I don't think you're going to find a way to do that built in to SQLAlchemy.
When you write "session.query(Parent)", SQLAlchemy constructs a query
against the "parent" table. But to filter by your "is_done" property,
it would suddenly need to join every child table into the query and
construct a complicated WHERE clause along the lines of "(type_ =
'child_one' AND child_one.requested_type_one =
child_one.delivered_type_one) OR (type_ = 'child_two' AND
child_two.requested_type_two = child_one.delivered_type_two)".
In theory, I think you *could* implement this as a hybrid property on
Parent, where the "expression" part of the hybrid property constructs
a subquery with a union or a join of all the child tables. It'll be
pretty messy, and might not perform particularly well.
Simon
On Wed, Feb 3, 2021 at 10:08 AM Mehrdad Pedramfar
<mehrdad1373pedram...@gmail.com> wrote:
>
> The structure I have created is like below:
>
>
> class Parent(Base):
> __tablename__ = 'parent'
>
> id = Field(
> Integer,
> primary_key=True
> )
>
> type_ = Field(
> String(50),
> readonly=True
> )
>
> __mapper_args__ = {
> 'polymorphic_on': type_,
> }
>
> class ChildOne(Parent):
> __tablename__ = 'child_one'
> __mapper_args__ = {
> 'polymorphic_identity': 'child_one'
> }
>
> id = Field(
> Integer,
> ForeignKey('parent.id'),
> primary_key=True
> )
>
> requested_type_one = Column(
> Integer,
> nullable=False,
> )
> delivered_type_one = Column(
> Integer,
> nullable=False,
> )
> is_done = column_property(
> requested_type_one == delivered_type_one
> )
>
> class ChildTwo(Parent):
> __tablename__ = 'child_two'
> __mapper_args__ = {
> 'polymorphic_identity': 'child_two'
> }
>
> id = Field(
> Integer,
> ForeignKey('parent.id'),
> primary_key=True
> )
>
> requested_type_two = Column(
> Integer,
> nullable=False,
> )
> delivered_type_two = Column(
> Integer,
> nullable=False,
> )
> is_done = column_property(
> requested_type_two == delivered_type_two
> )
>
>
> What I am looking for is to execute ORM query like this:
>
> session.query(Parent).filter(Parent.is_done.is_(True)).all()
>
> which raises `Parent class does not have is_done` error.
>
> I want that parent class gets Child class's `is_done` based on different
> types of child classes, I have tried to created `is_done` as
> `column_property` on parent but I couldn't make it work. Also I tried using
> `hybrid_property` and neither it is.
>
> What should I do to make Parent class get `is_done` from its children?
>
> stackoverflow link:
> https://stackoverflow.com/questions/66024558/creating-column-sqlalchemy-property-on-parent-class-based-on-child-column-proper
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
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--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
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