Then I perhaps did not understand how to use column_property correctly. I
mean one can do this:
n2 = aliased(Node)
sel = select([group_concat(...)]).where(n2.lft <= Node.lft).where(n2.rgt >=
Node.rgt).order_by(n2.lft).label('url')
Node.url = column_property(sel)
only when a class is defined, after class definition. And in this case it
is not possible to use url field in queries. Will try to read documentation
more carefully once again. Thanks a lot!
On Saturday, July 21, 2012 10:04:23 PM UTC+2, Michael Bayer wrote:
>
>
> On Jul 21, 2012, at 3:23 PM, Sergey Kucheryavski wrote:
>
> Thank you very much, Michael!
>
> I followed the link you gave and read about @hybrid_property. Seems like
> I can also use something like
>
> @hybrid_property
> def url(self):
> sel =
> select([func.group_concat(Node.urlname).label('url')]).where(Node.lft <=
> self.lft).\
> where(Node.rgt >= self.rgt).order_by(Node.lft)
> return db.session.execute(sel).first()['url'].replace(',', '/')
>
> Is it better or worse alternative to column_property in this case? Thanks
> a lot in advance!
>
>
>
> the hybrid property you have above will work given a Node instance since
> you're running the select() statement through execute(), but as written
> wouldn't work in an expression, like "Node.url == 'xyz'", unless you
> defined an @expression form that produced an inline expression.
>
> I almost always use hybrids, but column_property has the one advantage
> that it is automatically part of the SELECT emitted whenever you query()
> for Node.
>
>
>
>
> On Friday, July 20, 2012 4:00:17 PM UTC+2, Michael Bayer wrote:
>>
>>
>> On Jul 20, 2012, at 3:37 AM, Sergey Kucheryavski wrote:
>>
>> Hello,
>>
>> I want to use a table and ORM for keeping and managing tree of static web
>> pages with MPTT approach. The class/table itself is rather simple:
>>
>> class Node(db.Model):
>> __tablename__ = 'node'
>> id = db.Column(db.Integer, primary_key = True)
>> parent_id = db.Column(db.Integer, db.ForeignKey(id))
>> name = db.Column(db.String(150), nullable = False)
>> urlname = db.Column(db.String(150), nullable = False)
>> lft = db.Column(db.Integer, nullable = False)
>> rgt = db.Column(db.Integer, nullable = False)
>>
>> here urlname is the part of full URL a page has, e.g. if we have a page
>> Presentations with a list of presentations, with parent page Library and
>> available at '/library/presentations', the 'urlname' for this record will
>> be 'presentations' and urlname for the parent page, Library, will be
>> 'library'. However in this case, every time I need a full URL to the page I
>> have to make a query to get it. In standard SQL I can get a list of pages
>> with pseudo field 'url' as following:
>>
>> select n1.name, (select group_concat(n2.urlname SEPARATOR '/') from node
>> n2 where n2.lft <= n1.lft and n2.rgt >= n1.rgt order by n2.lft asc) as url
>> from node n1
>>
>> Is there any way to make SQLAlchemy select this pseudo field
>> automatically every time I make a query or fetch an object? Say I do
>> something like n = Node.query.filter(name = 'Presentations') and then just
>> use n.url?
>>
>> Thanks a lot in advance for any hints!
>>
>>
>> you'd get group_concat via func.group_concat(...), the "SEPARATOR" logic
>> you can probably get via op():
>>
>> from sqlalchemy import func, literal_column
>> func.group_concat(Node.urlname.op('SEPARATOR')(literal_column('/'))
>>
>> to hook it into the mapping to be automatically selected use
>> column_property():
>> http://docs.sqlalchemy.org/en/rel_0_7/orm/mapper_config.html?highlight=column_property#using-column-property-for-column-level-options
>>
>> since you're looking to fit a whole correlated subquery in there, you'll
>> want to build a select() against aliased:
>>
>> n2 = aliased(Node)
>> sel = select([group_concat(...)]).where(n2.lft <= Node.lft).where(n2.rgt
>> >= Node.rgt).order_by(n2.lft).label('url')
>>
>> Node.url = column_property(sel)
>>
>> select() will auto-correlate the FROM list when used as a scalar subquery
>> inside of a larger one.
>>
>>
>>
>
>
>
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