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An SqlAlchemy loader that solves the N+1 problem

TL;DR

What happens if you touch an unloaded column or relationship while looping over results from the DB?

users = ssn.query(User).all()

for user in users:
    user.articles  # load a relationship

Right. If you have 1000 users, you'll end up with 1000 queries.

Here's a simple solution for you that will lazy-load an attribute for all those users with just one query:

from nplus1loader import default_columns, nplus1loader

users = ssn.query(User).options(
    default_columns(User),
    nplus1loader('*')
).all()

for user in users:
    user.articles

It will only make 1 query to load all the users, then when it sees that you want articles, it will only make 1 additional query to load all articles¹ for those users.

¹: it will actually make 1 query per 500 users.

The N+1 Problem

First of all, what's the N+1 problem? Suppose you have the following code which loads a number of objects from the DB, but some fields are deferred:

users = ssn.query(User).options(
    load_only('id')  # worst case scenario
)

And then somewhere in the code some jsonify() function decides to check one of the unloaded properties:

for user in users:
    user.state  # load the attribute

Because the attribute hasn't been loaded on the whole collection... it will trigger N queries to load that attribute on every iteration of the loop. Crying shame.

In many cases we can tell SqlAlchemy to load exactly the attributes we are going to use (by using undefer() / joinedload() / selectinload()), but in other cases that's utterly impossible. We just don't know the names of those attributes in advance.

The Solution

This module offers a solution: when your code touches one unloaded attribute on a collection, it assumes that you're going to iterate through them, and loads all of them for you.

The solution is, however, not all-encompassing: you only enable it where you see fit. It's implemented as a load option, like this:

from nplus1loader import nplus1loader

users = ssn.query(User).options(
    load_only('id'),  # worst case scenario
    nplus1loader('*')  # include every column and every relationship
)

Now, whenever you touch any unloaded column or relationship on any of those loaded users, the attribute will be loaded for all User instance in that session. So now, you can safely iterate through them.

Warning: do not rely on this mechanism; only use it as a safeguard for production apps. It is recommended that you do something like this instead:

from nplus1loader import default_columns, raiseload_all, nplus1loader

users = ssn.query(User).options(
    default_columns(User),
    raiseload_all('*') if in_testing else nplus1loader('*')
)

Make sure you import the module first in order for those load options to get registered :)

import nplus1loader

One minor inconvenience

Did you notice the default_columns() load option? You'll have to insert it early into your query, I'm sorry.

This is because nplus1loader('*') is a catch-all column loader option. In other words, if no column strategies are given by you, there are no loaded columns, and this nplus1loader('*') will handle all columns!

This is not what we want. We would instead want to apply the defaults... then override them... and put this nplus1loader('*') in the end. But that's not possible with SqlAlchemy.

So we have to live with this default_columns(Model): it reads the default loading strategies from your Model and applies it early. In other words, it knows which columns are deferred by default.

If you don't put it there, nplus1loader() will remove all columns from the query, and you'll get this weird error:

sqlalchemy.exc.InvalidRequestError: Query contains no columns with which to SELECT from.

This is effectively the same as defer('*').

Now... why can't we just undefer('*') everything?

Because that's not right with some Column(deferred=True), and Column(deferred_group='password'), which have other preferences about being loaded. So default_columns(User) will respect those defaults.

Logging

All logging is done to the 'lib.db.nplus1loader.NPlusOneLazyColumnLoader' logger:

05:09:37 [W] nplus1loader.NPlusOneLazyColumnLoader: Number.es: N+1 loading of 4 instances
emitted by: _nplusone_lazy_loading() nplus1loader/strategies.py:76

Other solutions

Both projects served as a source of inspiration.

Database support

Tested with:

  • PostgreSQL
  • MySQL
  • SQLite

Other Included Tools

Handle specific attributes

The nplus1loader package has other tools that you might find useful.

First of all, you can be more specific about which attributes you want to have handled by the N+1 loader. You can opt to handle only relationships, all, or specific ones:

nplus1loader_rels('*')
nplus1loader_rels(relationship_name, ...)

or you can opt to handle only columns:

nplus1loader_cols('*')
nplus1loader_cols(column_name, ...)

nplus1loader('*', nested=True)

You may have noticed that the N+1 loader has the nested=True keyword argument. It controls which options should be put on those relationships that have been lazy-loaded in bulk.

By default, loader options in SqlAlchemy would only affect your top-level entities. For instance:

query(User).options(
    default_columns(User)
        .nplus1loader('*')
)

would only apply the N+1 loader on the immediate attributes of the User, but not to its related models. So, if the N+1 loader happens to load User's Articles, the attributes of Article would use the plain SqlAlchemy lazy-loading one by one.

With nested=True, the N+1 loader would also put an nplus1loader('*') on Article when those are loaded.

This nested=True is the default, and you don't have to bother about it. Moreover, there probably is no sane reason for disabling it, ever. So it's a good candidate for removal.

default_columns(Model)

This loader option takes the default defer()/undefer() settings from a model and sets them on a Query. In itself, this option is useless, but it enables you to alter the defaults by using other loading options.

In particular, nplus1loader('*') won't work unless you apply default_columns() first.

Note that nplus1loader('*') can be chained from default_columns(), or it can be written after a comma. Both will have the same effect.

raiseload_all('*')

Raiseload for both columns and relationships.

Example:

ssn.query(User).options(
    default_columns('id'),
    # Raise an error if any attribute at all is lazy-loaded
    raiseload_all('*')
)

The error that it throws is nplus1loader.LazyLoadingAttributeError(), so it's easy to catch in your code ;)

raiseload_col(*columns)

Raiseload for columns. Use to fine-tune.

Example:

ssn.query(User).options(
    load_only('id'),
    # Raise an error if any other column is lazy-loaded
    raiseload_col('*')
)

You can also specify individual columns as a list.

The error that it throws is nplus1loader.LazyLoadingAttributeError().

raiseload_rel(*relations)

Raiseload for relatioships. Use to fine-tune.

It's essentially the same as sqlalchemy.orm.raiseload(), but throws the nplus1loader.LazyLoadingAttributeError() error instead.

bulk_load_attribute_for_instance_states()

This is the heart of N+1 loader.

You give it a list of instances, name one unlaoded attribute, and it makes just one query to load this attribute's value from the database, and efficiently augments all the existing instances with the loaded value of this attribute.

This is an advanced feature. See the code: nplus1loader/bulk_load.py