Multiple Aggregate Functions with Peewee

As a technical blogger, I’ve come across several questions on Stack Overflow related to using Peewee, an Object-Relational Mapping (ORM) tool for Python. One such question revolved around performing multiple aggregate functions on two tables: A and B. In this article, we’ll delve into the world of Peewee, explore its capabilities, and provide a comprehensive solution to the problem at hand.

Background

For those unfamiliar with Peewee, it’s an ORM that abstracts away many database-related tasks, allowing developers to focus on writing application logic. Peewee supports various databases, including SQLite, which is commonly used in Python development.

In our scenario, we have two tables: A and B. Table A has multiple foreign keys referencing table B. Table B contains three fields: id, x, y, and z. We want to retrieve the minimum values for both x and y across all rows of table B for each row in table A, along with their corresponding z values.

Understanding the Problem

The problem can be broken down into two parts:

1. Retrieve the minimum value of x for each row in table A.
2. Retrieve the minimum value of y for each row in table A.

We then want to join these results with the original rows in table A, along with their corresponding z values.

Solution Overview

To solve this problem, we’ll employ Peewee’s capabilities to create complex queries and utilize common table expressions (CTEs).

Here’s a step-by-step approach:

1. Create tables A and B using Peewee.
2. Define two CTEs: one for the minimum value of x (zforx) and another for the minimum value of y (zfory).
3. Use these CTEs to create a final query that joins table A with both CTEs, along with their corresponding z values.

Creating Tables

# Define the tables
class A(Base):
    key = TextField()

class B(Base):
    a = ForeignKeyField(A)
    x = IntegerField()
    y = IntegerField()
    z = IntegerField()

Defining CTEs

To create two CTEs: zforx and zfory, we’ll define them separately.

# Create the first CTE for minimum value of x (zforx)
BX = B.alias()
subq_zforx = BX.select(BX.a, fn.MIN(BX.x).alias('minx')).group_by(BX.a)

# Create the second CTE for minimum value of y (zfory)
BY = B.alias()
subq_zfory = BY.select(BY.a, fn.MIN(BY.y).alias('miny')).group_by(BY.a)

Final Query

Now that we have our CTEs, let’s define the final query.

# Create a new CTE for each result set (x and y values)
z_for_x = (BX
           .select(BX.a, BX.z, subq_zforx.c.minx)
           .join(subq_zforx, on=((BX.a == subq_zforx.c.a_id) & (BX.x == subq_zforx.c.minx)))
           .group_by(BX.a, BX.z))

z_for_y = (BY
           .select(BY.a, BY.z, subq_zfory.c.miny)
           .join(subq_zfory, on=((BY.a == subq_zfory.c.a_id) & (BY.y == subq_zfory.c.miny)))
           .group_by(BY.a, BY.z))

# Define the final query with both CTEs
xcte = z_for_x.cte('zforx', columns=['ba', 'bz', 'minx'])
ycte = z_for_y.cte('zfory', columns=['ba', 'bz', 'miny'])

q = (A.select(A, xcte.c.minx, xcte.c.bz, ycte.c.miny, ycte.c.bz)
     .join_from(A, xcte, JOIN.INNER, on=(A.id == xcte.c.ba))
     .join_from(A, ycte, JOIN.INNER, on=(A.id == ycte.c.ba))
     .with_cte(xcte, ycte))

# Execute the final query
for row in q.tuples():
    print(row)

Output

When we execute this query, we get:

1  a1 1 10 1 2
2  a2 2 20 4 10

In conclusion, using Peewee’s ORM capabilities, we were able to solve the problem of retrieving multiple aggregate functions (minimum x and minimum y) across two tables (A and B). By employing CTEs and utilizing Peewee’s query builder, we can create complex queries that provide valuable insights into our data.

Last modified on 2024-02-18