Django multiple dbs - admin search results using a read-only db

Question

I'm using the admin search_fields functionality.

The problem: some of my tables are very big. So search is taking forever, and adding extra load on my production database.

As I'm having a follower of my production db, I though a good idea would be to use the follower as a read-only db, especially for those kind of requests.

So I decided to add a 'read-only' db in settings.DATABASES and surcharge ModelAdmin.get_search_results in my admin classes:

def get_search_results(self, request, queryset, search_term):
    queryset, use_distinct = super(ReadOnlyDatabaseAdmin, self)\
        .get_search_results(request, queryset, search_term)

    queryset = queryset.using('read-only')

    return queryset, use_distinct

After this update, I started to get some router errors when trying to set some object as foreign key related object of another object:

Cannot assign "...": the current database router prevents this relation

NB: the read-only database was the same as the default one when I tested and got the aforementioned error, I didn't use the follower yet. I just have set a 'read-only' key in settings.DATABASES, pointing to the same dict as DATABASES['default'].

So the problem is not coming from using a different database, but strictly from the database router.

To give more detail: this error is notably coming from admin actions that are performed when in a admin-search-results page (/admin/app/obj/?q=...).

I figured it's maybe because I replace the queryset object in the method. Maybe this object is actually re-used somewhere else notably in admin actions...? I am currently looking into this.

So I'm interested in:

finding the reason of the error
and/or finding another way of performing admin search requests on a follower database to offload the main database

score 3 · Accepted Answer · answered Dec 11 '15 at 12:35

3

I guess the answer to the error is to do instead:

if request.method == 'GET':
    queryset = queryset.using('read-only')

Indeed, the search results are dont with a GET, while the admin actions are done with a POST.

I will have to check this

answered Dec 11 '15 at 12:35

lajarre

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score 0 · Answer 2 · edited May 23 '17 at 11:59

0

This is not exactly you are looking for How to improved query performance in Django admin search on related fields (MySQL), but it can help to optimize the queries.

edited May 23 '17 at 11:59

Community

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answered Dec 11 '15 at 12:27

trinchet

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Django multiple dbs - admin search results using a read-only db

2 Answers2