[sqlalchemy] Composite columns and None

[jens.t...@gmail.com]

Hello everyone,

I wanted to follow up on the examples on Composite Column Types 
 and in particular 
setting the mapped composite value in Python-land to *None*.

For instance, this class
@dataclasses.dataclass
class Point:
x: int
y: int
is used in the following composite mapping and I’d like the two mapped 
properties to be optional (for the sake of the argument, whether it makes 
sense or not):
class Vertex(Base):
start: Mapped[Point] | None = composite(mapped_column("x1"), 
mapped_column("y1"))
end: Mapped[Point] | None = composite(mapped_column("x2"), mapped_column
("y2"))The optional would then have to map to *nullable=True* for both 
mapped columns, which in turn would need to be checked to ensure integrity 
— either both or neither of the two mapped columns for a *Point* can be 
NULL at the same time.

I wasn’t able to find a recipe 
 for this use case. 
What’s the recommendation to implement this? Will I have to roll most of 
this manually, or does SQLA provide support?

Much thanks!
Jens

-- 
SQLAlchemy - 
The Python SQL Toolkit and Object Relational Mapper

http://www.sqlalchemy.org/

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[sqlalchemy] Re: Composite columns and None

[jens.t...@gmail.com]

Come to think of it, mapping to *None* doesn’t make much sense because then 
there’s no mapper.

However, it probably would make sense to define an “invalid” mapped Point 
if all mapped columns are NULL. In that case, I’d still consider adding 
check constraints to mildly improve integrity, although if any one mapped 
properties is NULL then by definition to Point would be invalid.

Hmm 樂



On Wednesday, November 15, 2023 at 9:02:16 AM UTC+10 jens.t...@gmail.com 
wrote:

> Hello everyone,
>
> I wanted to follow up on the examples on Composite Column Types 
>  and in particular 
> setting the mapped composite value in Python-land to *None*.
>
> For instance, this class
> @dataclasses.dataclass
> class Point:
> x: int
> y: int
> is used in the following composite mapping and I’d like the two mapped 
> properties to be optional (for the sake of the argument, whether it makes 
> sense or not):
> class Vertex(Base):
> start: Mapped[Point] | None = composite(mapped_column("x1"), 
> mapped_column("y1"))
> end: Mapped[Point] | None = composite(mapped_column("x2"), 
> mapped_column("y2"))The optional would then have to map to *nullable=True* 
> for 
> both mapped columns, which in turn would need to be checked to ensure 
> integrity — either both or neither of the two mapped columns for a *Point* 
> can be NULL at the same time.
>
> I wasn’t able to find a recipe 
>  for this use case. 
> What’s the recommendation to implement this? Will I have to roll most of 
> this manually, or does SQLA provide support?
>
> Much thanks!
> Jens
>

-- 
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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