Check if Django model exists in an array

Question

So, I have a set of models in my database. Now I have made a script to web scrape certain items form a website. Now I wanted to delete all items from my database which are deleted in the website. So what I mean is if an item that I have web scraped is deleted in the website I want to do the same in my database. How do I check for this using a loop?

I was thinking of doing this:

items = ItemModel.objects.all()
for item in items:
  if item.tite not in webscrape_item[0]:
     item.delete

Im checking based on the title and deleting if the title does not exist in the web scrape array.

tim-mccurrach · Accepted Answer · 2021-01-21 13:35:57Z

1

Solution

No need to even do a loop, it's more efficient this way:

ItemModel.objects.exclude(tite__in=webscrape_item[0]).delete()

Explanation

Let's break the above down a bit: If you want to filter a queryset you would normally do:

ItemModel.objects.filter(some_property=some_value)

But we can just as well tell django what to exclude rather than what to include using .exclude. Hence:

ItemModel.objects.exclude(some_property=some_value)

Now in your case, we don't want to exclude just one value of tite but any value that is in your list. For this django provides __in= which is a way of saying "if the value is in this list". The following returns a querySet all of the values you want to delete:

ItemModel.objects.exclude(tite__in=webscrape_item[0])

But every querySet has a method delete which deletes all of the values in the queryset from the database. Hence the final expression:

ItemModel.objects.exclude(tite__in=webscrape_item[0]).delete()

Note though that if you have provided a custom delete method for your model, this will not necessarily be called. But otherwise this is a better way to go than a loop.

Why is it more efficient?

Why didn't we do this:

items_to_delete = ItemModel.objects.exclude(tite__in=webscrape_item[0])
for item in items_to_delete:
    item.delete()

Well the above loop, is one hit to the database to get items_to_delete, then an additional n hits to the database for every time we call item.delete(). Where as the solution I've suggested will attempt to do this in a single SQL statement.

edited Jan 21, 2021 at 13:35

answered Jan 21, 2021 at 10:38

tim-mccurrach

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3 Comments

Adam Baser Over a year ago

can you explain what the code is doing here

tim-mccurrach Over a year ago

@AdamBaser Sure :) I've updated the answer to include an explanation. I hope this helps

Adam Baser Over a year ago

Thank you so much man! Really appretiate it! :D

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