I have 3 classes;
'Company' top class its subclass 'Department' its subclass 'DepartmentalUnit'
I can access the values of all classes from the 'DepartmentalUnit' class to the top class 'Company'
What I could not do and understand despite reading the document is that;;
I cannot access other subclasses from the 'company' class
class Company(Base):
__tablename__ = 'company'
id = Column(Integer, primary_key=True)
name = Column(String)
departments = relationship('Department',backref='company')
class Department(Base):
__tablename__ = 'department'
id = Column(Integer, primary_key=True)
department_name = Column(String)
company_id = Column(Integer, ForeignKey('company.id'))
departmentalunits = relationship('DepartmentalUnit', backref='department')
class DepartmentalUnit(Base):
__tablename__ = 'departmentalunit'
id = Column(Integer, primary_key=True,nullable=False)
departmental_unit_name = Column(String)
departments_id = Column(Integer, ForeignKey('department.id'))
The code from which I access the upper classes from the subclasses:
query = session.query(DepartmentalUnit)
instance = query.all()
for i in instance:
print(i.department.company.name)
print(i.department.department_name)
print(i.departmental_unit_name)
The code I can't access other subclasses from the company class:
query = session.query(Company)
instance = query.all()
for i in instance:
print(i.department.department_name)
Your last query should be used differently:
there is a typo in the name of the relationship: should be departments instead of department
given that the relationship is 1-N, the result is a list, so you should iterate over children.
This should work:
query = session.query(Company)
for company in query.all():
print(company.name)
for dep in company.departments:
print(" ", dep.department_name)
for dep_unit in dep.departmentalunits:
print(" ", dep_unit.departmental_unit_name)
I solved the problem. I added a backref to relationships and now I can access all of them from the company. Not sure if it's a correct method? However, I am currently getting the return I want. I have no unanswered request yet.
Example solved:
class Company(Base):
__tablename__ = 'company'
id = Column(Integer, primary_key=True)
name = Column(String)
departments = relationship('Department',backref='company',uselist=False)
class Department(Base):
__tablename__ = 'department'
id = Column(Integer, primary_key=True)
department_name = Column(String)
company_id = Column(Integer, ForeignKey('company.id'))
departmentalunits = relationship('DepartmentalUnit', backref='department',uselist=False)
class DepartmentalUnit(Base):
__tablename__ = 'departmentalunit'
id = Column(Integer, primary_key=True,nullable=False)
departmental_unit_name = Column(String)
departments_id = Column(Integer, ForeignKey('department.id'))
query = session.query(Company)
instance = query.all()
for i in instance:
print(f"Company: {i.name}")
print(f"Department: {i.departments.department_name}")
print(f"Department Unit: {i.departments.departmentalunits.departmental_unit_name}")
print( f"Report Category : {i.departments.departmentalunits.reportcategoryoftheunit.report_category_name}")
Related
I am new to SQLAlchemy and I am having troubles implementing the concept of relationships into my logic.
I have two tables: Offices and Departments where departments include the office it exists in.
I tried to apply the information present in the documentation https://www.tutorialspoint.com/sqlalchemy/sqlalchemy_orm_building_relationship.htm but I am having struggles understanding where should I insert the Relationship and if back_populates attribute is actually useful in this case.
class Offices(Base):
__tablename__: 'offices'
id = Column(Integer, primary_key=True)
name = Column(String)
code = Column(String)
reader = Column(Integer)
class Departments(Base):
__tablename__ = 'departments'
id = Column(Integer, primary_key=True)
name = Column(String)
office_id = Column(Integer, ForeignKey('offices.id'))
office = relationship("Offices", back_populates="departments")
def __init__(self, name):
self.name = name
Also, will I have to define anything related to the office in the department's __init__ in order to save it when I try inserting new rows into the table?
You can specify them relationship in both models
class Offices(Base):
__tablename__: 'offices'
id = Column(Integer, primary_key=True)
name = Column(String)
code = Column(String)
reader = Column(Integer)
departments = relationship("Departments", back_populates='office')
class Departments(Base):
__tablename__ = 'departments'
id = Column(Integer, primary_key=True)
name = Column(String)
office_id = Column(Integer, ForeignKey('offices.id'))
office = relationship("Offices", back_populates="departments")
After that you can achieve this (assume department and office are objects)
department.offices # this will refer to office
office.departments # this will refer to departments set (department is one of them)
The alternative is backref. It will create reverse relationship by itself, so
class Offices(Base):
__tablename__: 'offices'
id = Column(Integer, primary_key=True)
name = Column(String)
code = Column(String)
reader = Column(Integer)
class Departments(Base):
__tablename__ = 'departments'
id = Column(Integer, primary_key=True)
name = Column(String)
office_id = Column(Integer, ForeignKey('offices.id'))
office = relationship("Offices", backref="departments")
will do the same the first does
Suppose we have a simple one-to-many relationship between Company and Employee, is there a way to query all companies and have a list of employees in the attribute of each company?
class Company(Base):
__tablename__ = 'company'
id = Column(Integer, primary_key=True)
name = Column(String)
class Employee(Base):
__tablename__ = 'employee'
id = Column(Integer, primary_key=True)
first_name = Column(String)
last_name = Column(String)
company_id = Column(Integer, ForeignKey(Company.id))
I'm looking for something like this:
>>> result = db.session.query(Company).join(Employee).all()
>>> result[0].Employee
[<Employee object at 0x...>, <Employee object at 0x...>]
The size of result should be same as the number of rows in company table.
I tried the following and it gives tuple of objects (which makes sense) instead of nice parent / child structure:
>>> db.session.query(Company, Employee).filter(Company.id = Employee.company_id).all()
It's not hard to convert this into my desired object structure but just wanted to see if there's any shortcut.
You have to configure the relationship in the parent class:
class Company(Base):
__tablename__ = 'company'
id = Column(Integer, primary_key=True)
name = Column(String)
employees = relationship('Employee', lazy='joined') # <<< Add this line
Then you can query it without a join:
companies = session.query(Company).all()
print(companies[0].employees)
Documentation:
https://docs.sqlalchemy.org/en/13/orm/loading_relationships.html
You could do something like this:
class Company(Base):
__tablename__ = 'company'
id = Column(Integer, primary_key=True)
name = Column(String)
employees = db.session.query(Company, Employee).filter(Company.id = self.id).all()
self.employee_list = ['{0} {1}'.format(c.first_name, c.last_name) for c in employees]
Then you could access a employee name with Company.employee_list[0]
I got 3 tables
class Article_Comment(Base):
__tablename__ = 'article_comment'
article_id = Column(Integer, ForeignKey('article.id'), primary_key=True)
comment_id = Column(Integer, ForeignKey('comment.id'), primary_key=True)
child = relationship("Comment", lazy="joined", innerjoin=True)
class Article(Base):
__tablename__ = 'article'
id = Column(Integer, primary_key=True)
title = Column(String)
children = relationship("Article_Comment", lazy="joined", innerjoin=True)
class Comment(Base):
__tablename__ = 'comment'
id = Column(Integer, primary_key=True)
text = Column(String)
I need to get specific Article with Comments. I do this like this:
Session = sessionmaker(bind=engine)
session = Session()
result = session.query(Article, Comment).join(Article_Comment).join(Comment).filter(Article_Comment.article_id == Article.id).filter(Article_Comment.comment_id == Comment.id).filter(Article.title=='music').all()
for i, j in result:
print i.title, j.text
But I want to make this query without using .join.
Can someone help me?
May be, I need to remake relationships?
My thx to univerio
Here is the full answer
set up a relationship on Article, comments = relationship(Comment, secondary=Article_Comment.__table__, lazy="joined")
session.query(Article).filter(Article.title=='music').one().comments
First, I used Sqlalchemy's polymorphic architecture.
ChildA and ChildB extends Child.
ChildA has name column.
ChildB has age column.
class Parent(Base):
__tablename__ = 'parent'
id = Column(Integer, primary_key=True)
class Child(Base):
__tablename__ = 'child'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('parent.id'))
parent = relationship(Parent, backref='children')
class ChildA(Child):
__tablename__ = 'child_a'
id = Column(Integer, primary_key=True)
name = Column(String(50))
class ChildB(Child):
__tablename__ = 'child_b'
id = Column(Integer, primary_key=True)
age = Column(Integer)
parent = DBSession.query(Parent).first()
subquery = parent.children.join(ChildA).subquery()
So I want to access ChildA.name column from subquery.
Something like subquery.c.ChildA.name == 'Tom'
If I understood properly what you are trying to do, you don't really need a subquery, it could be simply something like
In [13]:
f = session.query(Parent, ChildA).join(ChildA).first()
print(f.ChildA.name)
Pedro
For the use of subqueries, I would recommend you take a look to sqlalchemy tutorial.
On the other hand, I wasn't able to use the classes as you've defined them, I had to add a ForeignKey like this
class ChildA(Child):
__tablename__ = 'child_a'
id = Column(Integer, ForeignKey('child.id'), primary_key=True)
name = Column(String(50))
class ChildB(Child):
__tablename__ = 'child_b'
id = Column(Integer, ForeignKey('child.id'), primary_key=True)
age = Column(Integer)
I have no doubt that it works for you, this probably depends on the setup.
And finally, I would like to recommend you to use a column for the type of child. With this, it will be easier to recognize the children you are using. Something like this,
class Child(Base):
__tablename__ = 'child'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('parent.id'))
parent = relationship(Parent, backref='children')
type = Column(String(20))
__mapper_args__ = {
'polymorphic_identity':'child',
'polymorphic_on':type
}
class ChildA(Child):
__tablename__ = 'child_a'
id = Column(Integer, ForeignKey('child.id'), primary_key=True)
name = Column(String(50))
__mapper_args__ = {
'polymorphic_identity':'child_a',
}
class ChildB(Child):
__tablename__ = 'child_b'
id = Column(Integer, ForeignKey('child.id'), primary_key=True)
age = Column(Integer)
__mapper_args__ = {
'polymorphic_identity':'child_b',
}
Please take a look to sqlalchemy docs for details.
Hope it helps.
I want to map a Tag entity using declarative method with SQLAlchemy. A tag can have a parent (another Tag).
I have:
class Tag(Base):
__tablename__ = 'tag'
id = Column(Integer, primary_key=True)
label = Column(String)
def __init__(self, label, parentTag=None):
self.label = label
How can I add the "parent" relationship?
You add a ForeignKey referencing the parent, and then create a relationship that specifies the direction via remote_side. This is documented under adjacency list relationships. For declarative you'd do something like this:
class Tag(Base):
__tablename__ = 'tag'
id = Column(Integer, primary_key=True)
label = Column(String)
parent_id = Column(Integer, ForeignKey('tag.id'))
parent = relationship('Tag', remote_side=[id])
If you want the reverse relation also, add backref='children' to the relationship definition.
If you need children, you need to use uselist:
class Tag(Base):
__tablename__ = 'tag'
id = Column(Integer, primary_key=True)
label = Column(String)
child_id = Column(Integer, ForeignKey('tag.id'))
children = relation('Tag', remote_side=[id], uselist=True)
class Company(BaseModel):
__tablename__ = 'companies'
companyName = db.Column(db.String(50))
contactPerson = db.Column(db.String(50))
email = db.Column(db.String(50))
mobile = db.Column(db.String(20))
parentID = db.Column(db.Integer, db.ForeignKey('companies.id')) # parent company ID
childrenCompany = db.relationship('Company', remote_side='Company.id',
backref=db.backref('children_company')) # parent Company
use:
In [2]: company_query = Company.query.get_or_404(1)
In [3]: company_query.children_company
Out[3]:
[<app.models.user.Company at 0x10f527850>,
<app.models.user.Company at 0x10f527c10>]
parent = relation('Tag') — see http://www.sqlalchemy.org/docs/05/reference/ext/declarative.html#configuring-relations.