Hello everyone!
I've been stuck for about a week trying to figure this out. Been on
all sorts of blogs and asked questions on stackoverflow in hopes that it
would lead in the right direction. I would like to know what you think I
could do.
Let's say I have 4 models, a User model, Blog Model, Hashtag and a Comment
model. Users can create blogs, users can comment on them and blogs can be
tagged by the blog owner from the comments or the blog description itself.
If I we're to best describe this, think of Instagram. You can hashtag your
photo when your uploading it, and afterwards by commenting on it.
Here are the models:
#models.py
from django.db import models
from django.contrib.auth.models import User
class Blog(models.Model):
user = models.ForeignKey(User)
description = models.CharField(max_length=140, blank=True)
blog_text = models.CharField(max_length=5000, blank=True)
related_hashtags = models.ManyToManyField('Hashtag',
related_name = 'tagged_post', null=True, blank=True)
class Comment(models.Model):
user = models.ForeignKey(User)
comment = models.CharField(max_length=140, blank=True)
class Hashtag(models.Model):
user = models.ForeignKey(User)
comment = models.CharField(max_length=140, blank=True)
count = models.IntegerField(default=1)
Pretty simple. A blog can have multiple hashtags and a hashtag can belong
to multiple blogs. When a user creates a blog, I check the description
string for any hashtags using the following function below:
hash_tags = re.findall(r'#(\w+)', blog.description, re.UNICODE)[:30] #
each blog can have a maximum of 30 hashtags
if hash_tags:
for hash_tag in hash_tags:
try:
oldHashtag =
Hashtag.objects.get(hashtag = hash_tag)
oldHashtag.count =
oldHashtag.count + 1
oldHashtag.save()
blog.related_hashtag.add(oldHashtag)
except Hashtag.DoesNotExist:
newHashtag =
Hashtag.objects.create(hashtag = lowercase_hash_tag)
blog.related_hashtag.add(newHashtag)
I hope so far everything makes sense. I do the same as above whenever a
Comment object is created.
Now let's imagine the hashtag #acdc is really popular, about 20-40 blogs
are added to that hashtag about every millisecond.
If the user searches for #acdc, I would like to show the latest 20 blogs
that have just been added with #acdc, and as the user loads more, I'll show
the next 20. They are ordered in DESC order of insertion.
I had to resort to RAW SQL since Django couldn't order the queryset by the
pk of the ManyToManyField, many others suggested using a through table, but
for legacy reasons, I did it this way:
SELECT * FROM django_blog INNER JOIN django_blog_related_hashtag ON (
django_blog.id = django_blog_related_hashtag.blog_id ) INNER JOIN
django_hashtag ON ( django_blog_related_hashtag.hashtag_id =
django_hashtag.id ) WHERE ( django_hashtag.hashtag = 'acdc' ) ORDER BY
django_blog_related_hashtag.id DESC LIMIT 20
If the user wanted to view page two, I'd simply add an OFFSET at the end of
the query. A simple function like so:
OFFSET = (page_number -1) * 20
This all works great, but for web apps like Instagram or Twitter or this
Blog app I just made, if the queryset is constantly growing, the user is
not always in sync with the most recent data. Which is perfectly fine, I
don't care about that. But what I do care about is that the next page that
they request must contain the results that are suppose to come after the
ones they have just received. If I don't account for this, users might see
blogs that they have already seen from previous pages because of the
growing queryset. Someone mentioned caching the entire queryset, but
wouldn't that have a large memory over head?
I'm looking forward to hearing your thoughts and suggestions.
Kind Regards,
Riegie Godwin
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