SQLAlchemy equivalent of named scopes in ActiveRecord

Question

Is there an equivalent of AR's named scopes? Named scopes are basically filters, that can be wrapped in a method, and then chained.

Here's an example from http://archives.ryandaigle.com/articles/2008/8/20/named-scope-it-s-not-just-for-conditions-ya-know:

class Article < ActiveRecord::Base

  # Get all articles that have been published
  named_scope :published, :conditions => ['published = ?', true]

  # Get all articles that were created recently
  named_scope :recent, lambda { { :conditions => ['created_at >= ?', 1.week.ago] } }

end

# Get all recently created articles that have been published
Article.published.recent

Here's an example using Django ORM: http://furrybrains.com/2009/06/22/named-scopes-for-django/

Aeneas

Answer 1

SQLAlchemy has hybrid attributes, which you could use to build any sort of system:

from sqlalchemy.ext.hybrid import hybrid_property


class Article(Base):
    @hybrid_property
    def published(self):
        return self.is_published == True

    @hybrid_property
    def recent(self):
        # this relies on the date arithmetic of the target backend
        return datetime.now() - self.created_at >= datetime.timedelta(days=7)


articles = query(Article).filter(Article.published, Article.recent).all()

Farmhand

This doesn't seem right. It appears to be doing an in memory comparison, rather than altering the SQL itself for an optimized query – Paxton

@PeterEhrlich that's what it seems like, but if the object from which you call the methods is the Article class, not an Article object, these operations only produce SQL expressions. read hybrid attributes for more detail. – Farmhand