Django: Is it possible to get field values for a list of objects in bulk?-CodePudding

This is probably a silly question, but for some reason I just cannot find a way to do this:

I am working on a recommendation engine in Django, which recommends movies based on the similarities between your taste and other users' tastes. Since every database access takes time, I try to use bulk methods as much as possible. However, at one point I need to get the movie that is associated with a certain vote, and I just cannot figure out how to do this in bulk:

The relevant models are:

class Movie(models.Model):
id = models.AutoField(primary_key=True)
...
voters = models.ManyToManyField(Voter)
...

and

class MovieVote(models.Model):
    rating = models.FloatField()
    weight = models.FloatField()
    voter = models.ForeignKey(Voter, on_delete=models.CASCADE, null=True)
    movie = models.ForeignKey(Movie, on_delete=models.CASCADE, null=True)
    ...

And the one line that currently takes 80%(!) of the time of the whole recommendation process is:

for otherVote in listOfOtherVotes:
    ...    
    movie = otherVote.movie
    ...

Is there any way to look up the foreign key "movie" for the whole list of votes at once? And ideally return it as a dictionary that maps each vote to the movie?

CodePudding user response：

If listOfOtherVotes is a QuerySet, you can use .select_related(…) ^[Django-doc] to make a LEFT OUTER JOIN in the query, and thus fetch the details of the related .movies in the same query, thus avoiding to fetch each .movie with an extra query:

for otherVote in listOfOtherVotes.select_related('movie'):
    # …
    movie = otherVote.movie
    # …