Opened 3 years ago

Last modified 2 years ago

#22492 assigned New feature

provide a way to prevent database queries on model objects

Reported by: Chris Jerdonek Owned by: Raúl Cumplido
Component: Database layer (models, ORM) Version: master
Severity: Normal Keywords: model, queryset, defer, only
Cc: chris.jerdonek@…, raulcumplido@… Triage Stage: Accepted
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no


This is a feature request to provide a way to prevent database queries from happening in a block of code, especially on model objects (e.g. using a context manager, or an internal flag on model objects).

The motivation is for use with the QuerySet method `only()`, for example. (The only() method is used to prevent unnecessary fields from being loaded from a database when querying.) Consider the case of using only() to retrieve a list of many model objects, and then subsequently displaying them. If one executes some Django code after obtaining this list (e.g. by looping through the list of objects), it would be bad if this later code accidentally accessed some other field on each object. This could trigger the unintentional execution of many individual database queries (e.g. on a production database), with potentially bad consequences. I don't currently know an easy way to prevent this.

It would be good to have such a way. For example, Django could provide some sort of noQuery() context manager which would raise an exception if the database were queried inside it. Code after the only() line could be included in such a context manager. This could prevent accidentally hammering a database.

Alternatively, the QuerySet API could expose a way to return objects with some sort of no_query flag set. If attribute access on such a model object required a database query, objects with such a flag set could instead raise an exception. This would also suffice to prevent accidental queries.

Change History (4)

comment:1 Changed 3 years ago by Chris Jerdonek

Cc: chris.jerdonek@… added

comment:2 Changed 3 years ago by Tim Graham

Triage Stage: UnreviewedAccepted
Version: 1.6master

Seems like an interesting idea to at least explore.

comment:3 Changed 2 years ago by Raúl Cumplido

Cc: raulcumplido@… added
Owner: changed from nobody to Raúl Cumplido
Status: newassigned

comment:4 Changed 2 years ago by Raúl Cumplido

Is something like this a possible solution. I am thinking on the API, or do we prefer a solution where we have a context manager?

In [1]: from polls.models import Choice

In [2]: p = Choice.objects.defer('body_yes', 'id')

In [3]: choice = p[0]

In [4]: choice.no_query_deferred = True

In [5]: choice.body_yes

Traceback (most recent call last):
  File "<....>", line xxx, in <module>
QueryException: no_query flag is set to True and a query has been attempted.

In [6]: choice.no_query_deferred = False

In [7]: choice.body_yes
Out[7]: u'yes'

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