Is it possible to query SQLAlchemy Joined Table Inheritance with and condition

Question

Using flat models, it is possible to do the following join:
session.query( EmployeeModel.name, EmployeeDepartment.dept_name ).join( EmployeeDepartment, and_( EmployeeDepartment.employee_id == EmployeeModel.id, EmployeeDepartment.dept_code == 'P01' ) ).all()

In my case I have the following inheritance hierarchy:
`class Parent(Base):
tablename = ‘parent’

id = Column(BigInteger, primary_key=True)
cls = Column(String(255), nullable=False, index=True)
sub2_id = Column(ForeignKey('parent.id'),
                                  unique=True)
extended_id = Column(
    ForeignKey('extended.id'), index=True)
extended = relationship('Extended')

class Sub1(Parent):
tablename = ‘sub1’

sub1_id = Column(ForeignKey('parent.id'),
                           primary_key=True)

class Sub2(Sub1):
tablename = ‘sub2’

sub2_id = Column(
    ForeignKey('sub1.sub1_id'), primary_key=True)

class Extented(Base):
tablename = ‘extended’

id = Column(BigInteger, primary_key=True)
classname = Column(String(255), nullable=False, index=True)

`

I am trying to achieve the following SQL native query:

SELECT sub2.sub2_id AS sub2_sub2_id FROM parent JOIN sub1 ON parent.id = sub1.sub1_id AND parent.cls = 'MAIN' JOIN sub2 ON sub1.sub1_id = sub2.sub2_id JOIN extended ON extended.id = parent.extended_id

But the following query result will be:

query = db.session.query(Sub2.sub2_id).join(Extented)

SELECT sub2.sub2_id AS sub2_sub2_id FROM parent JOIN sub1 ON parent.id = sub1.sub1_id JOIN sub2 ON sub1.sub1_id = sub2.sub2_id JOIN extended ON extended.id = parent.extended_id

Is it possible to customize the Models or relationship to achieve the desired query?

Appreciating your help.

I have tried adding the and condition to a relationship but that did not help.

Asked By: Mohammad Shawahneh

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Source

Answer 1

Yes, it is possible to customize the models and relationships in order to achieve the desired query. You can use the filter() method or the filter_by() method to add a filtering condition to the join.

Here’s an example of how you can use the filter() method to add a filtering condition to the join:

from sqlalchemy import and_

query = db.session.query(Sub2.sub2_id).join(Extented, and_(Parent.cls == 'MAIN', Parent.id == Sub1.sub1_id))

This will add the filtering condition Parent.cls = ‘MAIN’ and Parent.id = Sub1.sub1_id to the join, resulting in the desired query.

Alternatively, you can use the filter_by() method to add a filtering condition to the join:

query = db.session.query(Sub2.sub2_id).join(Extented).filter_by(cls='MAIN', id=Sub1.sub1_id)

This will have the same effect as the first example.

It’s important to note that the filter method and filter_by method are not only used in join, but you could use them to filter any query.

See more : https://docs.sqlalchemy.org/en/14/core/sqlelement.html

Answered By: BandannaGecko

Is it possible to query SQLAlchemy Joined Table Inheritance with and condition

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