Need help with unique_together - on ForeignKey and Boolean field

[ThomasTheDjangoFan]

Hi guys,

I'd like to only be able to assign many ProcutImages to a Product, but only ONE Image as a default image per product (ProductImage.is_default_image=True).

I can't get this to work correnctly on Django 1.7 with a db.sqlite3: With the code below I can only assign ONE Image to a Product. After that I get the error "Product image with this Product und Is default image already exists.".

Am I getting something wrong?

from django.db import models

class Product(models.Model):
    title = models.CharField(max_length=255)
   
    is_active = models.BooleanField(default=True)


class ProductImage(models.Model):
    product = models.ForeignKey(Product)

    image = models.ImageField(upload_to='products/images/')

    is_default_image = models.BooleanField(default=False,null=False,blank=False)

    class Meta:
        # A product can have multiple ProductImages, but only one of them can be the "default"-image
        unique_together = ('product', 'is_default_image')

I would be really thankful for some tips.

Kind regards
Thomas

[Jani Tiainen]

Well boolean field can have only two values: "true" or "false".

Now your unique_together defines that for "product" combined with "is_default_image" must be unique. Which means that for each product you can have _two_ images, one where is_default_image is true and one where is_default_image is false. And this is forced on database level as well.

But apparently that is what you intended to do. There isn't really one single solution for your problem.

--

Jani Tiainen

[ThomasTheDjangoFan]

Hi Jani,

thanks for making it clear.

Actually I would like to assign a lot of images to a product and only have one image as the default image.

How would you do this? Can you give me a hint?

[Javier Guerra Giraldez]

On Mon, Nov 17, 2014 at 6:10 AM, ThomasTheDjangoFan
<stefan.eich...@googlemail.com> wrote:
>
> How would you do this? Can you give me a hint?

4 different ways:

- don't do it as a flag, add a 'defaultImage=ForeignKey(ProductImage)'
to the Product class.

- don't use False for non-default, use NULL. since two NULL values
aren't equal, they don't trip unique restriction

- don't use True/False, use an integer to give images some priority.
#1 is default, the others get 2...n

- use a partial index (supported by PosgreSQL and SQLite), it's not
directly supported by the ORM, but you can simply add as an "ALTER
TABLE CREATE UNIQUE INDEX ProductdefaultImage ON app_productimage
(image) WHERE is_default_image;" at creation and forget about it.


--
Javier

[ThomasTheDjangoFan]

Hi Javier,

oh yes! You seem to be a really creative and experienced programmer! 

My solution will be way number 5:
5) Do not set unique_together - forget about it and give the model a function "get_default_image", which returns the first "default_image=True" in the list.

Thanks a lot for your help! This was really inpiring!

Have a good one

[Chuka Nwadiogbu]

Hi, I know it has been a while since this question was asked but I have  a similar question. I have a Post model, and I want there to be one pinned post out of the list of all posts, I tried creating a new property thus, 

   

```pinned = models.BooleanField(default=False, unique=True)```

but it caused major problems. I want to have just one pinned post and the rest unpinned but that would mean pinned = True for one post/article and pinned=False for the rest and I don't think that is how unique is supposed to work. Unique is supposed to be different for all the objects right? not sure how to tackle this problem.

someone suggested I use a foreignkey field to link them all together but now if I'm being honest, I actually do not know where to go from here 

```class PinnedPost(models.Model):

pinned_post = models.ForeignKey(Post, related_name='pinned', on_delete=models.CASCADE)``` if there's anyone here that can help me with this please, I'd really appreciate it. 

[heriberto ochoa]

well, maybe using forms, and the parameter initial of the send form to the template
https://docs.djangoproject.com/en/2.1/ref/forms/fields/#initial
or directly with javascript

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[Manjunath]

How are you deciding which post is pinned?

The unique approach might not do any good to you.
A simple solution is to update the pinned values of each post to False & setting the required post pinned value to True when you are setting a pinned post.

There may be better solutions, but I hope this might help you.