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!
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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