<Post: mi post>” needs to have a value for field “id” before this many-to-many relationship can be used

Question

I'm trying to save data in a model that has a m2m relation from django admin, but when I save that error, try changing the relation to foreignkey but it's not what I need, any ideas?

models.py

class Post(models.Model):
    titulo = models.CharField(verbose_name="titulo del post", max_length=50)
    posting = HTMLField(verbose_name="posting",blank=True,null=True)
    categoria = models.ManyToManyField("posts.Categoria", verbose_name="categorias del post")
    slug = models.SlugField(verbose_name="slug del post", help_text="identificador unico del post", unique=True)

admin.py

@admin.register(Post)
class PostAdmin(admin.ModelAdmin):
    readonly_fields = ('slug',)

    def save_model(self,request,obj,form,change):
        if change:
            formato = "%d%S"
            es = " "
            if obj.titulo.find(es) >= 1:
                obj.slug = obj.titulo.replace(es, "-").lower() + "-" + obj.fecha_creacion.strftime(formato)
            else:
                obj.slug = obj.titulo.lower() + "-" + obj.fecha_creacion.strftime(formato)
            obj.save()

Answer 1

What this error is telling you is that your post, doesn't actually exist in the database for a many-to-many relationship to be created.

You said you were trying to save it in the admin? I can assume that you were assigning a category to it and then trying to save it.

The way to fix this is to save the parent model first, and then add the categories.

For example:

post = Post.objects.create(titulo="Test") # Create post first
post.categoria = Categoria.objects.last() # and then assign m2m relationship
post.save() # save post.

---

Another way you can do this in the admin is to set

    categoria = models.ManyToManyField("posts.Categoria", verbose_name="categorias del post")

to this:

categoria = models.ManyToManyField("posts.Categoria", verbose_name="categorias del post", blank=True)

Run your migrations, open the admin page and create a new post without assigning a category and click save.

Once the post has been created, you can now assign a category and click save again.