I know that I can encapsulate multiple columns into one using either
composite column or hybrid_property, but unfortunatelly neither suits me
100%.
Let's say I have a `File` model which includes fields `id`, 'file_name`,
`storage_type`,
and there is a `get_url` python function that when given `id`, 'file_name`
and `storage_type` returns the constructed url.
I want know to have a computed property `url` on the `File` model and be
able to make queries such as
```
f = session.query(File.url).filter_by(id=1).first()
f[0] # should be a url returned by `get_url(f.id, f.file_name,
f.storage_type)`
```
I obviously cannot use `hybrid_property` since it only allows sql (which is
expected).
What comes very close is `sqlalchemy.orm.composite`.
I can define composite class such as:
```
class UrlComposite:
def __init__(self, id, file_name, storage_type):
self.id = id
self.file_name = file_name
self.storage_type = storage_type
def __composite_values__(self):
return self.id, self.file_name, self.storage_type
def __eq__(self, other):
return self.id == other.id and self.file_name == other.file_name\
and self.storage_type == other.storage_type
def __ne__(self, other):
return not self.__eq__(other)
def get(self'):
return get_url(self.id, self.file_name, self.storage_type)
```
Then I can do:
```
f = session.query(File.url).filter_by(id=1).first()
f[0].get() # extra get!
```
it's almost what I wanted, but know to obtain the url I need to call a
getter method on `UrlComposite`, which ideally I'd like to avoid. Is this
possible?
I hope my question is clear, thanks in advance!
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