_identity': 'bob'}
bob_id = Column(Integer, ForeignKey('parents.id'), primary_key=True)
engine = create_engine("sqlite://")
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()
tag_a = Tag(name='a')
tag_b = Tag(name='b')
tag_c = Tag(name='c')
session.add(tag_a)
session.add(tag_b)
session.add(tag_c)
session.commit()
session.add(Alice(tags=[tag_a]))
session.add(Bob(tags=[tag_b]))
session.commit()
for tag in session.query(Tag).\
filter(Tag.parents.any(Parent.id == Alice.alice_id)).\
all():
print(tag.name)
If there's a good alternative approach, I'd still be interested. I can imagine sqlalchemy offering a more direct and elegant approach so that one could do, for example:
for tag in session.query(Tag).\
filter(Tag.alices.any()).\
all():
print(tag.name)

If you write your object classes a certain way you can use the blunt force method...Just search for them...Kind of like a crude query method:
all_tag_objects = session.query(Tag).all() ## All tag objects in your database
tags = []
for tag in all_tag_objects:
for parent in tag.parents:
if parent.alices != []: ## If there are alice objects in the tag parents alice reltionship instance variable...Then we append the tag because it meets our criteria.
flagged_tags.append(tag)
Sounds like you found a better way though, but I guess the ultimate test would be to actually do a speed test.

Many-to-many relations from table to itself in SQLAlchemy

I'm using SQLAlchemy to represent a relationship between authors. I'd like to have authors related to other authors (coauthorshp), with extra data in the relation, such that with an author a I can find their coauthors.
How this is done between two different objects is this:
class Association(Base):
__tablename__ = 'association'
left_id = Column(Integer, ForeignKey('left.id'), primary_key=True)
right_id = Column(Integer, ForeignKey('right.id'), primary_key=True)
extra_data = Column(String(80))
child = relationship('Child', backref='parent_assocs')
class Parent(Base):
__tablename__ = 'left'
id = Column(Integer, primary_key=True)
children = relationship('Association', backref='parent')
class Child(Base):
__tablename__ = 'right'
id = Column(Integer, primary_key=True)
but how would I do this in my case?
The nature of a coauthorship is that it is bidirectional. So, when you insert the tuple (id_left, id_right) into the coauthorship table through a coauthoship object, is there a way to also insert the reverse relation easily? I'm asking because I want to use association proxies.

if you'd like to literally have pairs of rows in association, that is, for every id_left, id_right that's inserted, you also insert an id_right, id_left, you'd use an attribute event to listen for append events on either side, and produce an append in the other direction.
If you just want to be able to navigate between Parent/Child in either direction, just a single row of id_left, id_right is sufficient. The examples in the docs regarding this kind of mapping illustrate the whole thing.

query to hierarchical mapper

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

SQLAlchemy One-to-Many relationship on single table inheritance - declarative

Basically, I have this model, where I mapped in a single table a "BaseNode" class, and two subclasses. The point is that I need one of the subclasses, to have a one-to-many relationship with the other subclass.
So in sort, it is a relationship with another row of different class (subclass), but in the same table.
How do you think I could write it using declarative syntax?.
Note: Due to other relationships in my model, if it is possible, I really need to stick with single table inheritance.
class BaseNode(DBBase):
__tablename__ = 'base_node'
id = Column(Integer, primary_key=True)
discriminator = Column('type', String(50))
__mapper_args__ = {'polymorphic_on': discriminator}
class NodeTypeA(BaseNode):
__mapper_args__ = {'polymorphic_identity': 'NodeTypeA'}
typeB_children = relationship('NodeTypeB', backref='parent_node')
class NodeTypeB(BaseNode):
__mapper_args__ = {'polymorphic_identity': 'NodeTypeB'}
parent_id = Column(Integer, ForeignKey('base_node.id'))
Using this code will throw:
sqlalchemy.exc.ArgumentError: NodeTypeA.typeB_children and
back-reference NodeTypeB.parent_node are both of the same direction
. Did you mean to set remote_side on the
many-to-one side ?
Any ideas or suggestions?

I was struggling through this myself earlier. I was able to get this self-referential relationship working:
class Employee(Base):
__tablename__ = 'employee'
id = Column(Integer, primary_key=True)
name = Column(String(64), nullable=False)
Employee.manager_id = Column(Integer, ForeignKey(Employee.id))
Employee.manager = relationship(Employee, backref='subordinates',
remote_side=Employee.id)
Note that the manager and manager_id are "monkey-patched" because you cannot make self-references within a class definition.
So in your example, I would guess this:
class NodeTypeA(BaseNode):
__mapper_args__ = {'polymorphic_identity': 'NodeTypeA'}
typeB_children = relationship('NodeTypeB', backref='parent_node',
remote_side='NodeTypeB.parent_id')
EDIT: Basically what your error is telling you is that the relationship and its backref are both identical. So whatever rules that SA is applying to figure out what the table-level relationships are, they don't jive with the information you are providing.
I learned that simply saying mycolumn=relationship(OtherTable) in your declarative class will result in mycolumn being a list, assuming that SA can detect an unambiguous relationship. So if you really want an object to have a link to its parent, rather than its children, you can define parent=relationship(OtherTable, backref='children', remote_side=OtherTable.id) in the child table. That defines both directions of the parent-child relationship.