django.db.utils.IntegrityError: UNIQUE constraint failed: rango_category__new.slug

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The reason for this constrain could be that you didn’t have any field called slug in Category class when you have initially migrated it (First Migration), and after adding this field in the model, when you ran makemigrations, you have set default value to something static value(i.e None or '' etc), and which broke the unique constrain for the Category’s table’s slug column in which slug should be unique but it isn’t because all the entry will get that default value.

To solve this, you can either drop the database and migration files and re-run makemigrations and migrate or set a unique default value like this:

slug = models.SlugField(unique=True, default=uuid.uuid1)

Edit:

According to Migrations that add unique fields, modify your migration file to overcome unique constrain. For example, modify your migration file (which added the slug field to the model) like this:

import uuid
from app.models import Category  # where app == tango_app_name

class Migration(migrations.Migration):

    dependencies = [
        ('yourproject', '0003_remove_category_slug'),
    ]

    def gen_uuid(apps, schema_editor):
        for row in Category.objects.all():
            row.slug = uuid.uuid4()
            row.save()

    operations = [
        migrations.AddField(
            model_name="category",
            name="slug",
            field=models.SlugField(default=uuid.uuid4),
            preserve_default=True,
        ),
        migrations.RunPython(gen_uuid),

        migrations.AlterField(
            model_name="category",
            name="slug",
            field=models.SlugField(default=uuid.uuid4, unique=True),
        ),
    ]

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