Relational algebra for Django, a la ARel, SQLAlchemy. Because sometimes you need a left join.

This (as of writing) is a young project. It probably has bugs. The interface may also change drastically between versions.

I highly recommend using SQLAlchemy! However, you may find this project useful if the decision to use the Django ORM is out of your control, or if you like the benefits of sticking with the Django ORM (admin etc.) but would occasionally like to dip down into something lower level that it can't express easily.

Given the following Models:

class BlogUser(models.Model):
    username = models.CharField(max_length=200)

class BlogPost(models.Model):
    title = models.CharField(max_length=200)
    body = models.TextField()
    published = models.DateTimeField(auto_now_add=True)
    user = models.ForeignKey(BlogUser)

First, create a wrapper around them:

import drel as d
user = d.table(BlogUser)
post = d.table(BlogPost)

All DRel constructs are immutable and stateless, and so can be used at the module level.

Table fields can be referenced using their Django Model name, or their database column name.

The basic way to construct a query from a DRel table is using the .project(*expressions), .join(table, expression), .leftjoin(table, expression), .crossjoin(table), .where(expression), .group(*expressions), .order(*expressions) methods. Select queries can themselves be used as an expression or table using .subquery.

All fields and expressions in the .project list must have a name -- expressions support .label(name) to give them one. Labelled expressions can be referred to in other parts of the query using d.label(name).

Expressions have operator overloading to support comparison, arithmetic, and (&), or (|).

Insert values into your queries with d.const(value).

Evaluate your queries with .all() or .one(). .all() returns a generator yielding named tuples.

Some example queries using the Blog models.

# All titles
post.project(post.title).all()

# All posts with usernames
post.join(user, user.id == post.user_id)
    .project(post.title, user.username)
    .order(post.published.desc)
    .all()

# Post counts
user.leftjoin(post, post.user_id == user.id)
    .group(user.username)
    .project(user.username, d.count(post.id).label("postcount")))
    .all()
    
# Total posts
post.project(d.count().label("total")).one()

# Latest post titles for all users
# Use another table for a self join
post2 = d.table(BlogPost)

user.leftjoin(post, post.user == user.id)
    .leftjoin(post2, (post2.user == user.id) & (post.published < post2.published))
    .where(post2.user.is_null)
    .project(user.username, post.title)
    .all()

• Missing SQL expressiveness
• Inserts
• Updates
• Documentation
• More tests

Copyright (c) 2011, Kevin Mahoney

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