queryset_generator and queryset_list_generator

queryset_generators.py

def queryset_generator(queryset, chunksize=1000):
    """ 
    Iterate over a Django Queryset ordered by the primary key

    This method loads a maximum of chunksize (default: 1000) rows in its
    memory at the same time while django normally would load all rows in its
    memory. Using the iterator() method only causes it to not preload all the
    classes.

    Note that the implementation of the generator does not support ordered query sets.

    """
    last_pk = queryset.order_by('-pk')[0].pk
    queryset = queryset.order_by('pk')
    pk = queryset[0].pk - 1
    while pk < last_pk:
        for row in queryset.filter(pk__gt=pk)[:chunksize]:
            pk = row.pk
            yield row 
        gc.collect()


def queryset_list_generator(queryset, listsize=10000, chunksize=1000):
    """ 
    Iterate over a Django Queryset ordered by the primary key and return a
    list of model objects of the size 'listsize'.
    
    This method loads a maximum of chunksize (default: 1000) rows in its memory
    at the same time while django normally would load all rows in its memory.
    In contrast to the queryset_generator, it doesn't return each row on its own,
    but returns a list of listsize (default: 10000) rows at a time.

    Note that the implementation of the generator does not support ordered query sets.
    
    """
    it = queryset_generator(queryset, chunksize)
    i = 0 
    row_list = []
    for row in it: 
        i += 1
        row_list.append(row)
        if i >= listsize:
            yield row_list
            i = 0 
            row_list = []

Hello,

I am trying this out in a large-scale django app I am building, and it is not processing the very first record. I believe it is because of the way pk is first initialized. You set it to the first ID, then you do a pk__gt filter on pk, which skips that first ID. Am I mistaken? I am setting pk to pk-1 initially in my code, and it is now getting the initial pk value in the first chunk where it would not have before. Is this a reasonable fix?

Thanks,

Jon Morgan

Jon: Looks like you're right. A cleaner way to fix this than to set pk to pk-1 is to use the queryset filter "__gte": queryset.filter(pk__gte=pk)[:chunksize]. Thanks for the comment.

But then would you be including the last row of each chunk twice, both in the previous and the next chunk?

Oops, you're right. I guess your fix is quite ok then :) I changed the gist to pk = queryset[0].pk - 1.

Just making sure. Thanks for writing and updating it! It is an exceptionally helpful code snippet! Jon

On Fri, May 6, 2011 at 5:30 AM, gwrtheyrn < ***@***.***>wrote: Oops, you're right. I guess your fix is quite ok then :) I changed the gist to `pk = queryset[0].pk - 1`. ## Reply to this email directly or view it on GitHub: https://gist.github.com/897894

## "The man with the new idea is a Crank until the idea succeeds." - Mark Twain, from 'Following the Equator: A Journey Around the World'

Thanks. By the way, I just realized the naming of the functions is kind of wrong. Those are generators, not iterators. I'll change the snippet accordingly :)

Thanks, very useful improvements! Most of the "it's" should be changed to "its", though!

@atdt: Thanks for the hint. The gist is based on this Code, so I simply copied most of the comment without noticing the grammar errors. I'll fix it :)

@gwrtheryrn, I added a date-based queryset generator if you're interested in 'pulling' it in. It's here.

@mlissner that's already pretty specialized, so i'll leave it away for now. but it's now linked in your comment and in the "forks" list, so people needing it will find it :) thanks for the contribution.