SQLConstruct

SQLConstruct is a functional approach to query database using SQLAlchemy library. It was written to reach more speed without introducing unmaintainable and verbose code. On the contrary, code becomes simpler, so there are less chances of shooting yourself in the foot.

Main problems it aims to solve:

ORM overhead in read-only SELECT queries;
Network traffic when loading unnecessary columns;
Code complexity;
N+1 problem.

Final

You describe what you want to get from the database:

from sqlconstruct import Construct, if_

product_struct = Construct({
    'name': Product.name,
    'url': url_for_product.defn(Product),
    'image_url': if_(
        Image.id,
        then_=url_for_image.defn(Image, 100, 100),
        else_=None,
    ),
})

And you get it. SQLConstruct knows which columns you need and how transform them into suitable to use format:

>>> product = (
...     session.query(product_struct)
...     .outerjoin(Product.image)
...     .first()
... )
...
>>> product.name
'Foo product'
>>> product.url
'/p1-foo-product.html'
>>> product.image_url
'//images.example.st/123-100x100-foo.jpg'

Full story

Basic preparations:

from sqlalchemy import create_engine
from sqlalchemy import Column, Integer, String, Text, ForeignKey
from sqlalchemy.orm import Session, relationship, eagerload
from sqlalchemy.ext.declarative import declarative_base

engine = create_engine('sqlite://')
Base = declarative_base()

class Image(Base):
    __tablename__ = 'image'

    id = Column(Integer, primary_key=True)
    name = Column(String)

class Product(Base):
    __tablename__ = 'product'

    id = Column(Integer, primary_key=True)
    name = Column(String)
    image_id = Column(Integer, ForeignKey(Image.id))
    description = Column(Text)

    image = relationship(Image)

Base.metadata.create_all(engine)

session = Session(engine)
session.add(Product(name='Foo product', image=Image(name='Foo.jpg')))
session.commit()

def slugify(name):
    # very dumb implementation, just for an example
    return name.lower().replace(' ', '-')

def url_for_product(product):
    return '/p{id}-{name}.html'.format(
        id=product.id,
        name=slugify(product.name),
    )

def url_for_image(image, width, height):
    return '//images.example.st/{id}-{width}x{height}-{name}'.format(
        id=image.id,
        width=width,
        height=height,
        name=slugify(image.name),
    )

Usual way:

>>> product = (
...     session.query(Product)
...     .options(eagerload(Product.image))
...     .first()
... )
...
>>> product.name
u'Foo product'
>>> url_for_product(product)
'/p1-foo-product.html'
>>> url_for_image(product.image, 100, 100) if product.image else None
'//images.example.st/1-100x100-foo.jpg'

Disadvantages:

description column isn't deferred, it will be loaded every time;
if you will mark description column as deferred, this can introduce N+1 problem somewhere else in your project;
if you forgot to eagerload Product.image you will also get N+1 problem;
you have to pass model instances as arguments everywhere in the project and this tends to code complexity, because you don't know how they will be used in the future;
model instances creation isn't cheap, CPU time grows with number of columns, even if they are all deferred.

Initial solution:

from sqlconstruct import Construct, apply_, if_

def url_for_product(product_id, product_name):
    return '/p{id}-{name}.html'.format(
        id=product_id,
        name=slugify(product_name),
    )

def url_for_image(image_id, image_name, width, height):
    return '//images.example.st/{id}-{width}x{height}-{name}'.format(
        id=image_id,
        width=width,
        height=height,
        name=slugify(image_name),
    )

product_struct = Construct({
    'name': Product.name,
    'url': apply_(url_for_product, args=[Product.id, Product.name]),
    'image_url': if_(
        Image.id,
        then_=apply_(url_for_image, args=[Image.id, Image.name, 100, 100]),
        else_=None,
    ),
})

Usage:

>>> product = (
...     session.query(product_struct)
...     .outerjoin(Product.image)
...     .first()
... )
...
>>> product.name
u'Foo product'
>>> product.url
'/p1-foo-product.html'
>>> product.image_url
'//images.example.st/1-100x100-foo.jpg'

Advantages:

you're loading only what you need, no extra network traffic, no need to defer/undefer columns;
url_for_product and url_for_image functions can't add complexity, because they are forced to define all needed columns as arguments;
you're working with precomputed values (urls in this example).

Disadvantages:

code of functions is hard to refactor and reuse, because you should specify or pass all the arguments every time;
you should be careful with joins, because if you wouldn't specify them explicitly, SQLAlchemy will produce cartesian product of the tables (SELECT ... FROM product, image WHERE ...), which will return wrong results and hurt your performance.

To address first disadvantage, SQLConstruct provides define decorator, which gives you ability to define hybrid functions to use them in different ways:

from sqlconstruct import define

@define
def url_for_product(product):
    def body(product_id, product_name):
        return '/p{id}-{name}.html'.format(
            id=product_id,
            name=slugify(product_name),
        )
    return body, [product.id, product.name]

@define
def url_for_image(image, width, height):
    def body(image_id, image_name, width, height):
        return '//images.example.st/{id}-{width}x{height}-{name}'.format(
            id=image_id,
            width=width,
            height=height,
            name=slugify(image_name),
        )
    return body, [image.id, image.name, width, height]

Now these functions can be used in these ways:

>>> product = session.query(Product).first()
>>> url_for_product(product)  # objective style
'/p1-foo-product.html'
>>> url_for_product.defn(Product)  # apply_ declaration
<sqlconstruct.apply_ at 0x000000000>
>>> url_for_product.func(product.id, product.name)  # functional style
'/p1-foo-product.html'

Modified final Construct definition:

product_struct = Construct({
    'name': Product.name,
    'url': url_for_product.defn(Product),
    'image_url': if_(
        Image.id,
        then_=url_for_image.defn(Image, 100, 100),
        else_=None,
    ),
})

Installation

To install SQLConstruct, simply:

$ pip install sqlconstruct

Tested Python versions: 2.7, 3.4, 3.8.

Tested SQLAlchemy versions: 1.0, 1.1, 1.2, 1.3.

Examples above are using SQLAlchemy >= 0.9, if you are using older versions, you will have to do next changes in your project configuration:

from sqlconstruct import QueryMixin
from sqlalchemy.orm.query import Query as BaseQuery

class Query(QueryMixin, BaseQuery):
    pass

session = Session(engine, query_cls=Query)

Flask-SQLAlchemy:

from flask.ext.sqlalchemy import SQLAlchemy

db = SQLAlchemy(app, session_options={'query_cls': Query})

db = SQLAlchemy(session_options={'query_cls': Query})
db.init_app(app)

License

SQLConstruct is distributed under the BSD license. See LICENSE.txt for more details.

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