def connect(self, index):
        """Connect signals needed for dependency updates."""
        # Determine which model is the target model as either side of the relation
        # may be passed as `field`.
        if index.object_type == self.field.rel.model:
            self.model = self.field.rel.related_model
            self.accessor = self.field.rel.field.attname
        else:
            self.model = self.field.rel.model
            if self.field.rel.symmetrical:
                # Symmetrical m2m relation on self has no reverse accessor.
                raise NotImplementedError(
                    'Dependencies on symmetrical M2M relations are not supported due '
                    'to strange handling of the m2m_changed signal which only makes '
                    'half of the relation visible during signal execution. For now you '
                    'need to use symmetrical=False on the M2M field definition.'
                )
            else:
                self.accessor = self.field.rel.get_accessor_name()

        # Connect signals.
        signals = super().connect(index)

        m2m_signal = ElasticSignal(self, 'process_m2m', pass_kwargs=True)
        m2m_signal.connect(m2m_changed, sender=self.field.through)
        signals.append(m2m_signal)

        # If the relation has a custom through model, we need to subscribe to it.
        if not self.field.rel.through._meta.auto_created:  # pylint: disable=protected-access
            signal = ElasticSignal(self, 'process_m2m_through_save', pass_kwargs=True)
            signal.connect(post_save, sender=self.field.rel.through)
            signals.append(signal)

            signal = ElasticSignal(self, 'process_m2m_through_pre_delete', pass_kwargs=True)
            signal.connect(pre_delete, sender=self.field.rel.through)
            signals.append(signal)

            signal = ElasticSignal(self, 'process_m2m_through_post_delete', pass_kwargs=True)
            signal.connect(post_delete, sender=self.field.rel.through)
            signals.append(signal)

        return signals