Can’t make the Composite Comparator work…

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

Hello,

I’m noodling through the Composite Column Types examples, and can’t make it work. Based on the code on that page I put together a minimal, reproducible example (attached) which fails with

Traceback (most recent call last):
  File "/path/to/test.py", line 75, in <module>
    assert v1.start > v2.start
TypeError: '>' not supported between instances of 'Point' and 'Point'

Why is that? What am I missing? I expected to see SQL generated that implements the “greater than” between two Point instances (or perhaps Vertex instance, not sure, probably not).

Much thanks!

Jens

[Simon King]

(I haven't used any of these features, so the following is just a guess)

In your assertion, you are comparing two *Point instances*. The SQLAlchemy comparator_factory mechanism has not made any changes to the Point class itself, and Point doesn't define __gt__, hence your TypeError.

The point of the comparator_factory is to add class-level behaviour when you are building SQL expressions. In this case, you could write a query like this:

    query = dbsession.query(Vertex).filter(Vertex.start > Point(2, 2))

Hope that helps,

Simon

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

Thank you Simon!

That worked indeed and makes a lot of sense. I think I misinterpreted the documentation a bit in that sense; it did puzzle that SQLA would patch all these mapped objects…

Based on your example:

>>> dbsession.query(Vertex).filter(Vertex.start > Point(2, 2)).all()

[<__main__.Vertex object at 0x1065f5690>]

Cheers,

Jens