What criteria should I use to determine whether I need a distinct class-level expression for my hybrid attributes?

[Jinghui Niu]

I'm creating a mixin for my timestamp columns throughout my projects. Internally, the mixin uses UTC strings to store timestamps, externally it converts back and forth into local time using a second column that stores timezone information. I'm studying the hybrid attribute section in the documentation, but not sure if this use case would involve a different expression between the instance level and class level.

As a general rule, what should I look into to determine whether I need to use the hybrid_property.expression()?

Do I need to first familiarize myself with the SQLalchemy's SQL functions, such as func() etc., or there is some simpler rules? Thanks.

[Mike Bayer]

On 08/22/2016 03:20 AM, Jinghui Niu wrote:
> I'm creating a mixin for my timestamp columns throughout my projects.
> Internally, the mixin uses UTC strings to store timestamps, externally
> it converts back and forth into local time using a second column that
> stores timezone information. I'm studying the hybrid attribute section
> in the documentation, but not sure if this use case would involve a
> different expression between the instance level and class level.
>
> As a general rule, what should I look into to determine whether I need
> to use the hybrid_property.expression()?

expression is needed when you A. want to use your attribute in a Query
as part of filter() or similar and B. if the Python inside your hybrid
doesn't work as a SQLAlchemy expression.

Example 1: doesn't need expression - simple addition:

@hybrid_property
def a_plus_b(self):
return self.a + self.b


Example 2: does need expression - Python "if" statement needs to be CAST
on SQL

@hybrid_property
def conditional_a(self):
return "foo" if self.a == 'f' else "bar"


@conditional_a.expression
def conditional_a(cls):
return sql.case(("foo", cls.a == 'f'), else_="bar")

>
> Do I need to first familiarize myself with the SQLalchemy's SQL
> functions, such as func() etc., or there is some simpler rules? Thanks.

if you're working with date / timezone conversions in SQL then you
should figure out the SQL you want first, then at that point you
probably would need to use func. to get some SQL functions out of it,
it's pretty easy at that level.


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[Jinghui Niu]

Thanks Mike. In future is it likely to have the instance level expression and class level expression automatically translated by ORM? It will be so much easier!

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[Mike Bayer]

On 08/23/2016 11:31 AM, Jinghui Niu wrote:
> Thanks Mike. In future is it likely to have the instance level
> expression and class level expression automatically translated by ORM?
> It will be so much easier!

well a very small subset of them already are, but beyond that it's an
issue of increasingly diminished returns.

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