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Database constraints don't affect form validation. They will just raise django.db.utils.IntegrityError if any data doesn't pass its conditions when saving entries to the database.

If you want to have data validated at the forms, you have to implement the validation in addition yourself, for example, in the clean() method of the model. That would look like this for the Idea model:

# myproject/apps/ideas/models.py
from django.db import models
from django.conf import settings
from django.core.exceptions import ValidationError
from django.utils.translation import gettext_lazy as _


class Idea(models.Model):
    author = models.ForeignKey(
        settings.AUTH_USER_MODEL,
        verbose_name=_("Author"),
        on_delete=models.SET_NULL,
        blank=True,
        null=True,
        related_name="authored_ideas2",
    )
    title = models.CharField(
        _("Title"),
        max_length=200,
    )

    # other fields and attributes…

    class Meta:
        verbose_name = _("Idea")
        verbose_name_plural = _("Ideas")
        constraints = [
            models.UniqueConstraint(
                fields=["title"],
                condition=~models.Q(author=None),
                name="unique_titles_for_each_author2",
            ),
            models.CheckConstraint(
                check=models.Q(
                    title__iregex=r"^\S.*\S$"
                    # starts with non-whitespace,
                    # ends with non-whitespace,
                    # anything in the middle
                ),
                name="title_has_no_leading_and_trailing_whitespaces2",
            )
        ]

    def clean(self):
        import re
        if self.author and Idea.objects.exclude(pk=self.pk).filter(
            author=self.author,
            title=self.title,
        ).exists():
            raise ValidationError(
                _("Each idea of the same user should have a unique title.")
            )
        if not re.match(r"^\S.*\S$", self.title):
            raise ValidationError(
                _("The title cannot start or end with a whitespace.")
            )

    # other properties and methods…