nplusone 0.7.0

Django

Note: nplusone supports Django 1.8 and 1.9.

Add nplusone to INSTALLED_APPS:

INSTALLED_APPS = (
    ...
    'nplusone.ext.django',
)

Add NPlusOneMiddleware:

MIDDLEWARE_CLASSES = (
    'nplusone.ext.django.NPlusOneMiddleware',
    ...
)

Optionally configure logging settings:

NPLUSONE_LOGGER = logging.getLogger('nplusone')
NPLUSONE_LOG_LEVEL = logging.WARN

Configure logging handlers:

LOGGING = {
    'version': 1,
    'handlers': {
        'console': {
            'class': 'logging.StreamHandler',
        },
    },
    'loggers': {
        'nplusone': {
            'handlers': ['console'],
            'level': 'WARN',
        },
    },
}

When your app loads data lazily, nplusone will emit a log message:

Potential n+1 query detected on `<model>.<field>`

Consider using select_related or prefetch_related in this case.

When your app eagerly loads related data without accessing it, nplusone will log a warning:

Potential unnecessary eager load detected on `<model>.<field>`

Flask-SQLAlchemy

Wrap application with NPlusOne:

from flask import Flask
from nplusone.ext.flask_sqlalchemy import NPlusOne

app = Flask(__name__)
NPlusOne(app)

Optionally configure logging settings:

app = Flask(__name__)
app.config['NPLUSONE_LOGGER'] = logging.getLogger('app.nplusone')
app.config['NPLUSONE_LOG_LEVEL'] = logging.ERROR
NPlusOne(app)

When your app loads data lazily, nplusone will emit a log message:

Potential n+1 query detected on `<model>.<field>`

Consider using subqueryload or joinedload in this case; see SQLAlchemy’s guide to relationship loading for complete documentation.

When your app eagerly loads related data without accessing it, nplusone will log a warning:

Potential unnecessary eager load detected on `<model>.<field>`

Customizing notifications

By default, nplusone logs all potentially unnecessary queries using a logger named “nplusone”. When the NPLUSONE_RAISE configuration option is set, nplusone will also raise an NPlusOneError. This can be used to force all automated tests involving unnecessary queries to fail.

# Django config
NPLUSONE_RAISE = True

# Flask config
app.config['NPLUSONE_RAISE'] = True

The exception type can also be specified, if desired, using the NPLUSONE_ERROR option.

Ignoring notifications

To ignore notifications from nplusone globally, configure the whitelist using the NPLUSONE_WHITELIST option:

# Django config
NPLUSONE_WHITELIST = [
    {'label': 'n_plus_one', 'model': 'myapp.MyModel'},
]

# Flask-SQLAlchemy config
app.config['NPLUSONE_WHITELIST'] = [
    {'label': 'unused_eager_load', 'model': 'MyModel', 'field': 'my_field'}
]

To suppress notifications locally, use the ignore context manager:

from nplusone.core import signals

with signals.ignore(signals.lazy_load):
    # lazy-load rows
    # ...