class UniqueConstraint

    def constraint_sql(self, model, schema_editor):
        fields = [model._meta.get_field(field_name) for field_name in self.fields]
        include = [
            model._meta.get_field(field_name).column for field_name in self.include
        ]
        condition = self._get_condition_sql(model, schema_editor)
        expressions = self._get_index_expressions(model, schema_editor)
        return schema_editor._unique_sql(
            model,
            fields,
            self.name,
            condition=condition,
            deferrable=self.deferrable,
            include=include,
            opclasses=self.opclasses,
            expressions=expressions,
            nulls_distinct=self.nulls_distinct,
        )

    def constraint_sql(self, model, schema_editor):
        raise NotImplementedError("This method must be implemented by a subclass.")

    def create_sql(self, model, schema_editor):
        fields = [model._meta.get_field(field_name) for field_name in self.fields]
        include = [
            model._meta.get_field(field_name).column for field_name in self.include
        ]
        condition = self._get_condition_sql(model, schema_editor)
        expressions = self._get_index_expressions(model, schema_editor)
        return schema_editor._create_unique_sql(
            model,
            fields,
            self.name,
            condition=condition,
            deferrable=self.deferrable,
            include=include,
            opclasses=self.opclasses,
            expressions=expressions,
            nulls_distinct=self.nulls_distinct,
        )

    def create_sql(self, model, schema_editor):
        raise NotImplementedError("This method must be implemented by a subclass.")

    def deconstruct(self):
        path, args, kwargs = super().deconstruct()
        if self.fields:
            kwargs["fields"] = self.fields
        if self.condition:
            kwargs["condition"] = self.condition
        if self.deferrable:
            kwargs["deferrable"] = self.deferrable
        if self.include:
            kwargs["include"] = self.include
        if self.opclasses:
            kwargs["opclasses"] = self.opclasses
        if self.nulls_distinct is not None:
            kwargs["nulls_distinct"] = self.nulls_distinct
        return path, self.expressions, kwargs

    def deconstruct(self):
        path = "%s.%s" % (self.__class__.__module__, self.__class__.__name__)
        path = path.replace("django.db.models.constraints", "django.db.models")
        kwargs = {"name": self.name}
        if (
            self.violation_error_message is not None
            and self.violation_error_message != self.default_violation_error_message
        ):
            kwargs["violation_error_message"] = self.violation_error_message
        if self.violation_error_code is not None:
            kwargs["violation_error_code"] = self.violation_error_code
        return (path, (), kwargs)

    def remove_sql(self, model, schema_editor):
        condition = self._get_condition_sql(model, schema_editor)
        include = [
            model._meta.get_field(field_name).column for field_name in self.include
        ]
        expressions = self._get_index_expressions(model, schema_editor)
        return schema_editor._delete_unique_sql(
            model,
            self.name,
            condition=condition,
            deferrable=self.deferrable,
            include=include,
            opclasses=self.opclasses,
            expressions=expressions,
            nulls_distinct=self.nulls_distinct,
        )

    def remove_sql(self, model, schema_editor):
        raise NotImplementedError("This method must be implemented by a subclass.")

    def validate(self, model, instance, exclude=None, using=DEFAULT_DB_ALIAS):
        queryset = model._default_manager.using(using)
        if self.fields:
            lookup_kwargs = {}
            for field_name in self.fields:
                if exclude and field_name in exclude:
                    return
                field = model._meta.get_field(field_name)
                lookup_value = getattr(instance, field.attname)
                if (
                    self.nulls_distinct is not False
                    and lookup_value is None
                    or (
                        lookup_value == ""
                        and connections[
                            using
                        ].features.interprets_empty_strings_as_nulls
                    )
                ):
                    # A composite constraint containing NULL value cannot cause
                    # a violation since NULL != NULL in SQL.
                    return
                lookup_kwargs[field.name] = lookup_value
            queryset = queryset.filter(**lookup_kwargs)
        else:
            # Ignore constraints with excluded fields.
            if exclude:
                for expression in self.expressions:
                    if hasattr(expression, "flatten"):
                        for expr in expression.flatten():
                            if isinstance(expr, F) and expr.name in exclude:
                                return
                    elif isinstance(expression, F) and expression.name in exclude:
                        return
            replacements = {
                F(field): value
                for field, value in instance._get_field_value_map(
                    meta=model._meta, exclude=exclude
                ).items()
            }
            expressions = []
            for expr in self.expressions:
                # Ignore ordering.
                if isinstance(expr, OrderBy):
                    expr = expr.expression
                expressions.append(Exact(expr, expr.replace_expressions(replacements)))
            queryset = queryset.filter(*expressions)
        model_class_pk = instance._get_pk_val(model._meta)
        if not instance._state.adding and model_class_pk is not None:
            queryset = queryset.exclude(pk=model_class_pk)
        if not self.condition:
            if queryset.exists():
                if self.expressions:
                    raise ValidationError(
                        self.get_violation_error_message(),
                        code=self.violation_error_code,
                    )
                # When fields are defined, use the unique_error_message() for
                # backward compatibility.
                for model, constraints in instance.get_constraints():
                    for constraint in constraints:
                        if constraint is self:
                            raise ValidationError(
                                instance.unique_error_message(model, self.fields),
                            )
        else:
            against = instance._get_field_value_map(meta=model._meta, exclude=exclude)
            try:
                if (self.condition & Exists(queryset.filter(self.condition))).check(
                    against, using=using
                ):
                    raise ValidationError(
                        self.get_violation_error_message(),
                        code=self.violation_error_code,
                    )
            except FieldError:
                pass

    def validate(self, model, instance, exclude=None, using=DEFAULT_DB_ALIAS):
        raise NotImplementedError("This method must be implemented by a subclass.")