Django: values_list() multiple fields concatenated

Question

I have a Person model and I am using a django form to edit another object with a foreign key to Person. The person model has first_name and last_name fields. I want to run a method to filter results for the drop down box of the foreign reference.

I am trying to use values_list() to override the form field options (choices property) like so:

data.form.fields['person'].choices = GetPersons().values_list('id', 'first_name')

GetPersons() just filters the Person class like

return Person.objects.filter(id__gt=1000)`

for example, so I only get people I want to show up. How can I use values_list() to return the concatenation of first_name and last_name without having to return a dictionary and splitting everything manually?

Answer 1

I have in mind 2 sugestions for you:

• First one is to concatenate fields in database with extra . For me is a dirty solutions but can run.

Sample:

persons =  GetPersons().extra(select={'full_name': "concatenate( first, last) "} )
choices = persons.values_list('id', 'full_name')

and ...

• the second one use list comprehension:

Sample:

choices = [ ( p.id, '{0} {1}'.format( p.first, p.last ),) for p in GetPersons() ]

Edited 2018

Concat is now available as database function:

>>> from django.db.models import CharField, Value as V
>>> from django.db.models.functions import Concat
>>> persons = GetPersons().annotate(
...     full_name=Concat(
...         'last', V(', '), 'first', V('.'),
...         output_field=CharField()
...     )
... )

Answer 2

It sounds like the annotate() function got more flexible in Django 1.8. You can combine two fields with a Concat expression and then annotate the queryset with that expression.

# Tested with Django 1.9.2
import sys

import django
from django.apps import apps
from django.apps.config import AppConfig
from django.conf import settings
from django.db import connections, models, DEFAULT_DB_ALIAS
from django.db.models.base import ModelBase
from django.db.models.functions import Concat, Value

NAME = 'udjango'


def main():
    setup()

    class Person(models.Model):
        first_name = models.CharField(max_length=30)
        last_name = models.CharField(max_length=30)

    syncdb(Person)

    Person.objects.create(first_name='Jimmy', last_name='Jones')
    Person.objects.create(first_name='Bob', last_name='Brown')

    print(Person.objects.annotate(
        full_name=Concat('first_name',
                         Value(' '),
                         'last_name')).values_list('id', 'full_name'))
    # >>> [(1, u'Jimmy Jones'), (2, u'Bob Brown')]


def setup():
    DB_FILE = NAME + '.db'
    with open(DB_FILE, 'w'):
        pass  # wipe the database
    settings.configure(
        DEBUG=True,
        DATABASES={
            DEFAULT_DB_ALIAS: {
                'ENGINE': 'django.db.backends.sqlite3',
                'NAME': DB_FILE}},
        LOGGING={'version': 1,
                 'disable_existing_loggers': False,
                 'formatters': {
                    'debug': {
                        'format': '%(asctime)s[%(levelname)s]'
                                  '%(name)s.%(funcName)s(): %(message)s',
                        'datefmt': '%Y-%m-%d %H:%M:%S'}},
                 'handlers': {
                    'console': {
                        'level': 'DEBUG',
                        'class': 'logging.StreamHandler',
                        'formatter': 'debug'}},
                 'root': {
                    'handlers': ['console'],
                    'level': 'WARN'},
                 'loggers': {
                    "django.db": {"level": "WARN"}}})
    app_config = AppConfig(NAME, sys.modules['__main__'])
    apps.populate([app_config])
    django.setup()
    original_new_func = ModelBase.__new__

    @staticmethod
    def patched_new(cls, name, bases, attrs):
        if 'Meta' not in attrs:
            class Meta:
                app_label = NAME
            attrs['Meta'] = Meta
        return original_new_func(cls, name, bases, attrs)
    ModelBase.__new__ = patched_new


def syncdb(model):
    """ Standard syncdb expects models to be in reliable locations.

    Based on https://github.com/django/django/blob/1.9.3
    /django/core/management/commands/migrate.py#L285
    """
    connection = connections[DEFAULT_DB_ALIAS]
    with connection.schema_editor() as editor:
        editor.create_model(model)

main()