[Sqlalchemy-tickets] [sqlalchemy] #2824: Add ability to query column sets as one entity

[sqlalchemy <mi...@zz...>]

#2824: Add ability to query column sets as one entity
-------------------------+-----------------------------------------
 Reporter:  vmagamedov   |           Owner:  zzzeek
     Type:  enhancement  |          Status:  new
 Priority:  medium       |       Milestone:
Component:  orm          |        Severity:  no triage selected yet
 Keywords:               |  Progress State:  awaiting triage
-------------------------+-----------------------------------------
 In our company we have a tool called {{{construct}}}, which is doing next
 things:

 1. You declare what you want to show in your template (we use Mako, but we
 want to make templates as dumb as possible):
 {{{#!python
 product_struct = Construct(dict(
     name=Product.name,
     url=apply_(
         get_product_url,
         args=[Product.id, Product.name],
     ),
     image_url=if_(
         Product.main_image_id,
         then_=apply_(get_image_url,
                      args=[Image.id, Image.file_name, Image.store_type,
                            100, 100]),
         else_=None,
     ),
 ))
 }}}

 2. You query this structure using {{{session.query}}}
 {{{#!python
 products = (
     db.session.query(product_struct)
     .outerjoin(Product.main_image)
     .limit(10)
     .all()
 )
 }}}

 3. And you get these results:
 {{{#!python
 [
     Object(name=u'Foo',
            url=u'/p1-foo.html',
            image_url=u'http://images.example.st/123-foo-100x100.jpg'),
     Object(name=u'Bar',
            url=u'/p2-bar.html',
            image_url=None),

     # ...

     Object(name=u'Baz',
            url=u'/p10-baz.html',
            image_url=u'http://images.example.st/789-baz-100x100.jpg'),
 ]
 }}}
 {{{Object}}} is a namedtuple-like data structure.

 At this time, construct requires custom {{{Query}}} subclass, which
 overrides {{{_set_entities}}} and {{{instances}}} methods (plus some other
 methods). I find this hacky and I can't find any other solution in how to
 make construct and {{{Query}}} work better together.

 I've tried to subclass {{{ClauseList}}} and {{{ColumnElement}}} classes,
 to provide necessary columns to query (this works), but I can't figure out
 how to gather these columns as one entity/structure in the resulting row.
 Looks like {{{_ColumnEntity}}} wrapper doesn't support this functionality
 or implementation would be also very hacky, {{{_MapperEntity}}} is tied to
 mapped classes and {{{Query}}} doesn't support any other option.

 Is it possible to support so weird option?

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Ticket URL: <http://www.sqlalchemy.org/trac/ticket/2824>
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