Query's str() method fails when 'default' database is empty

[liboz]

According to the ​docs, we can have

default database...[with]...parameters dictionary...blank if it will not be used.

However, when trying to print a query with something like:

print Question.objects.all().query

you get the error that

settings.DATABASES is improperly configured. Please supply the ENGINE value. 

even though the query itself can return results.

You can replicate this by creating a new project, creating a router that routes everything to a test database like so:

class Router(object):
    def db_for_read(self, model, **hints):
        """
        Reads go to a randomly-chosen replica.
        """
        return 'test'

    def db_for_write(self, model, **hints):
        """
        Writes always go to primary.
        """
        return 'test'

    def allow_relation(self, obj1, obj2, **hints):
        """
        Relations between objects are allowed if both objects are
        in the primary/replica pool.
        """
        return True

    def allow_migrate(self, db, app_label, model=None, **hints):
        """
        All non-auth models end up in this pool.
        """
        return True


# Database
# https://docs.djangoproject.com/en/1.8/ref/settings/#databases
DATABASE_ROUTERS = ['test123.settings.Router']
DATABASES = {
    'default': {},
    'test': {
        'ENGINE': 'django.db.backends.sqlite3',
        'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),
    }
}

Create a simple model like this one:

from django.db import models

# Create your models here.
class Question(models.Model):
    question_text = models.CharField(max_length=200)
    pub_date = models.DateTimeField('date published')

and run the appropriate migrations on the test database.

Then attempting to print the query will fail, but the query itself will work. I believe the error is because the sql_with_params method in django.db.models.sql.query forces the uses of the DEFAULT_DB_ALIAS:

def sql_with_params(self):
    """
    Returns the query as an SQL string and the parameters that will be
    substituted into the query.
    """
    return self.get_compiler(DEFAULT_DB_ALIAS).as_sql()

[Tim Graham]

I guess this might be tricky to fix for reasons similar to this comment in the file: "We need to use DEFAULT_DB_ALIAS here, as QuerySet does not have (nor should it have) knowledge of which connection is going to be used."

[liboz]

I looked into this a bit and Django seems to crash when running quote_name_unless_alias in django.db.models.sql.compiler because it uses DatabaseOperations' quote_name function. Could changing the DatabaseOperations for django.db.backends.dummy to have a quote_name function like that of sqlite?

I'm thinking of something like:

def quote_name(self, name):
        if name.startswith('"') and name.endswith('"'):
            return name  # Quoting once is enough.
        return '"%s"' % name

[Teddy Ni]

Patch is provided in ​https://github.com/django/django/pull/15142.

[jayvynl]

I think it's reasonable to use QuerySet.db() to replace DEFAULT_DB_ALIAS in sql_with_db method. To achieve this, QuerySet should pass itself to Query, this only need just a few lines change.

[jayvynl]

Passing QuseySet to Query will make QuerySet object be evaluated when pickling Query object and make pickle results big.

Related ticket #27159

It should have some reason why Query will be pickled. I will come up with another approch to solve this issue.

[jayvynl]

GitHub PR ​https://github.com/django/django/pull/18039

[Natalia Bidart]

Following a chat with Sarah about this ticket and PR, I can confirm this issue is still valid. For the sake of explicitness, these are the steps to reproduce the original report:

0. ensure that you have latest django installed and venv enabled if that applies
1. create an empty test project django-admin startproject projectmultidb
2. change into the project directory cd projectmultidb
3. create a router definition, for example in dbrouter.testrouter.TestRouter, use the router as provided above (no (object) inheritance though)

   class TestRouter:
   
       db_alias = "test"
   
       def db_for_read(self, model, **hints):
           return self.db_alias
   
       def db_for_write(self, model, **hints):
           return self.db_alias
   
       def allow_relation(self, obj1, obj2, **hints):
           return True
   
       def allow_migrate(self, db, app_label, model_name=None, **hints):
           return True
   

4. tweak the settings to the following:

   DATABASE_ROUTERS = ["dbrouter.testrouter.TestRouter"]
   DATABASES = {
       "default": {},
       'test': {
           'ENGINE': 'django.db.backends.sqlite3',
           'NAME': BASE_DIR / 'test.sqlite3',
       },
   }
   

5. Double check no migrations are pending python -Wall manage.py makemigrations should return No changes detected
6. Apply migrations from apps python -Wall manage.py migrate --database=test with output:

   Operations to perform:
     Apply all migrations: admin, auth, contenttypes, sessions
   Running migrations:
     Applying contenttypes.0001_initial... OK
   ...
     Applying auth.0012_alter_user_first_name_max_length... OK
     Applying sessions.0001_initial... OK
   

7. Start a new app testapp, add a Question model as described above, generate migration, and apply it:

   (django-3.12) [projectmultidb]$ python -Wall manage.py startapp testapp
   (django-3.12) [projectmultidb]$ python -Wall manage.py makemigrations
   Migrations for 'testapp':
     testapp/migrations/0001_initial.py
       + Create model Question
   (django-3.12) [projectmultidb]$ python -Wall manage.py migrate --database=test
   Operations to perform:
     Apply all migrations: admin, auth, contenttypes, sessions, testapp
   Running migrations:
     Applying testapp.0001_initial... OK
   

8. Open the shell python -Wall manage.py shell and run:

   In [1]: from django.db import connections
   
   In [2]: from testapp.models import Question
   
   In [3]: Question.objects.all()
   Out[3]: <QuerySet []>
   
   In [4]: connections["test"].queries
   Out[4]: 
   [{'sql': 'SELECT "testapp_question"."id", "testapp_question"."question_text", "testapp_question"."pub_date" FROM "testapp_question" LIMIT 21',
     'time': '0.001'}]
   
   In [5]: print(Question.objects.all().query)
   ---------------------------------------------------------------------------
   ImproperlyConfigured                      Traceback (most recent call last)
   ...
   ImproperlyConfigured: settings.DATABASES is improperly configured. Please supply the ENGINE value. Check settings documentation for more details.