import logging

from django.contrib.contenttypes.fields import GenericRelation
from django.core.exceptions import ObjectDoesNotExist, ValidationError
from django.core.validators import MaxValueValidator, MinValueValidator
from django.db import models
from django.db.models import Sum
from django.urls import reverse
from taggit.managers import TaggableManager

from dcim.choices import *
from dcim.constants import *
from dcim.exceptions import CableTraceSplit
from dcim.fields import MACAddressField
from extras.models import ObjectChange, TaggedItem
from extras.utils import extras_features
from utilities.fields import NaturalOrderingField
from utilities.ordering import naturalize_interface
from utilities.querysets import RestrictedQuerySet
from utilities.query_functions import CollateAsChar
from utilities.utils import serialize_object


__all__ = (
    'CableTermination',
    'ConsolePort',
    'ConsoleServerPort',
    'DeviceBay',
    'FrontPort',
    'Interface',
    'InventoryItem',
    'PowerOutlet',
    'PowerPort',
    'RearPort',
)


class ComponentModel(models.Model):
    device = models.ForeignKey(
        to='dcim.Device',
        on_delete=models.CASCADE,
        related_name='%(class)ss'
    )
    name = models.CharField(
        max_length=64
    )
    _name = NaturalOrderingField(
        target_field='name',
        max_length=100,
        blank=True
    )
    label = models.CharField(
        max_length=64,
        blank=True,
        help_text="Physical label"
    )
    description = models.CharField(
        max_length=200,
        blank=True
    )

    objects = RestrictedQuerySet.as_manager()

    class Meta:
        abstract = True

    def __str__(self):
        if self.label:
            return f"{self.name} ({self.label})"
        return self.name

    def to_objectchange(self, action):
        # Annotate the parent Device
        try:
            device = self.device
        except ObjectDoesNotExist:
            # The parent Device has already been deleted
            device = None
        return ObjectChange(
            changed_object=self,
            object_repr=str(self),
            action=action,
            related_object=device,
            object_data=serialize_object(self)
        )

    @property
    def parent(self):
        return getattr(self, 'device', None)


class CableTermination(models.Model):
    cable = models.ForeignKey(
        to='dcim.Cable',
        on_delete=models.SET_NULL,
        related_name='+',
        blank=True,
        null=True
    )

    # Generic relations to Cable. These ensure that an attached Cable is deleted if the terminated object is deleted.
    _cabled_as_a = GenericRelation(
        to='dcim.Cable',
        content_type_field='termination_a_type',
        object_id_field='termination_a_id'
    )
    _cabled_as_b = GenericRelation(
        to='dcim.Cable',
        content_type_field='termination_b_type',
        object_id_field='termination_b_id'
    )

    class Meta:
        abstract = True

    def trace(self):
        """
        Return three items: the traceable portion of a cable path, the termination points where it splits (if any), and
        the remaining positions on the position stack (if any). Splits occur when the trace is initiated from a midpoint
        along a path which traverses a RearPort. In cases where the originating endpoint is unknown, it is not possible
        to know which corresponding FrontPort to follow. Remaining positions occur when tracing a path that traverses
        a FrontPort without traversing a RearPort again.

        The path is a list representing a complete cable path, with each individual segment represented as a
        three-tuple:

            [
                (termination A, cable, termination B),
                (termination C, cable, termination D),
                (termination E, cable, termination F)
            ]
        """
        endpoint = self
        path = []
        position_stack = []

        def get_peer_port(termination):
            from circuits.models import CircuitTermination

            # Map a front port to its corresponding rear port
            if isinstance(termination, FrontPort):
                # Retrieve the corresponding RearPort from database to ensure we have an up-to-date instance
                peer_port = RearPort.objects.get(pk=termination.rear_port.pk)

                # Don't use the stack for RearPorts with a single position. Only remember the position at
                # many-to-one points so we can select the correct FrontPort when we reach the corresponding
                # one-to-many point.
                if peer_port.positions > 1:
                    position_stack.append(termination)

                return peer_port

            # Map a rear port/position to its corresponding front port
            elif isinstance(termination, RearPort):
                if termination.positions > 1:
                    # Can't map to a FrontPort without a position if there are multiple options
                    if not position_stack:
                        raise CableTraceSplit(termination)

                    front_port = position_stack.pop()
                    position = front_port.rear_port_position

                    # Validate the position
                    if position not in range(1, termination.positions + 1):
                        raise Exception("Invalid position for {} ({} positions): {})".format(
                            termination, termination.positions, position
                        ))
                else:
                    # Don't use the stack for RearPorts with a single position. The only possible position is 1.
                    position = 1

                try:
                    peer_port = FrontPort.objects.get(
                        rear_port=termination,
                        rear_port_position=position,
                    )
                    return peer_port
                except ObjectDoesNotExist:
                    return None

            # Follow a circuit to its other termination
            elif isinstance(termination, CircuitTermination):
                peer_termination = termination.get_peer_termination()
                if peer_termination is None:
                    return None
                return peer_termination

            # Termination is not a pass-through port
            else:
                return None

        logger = logging.getLogger('netbox.dcim.cable.trace')
        logger.debug("Tracing cable from {} {}".format(self.parent, self))

        while endpoint is not None:

            # No cable connected; nothing to trace
            if not endpoint.cable:
                path.append((endpoint, None, None))
                logger.debug("No cable connected")
                return path, None, position_stack

            # Check for loops
            if endpoint.cable in [segment[1] for segment in path]:
                logger.debug("Loop detected!")
                return path, None, position_stack

            # Record the current segment in the path
            far_end = endpoint.get_cable_peer()
            path.append((endpoint, endpoint.cable, far_end))
            logger.debug("{}[{}] --- Cable {} ---> {}[{}]".format(
                endpoint.parent, endpoint, endpoint.cable.pk, far_end.parent, far_end
            ))

            # Get the peer port of the far end termination
            try:
                endpoint = get_peer_port(far_end)
            except CableTraceSplit as e:
                return path, e.termination.frontports.all(), position_stack

            if endpoint is None:
                return path, None, position_stack

    def get_cable_peer(self):
        if self.cable is None:
            return None
        if self._cabled_as_a.exists():
            return self.cable.termination_b
        if self._cabled_as_b.exists():
            return self.cable.termination_a

    def get_path_endpoints(self):
        """
        Return all endpoints of paths which traverse this object.
        """
        endpoints = []

        # Get the far end of the last path segment
        path, split_ends, position_stack = self.trace()
        endpoint = path[-1][2]
        if split_ends is not None:
            for termination in split_ends:
                endpoints.extend(termination.get_path_endpoints())
        elif endpoint is not None:
            endpoints.append(endpoint)

        return endpoints


#
# Console ports
#

@extras_features('export_templates', 'webhooks')
class ConsolePort(CableTermination, ComponentModel):
    """
    A physical console port within a Device. ConsolePorts connect to ConsoleServerPorts.
    """
    type = models.CharField(
        max_length=50,
        choices=ConsolePortTypeChoices,
        blank=True,
        help_text='Physical port type'
    )
    connected_endpoint = models.OneToOneField(
        to='dcim.ConsoleServerPort',
        on_delete=models.SET_NULL,
        related_name='connected_endpoint',
        blank=True,
        null=True
    )
    connection_status = models.BooleanField(
        choices=CONNECTION_STATUS_CHOICES,
        blank=True,
        null=True
    )
    tags = TaggableManager(through=TaggedItem)

    csv_headers = ['device', 'name', 'label', 'type', 'description']

    class Meta:
        ordering = ('device', '_name')
        unique_together = ('device', 'name')

    def get_absolute_url(self):
        return reverse('dcim:consoleport', kwargs={'pk': self.pk})

    def to_csv(self):
        return (
            self.device.identifier,
            self.name,
            self.label,
            self.type,
            self.description,
        )


#
# Console server ports
#

@extras_features('webhooks')
class ConsoleServerPort(CableTermination, ComponentModel):
    """
    A physical port within a Device (typically a designated console server) which provides access to ConsolePorts.
    """
    type = models.CharField(
        max_length=50,
        choices=ConsolePortTypeChoices,
        blank=True,
        help_text='Physical port type'
    )
    connection_status = models.BooleanField(
        choices=CONNECTION_STATUS_CHOICES,
        blank=True,
        null=True
    )
    tags = TaggableManager(through=TaggedItem)

    csv_headers = ['device', 'name', 'label', 'type', 'description']

    class Meta:
        ordering = ('device', '_name')
        unique_together = ('device', 'name')

    def get_absolute_url(self):
        return reverse('dcim:consoleserverport', kwargs={'pk': self.pk})

    def to_csv(self):
        return (
            self.device.identifier,
            self.name,
            self.label,
            self.type,
            self.description,
        )


#
# Power ports
#

@extras_features('export_templates', 'webhooks')
class PowerPort(CableTermination, ComponentModel):
    """
    A physical power supply (intake) port within a Device. PowerPorts connect to PowerOutlets.
    """
    type = models.CharField(
        max_length=50,
        choices=PowerPortTypeChoices,
        blank=True,
        help_text='Physical port type'
    )
    maximum_draw = models.PositiveSmallIntegerField(
        blank=True,
        null=True,
        validators=[MinValueValidator(1)],
        help_text="Maximum power draw (watts)"
    )
    allocated_draw = models.PositiveSmallIntegerField(
        blank=True,
        null=True,
        validators=[MinValueValidator(1)],
        help_text="Allocated power draw (watts)"
    )
    _connected_poweroutlet = models.OneToOneField(
        to='dcim.PowerOutlet',
        on_delete=models.SET_NULL,
        related_name='connected_endpoint',
        blank=True,
        null=True
    )
    _connected_powerfeed = models.OneToOneField(
        to='dcim.PowerFeed',
        on_delete=models.SET_NULL,
        related_name='+',
        blank=True,
        null=True
    )
    connection_status = models.BooleanField(
        choices=CONNECTION_STATUS_CHOICES,
        blank=True,
        null=True
    )
    tags = TaggableManager(through=TaggedItem)

    csv_headers = ['device', 'name', 'label', 'type', 'maximum_draw', 'allocated_draw', 'description']

    class Meta:
        ordering = ('device', '_name')
        unique_together = ('device', 'name')

    def get_absolute_url(self):
        return reverse('dcim:powerport', kwargs={'pk': self.pk})

    def to_csv(self):
        return (
            self.device.identifier,
            self.name,
            self.label,
            self.get_type_display(),
            self.maximum_draw,
            self.allocated_draw,
            self.description,
        )

    @property
    def connected_endpoint(self):
        """
        Return the connected PowerOutlet, if it exists, or the connected PowerFeed, if it exists. We have to check for
        ObjectDoesNotExist in case the referenced object has been deleted from the database.
        """
        try:
            if self._connected_poweroutlet:
                return self._connected_poweroutlet
        except ObjectDoesNotExist:
            pass
        try:
            if self._connected_powerfeed:
                return self._connected_powerfeed
        except ObjectDoesNotExist:
            pass
        return None

    @connected_endpoint.setter
    def connected_endpoint(self, value):
        # TODO: Fix circular import
        from . import PowerFeed

        if value is None:
            self._connected_poweroutlet = None
            self._connected_powerfeed = None
        elif isinstance(value, PowerOutlet):
            self._connected_poweroutlet = value
            self._connected_powerfeed = None
        elif isinstance(value, PowerFeed):
            self._connected_poweroutlet = None
            self._connected_powerfeed = value
        else:
            raise ValueError(
                "Connected endpoint must be a PowerOutlet or PowerFeed, not {}.".format(type(value))
            )

    def get_power_draw(self):
        """
        Return the allocated and maximum power draw (in VA) and child PowerOutlet count for this PowerPort.
        """
        # Calculate aggregate draw of all child power outlets if no numbers have been defined manually
        if self.allocated_draw is None and self.maximum_draw is None:
            outlet_ids = PowerOutlet.objects.filter(power_port=self).values_list('pk', flat=True)
            utilization = PowerPort.objects.filter(_connected_poweroutlet_id__in=outlet_ids).aggregate(
                maximum_draw_total=Sum('maximum_draw'),
                allocated_draw_total=Sum('allocated_draw'),
            )
            ret = {
                'allocated': utilization['allocated_draw_total'] or 0,
                'maximum': utilization['maximum_draw_total'] or 0,
                'outlet_count': len(outlet_ids),
                'legs': [],
            }

            # Calculate per-leg aggregates for three-phase feeds
            if self._connected_powerfeed and self._connected_powerfeed.phase == PowerFeedPhaseChoices.PHASE_3PHASE:
                for leg, leg_name in PowerOutletFeedLegChoices:
                    outlet_ids = PowerOutlet.objects.filter(power_port=self, feed_leg=leg).values_list('pk', flat=True)
                    utilization = PowerPort.objects.filter(_connected_poweroutlet_id__in=outlet_ids).aggregate(
                        maximum_draw_total=Sum('maximum_draw'),
                        allocated_draw_total=Sum('allocated_draw'),
                    )
                    ret['legs'].append({
                        'name': leg_name,
                        'allocated': utilization['allocated_draw_total'] or 0,
                        'maximum': utilization['maximum_draw_total'] or 0,
                        'outlet_count': len(outlet_ids),
                    })

            return ret

        # Default to administratively defined values
        return {
            'allocated': self.allocated_draw or 0,
            'maximum': self.maximum_draw or 0,
            'outlet_count': PowerOutlet.objects.filter(power_port=self).count(),
            'legs': [],
        }


#
# Power outlets
#

@extras_features('webhooks')
class PowerOutlet(CableTermination, ComponentModel):
    """
    A physical power outlet (output) within a Device which provides power to a PowerPort.
    """
    type = models.CharField(
        max_length=50,
        choices=PowerOutletTypeChoices,
        blank=True,
        help_text='Physical port type'
    )
    power_port = models.ForeignKey(
        to='dcim.PowerPort',
        on_delete=models.SET_NULL,
        blank=True,
        null=True,
        related_name='poweroutlets'
    )
    feed_leg = models.CharField(
        max_length=50,
        choices=PowerOutletFeedLegChoices,
        blank=True,
        help_text="Phase (for three-phase feeds)"
    )
    connection_status = models.BooleanField(
        choices=CONNECTION_STATUS_CHOICES,
        blank=True,
        null=True
    )
    tags = TaggableManager(through=TaggedItem)

    csv_headers = ['device', 'name', 'label', 'type', 'power_port', 'feed_leg', 'description']

    class Meta:
        ordering = ('device', '_name')
        unique_together = ('device', 'name')

    def get_absolute_url(self):
        return reverse('dcim:poweroutlet', kwargs={'pk': self.pk})

    def to_csv(self):
        return (
            self.device.identifier,
            self.name,
            self.label,
            self.get_type_display(),
            self.power_port.name if self.power_port else None,
            self.get_feed_leg_display(),
            self.description,
        )

    def clean(self):

        # Validate power port assignment
        if self.power_port and self.power_port.device != self.device:
            raise ValidationError(
                "Parent power port ({}) must belong to the same device".format(self.power_port)
            )


#
# Interfaces
#

class BaseInterface(models.Model):
    """
    Abstract base class for fields shared by dcim.Interface and virtualization.VMInterface.
    """
    enabled = models.BooleanField(
        default=True
    )
    mac_address = MACAddressField(
        null=True,
        blank=True,
        verbose_name='MAC Address'
    )
    mtu = models.PositiveIntegerField(
        blank=True,
        null=True,
        validators=[MinValueValidator(1), MaxValueValidator(65536)],
        verbose_name='MTU'
    )
    mode = models.CharField(
        max_length=50,
        choices=InterfaceModeChoices,
        blank=True
    )

    class Meta:
        abstract = True


@extras_features('graphs', 'export_templates', 'webhooks')
class Interface(CableTermination, ComponentModel, BaseInterface):
    """
    A network interface within a Device. A physical Interface can connect to exactly one other Interface.
    """
    # Override ComponentModel._name to specify naturalize_interface function
    _name = NaturalOrderingField(
        target_field='name',
        naturalize_function=naturalize_interface,
        max_length=100,
        blank=True
    )
    _connected_interface = models.OneToOneField(
        to='self',
        on_delete=models.SET_NULL,
        related_name='+',
        blank=True,
        null=True
    )
    _connected_circuittermination = models.OneToOneField(
        to='circuits.CircuitTermination',
        on_delete=models.SET_NULL,
        related_name='+',
        blank=True,
        null=True
    )
    connection_status = models.BooleanField(
        choices=CONNECTION_STATUS_CHOICES,
        blank=True,
        null=True
    )
    lag = models.ForeignKey(
        to='self',
        on_delete=models.SET_NULL,
        related_name='member_interfaces',
        null=True,
        blank=True,
        verbose_name='Parent LAG'
    )
    type = models.CharField(
        max_length=50,
        choices=InterfaceTypeChoices
    )
    mgmt_only = models.BooleanField(
        default=False,
        verbose_name='OOB Management',
        help_text='This interface is used only for out-of-band management'
    )
    untagged_vlan = models.ForeignKey(
        to='ipam.VLAN',
        on_delete=models.SET_NULL,
        related_name='interfaces_as_untagged',
        null=True,
        blank=True,
        verbose_name='Untagged VLAN'
    )
    tagged_vlans = models.ManyToManyField(
        to='ipam.VLAN',
        related_name='interfaces_as_tagged',
        blank=True,
        verbose_name='Tagged VLANs'
    )
    ip_addresses = GenericRelation(
        to='ipam.IPAddress',
        content_type_field='assigned_object_type',
        object_id_field='assigned_object_id',
        related_query_name='interface'
    )
    tags = TaggableManager(through=TaggedItem)

    csv_headers = [
        'device', 'name', 'label', 'lag', 'type', 'enabled', 'mac_address', 'mtu', 'mgmt_only', 'description', 'mode',
    ]

    class Meta:
        ordering = ('device', CollateAsChar('_name'))
        unique_together = ('device', 'name')

    def get_absolute_url(self):
        return reverse('dcim:interface', kwargs={'pk': self.pk})

    def to_csv(self):
        return (
            self.device.identifier if self.device else None,
            self.name,
            self.label,
            self.lag.name if self.lag else None,
            self.get_type_display(),
            self.enabled,
            self.mac_address,
            self.mtu,
            self.mgmt_only,
            self.description,
            self.get_mode_display(),
        )

    def clean(self):

        # Virtual interfaces cannot be connected
        if self.type in NONCONNECTABLE_IFACE_TYPES and (
                self.cable or getattr(self, 'circuit_termination', False)
        ):
            raise ValidationError({
                'type': "Virtual and wireless interfaces cannot be connected to another interface or circuit. "
                        "Disconnect the interface or choose a suitable type."
            })

        # An interface's LAG must belong to the same device (or VC master)
        if self.lag and self.lag.device not in [self.device, self.device.get_vc_master()]:
            raise ValidationError({
                'lag': "The selected LAG interface ({}) belongs to a different device ({}).".format(
                    self.lag.name, self.lag.device.name
                )
            })

        # A virtual interface cannot have a parent LAG
        if self.type in NONCONNECTABLE_IFACE_TYPES and self.lag is not None:
            raise ValidationError({
                'lag': "{} interfaces cannot have a parent LAG interface.".format(self.get_type_display())
            })

        # Only a LAG can have LAG members
        if self.type != InterfaceTypeChoices.TYPE_LAG and self.member_interfaces.exists():
            raise ValidationError({
                'type': "Cannot change interface type; it has LAG members ({}).".format(
                    ", ".join([iface.name for iface in self.member_interfaces.all()])
                )
            })

        # Validate untagged VLAN
        if self.untagged_vlan and self.untagged_vlan.site not in [self.parent.site, None]:
            raise ValidationError({
                'untagged_vlan': "The untagged VLAN ({}) must belong to the same site as the interface's parent "
                                 "device, or it must be global".format(self.untagged_vlan)
            })

    def save(self, *args, **kwargs):

        # Remove untagged VLAN assignment for non-802.1Q interfaces
        if self.mode is None:
            self.untagged_vlan = None

        # Only "tagged" interfaces may have tagged VLANs assigned. ("tagged all" implies all VLANs are assigned.)
        if self.pk and self.mode != InterfaceModeChoices.MODE_TAGGED:
            self.tagged_vlans.clear()

        return super().save(*args, **kwargs)

    @property
    def connected_endpoint(self):
        """
        Return the connected Interface, if it exists, or the connected CircuitTermination, if it exists. We have to
        check for ObjectDoesNotExist in case the referenced object has been deleted from the database.
        """
        try:
            if self._connected_interface:
                return self._connected_interface
        except ObjectDoesNotExist:
            pass
        try:
            if self._connected_circuittermination:
                return self._connected_circuittermination
        except ObjectDoesNotExist:
            pass
        return None

    @connected_endpoint.setter
    def connected_endpoint(self, value):
        from circuits.models import CircuitTermination

        if value is None:
            self._connected_interface = None
            self._connected_circuittermination = None
        elif isinstance(value, Interface):
            self._connected_interface = value
            self._connected_circuittermination = None
        elif isinstance(value, CircuitTermination):
            self._connected_interface = None
            self._connected_circuittermination = value
        else:
            raise ValueError(
                "Connected endpoint must be an Interface or CircuitTermination, not {}.".format(type(value))
            )

    @property
    def parent(self):
        return self.device

    @property
    def is_connectable(self):
        return self.type not in NONCONNECTABLE_IFACE_TYPES

    @property
    def is_virtual(self):
        return self.type in VIRTUAL_IFACE_TYPES

    @property
    def is_wireless(self):
        return self.type in WIRELESS_IFACE_TYPES

    @property
    def is_lag(self):
        return self.type == InterfaceTypeChoices.TYPE_LAG

    @property
    def count_ipaddresses(self):
        return self.ip_addresses.count()


#
# Pass-through ports
#

@extras_features('webhooks')
class FrontPort(CableTermination, ComponentModel):
    """
    A pass-through port on the front of a Device.
    """
    type = models.CharField(
        max_length=50,
        choices=PortTypeChoices
    )
    rear_port = models.ForeignKey(
        to='dcim.RearPort',
        on_delete=models.CASCADE,
        related_name='frontports'
    )
    rear_port_position = models.PositiveSmallIntegerField(
        default=1,
        validators=[MinValueValidator(1), MaxValueValidator(64)]
    )
    tags = TaggableManager(through=TaggedItem)

    csv_headers = ['device', 'name', 'label', 'type', 'rear_port', 'rear_port_position', 'description']

    class Meta:
        ordering = ('device', '_name')
        unique_together = (
            ('device', 'name'),
            ('rear_port', 'rear_port_position'),
        )

    def get_absolute_url(self):
        return reverse('dcim:frontport', kwargs={'pk': self.pk})

    def to_csv(self):
        return (
            self.device.identifier,
            self.name,
            self.label,
            self.get_type_display(),
            self.rear_port.name,
            self.rear_port_position,
            self.description,
        )

    def clean(self):

        # Validate rear port assignment
        if self.rear_port.device != self.device:
            raise ValidationError(
                "Rear port ({}) must belong to the same device".format(self.rear_port)
            )

        # Validate rear port position assignment
        if self.rear_port_position > self.rear_port.positions:
            raise ValidationError(
                "Invalid rear port position ({}); rear port {} has only {} positions".format(
                    self.rear_port_position, self.rear_port.name, self.rear_port.positions
                )
            )


@extras_features('webhooks')
class RearPort(CableTermination, ComponentModel):
    """
    A pass-through port on the rear of a Device.
    """
    type = models.CharField(
        max_length=50,
        choices=PortTypeChoices
    )
    positions = models.PositiveSmallIntegerField(
        default=1,
        validators=[MinValueValidator(1), MaxValueValidator(64)]
    )
    tags = TaggableManager(through=TaggedItem)

    csv_headers = ['device', 'name', 'label', 'type', 'positions', 'description']

    class Meta:
        ordering = ('device', '_name')
        unique_together = ('device', 'name')

    def get_absolute_url(self):
        return reverse('dcim:rearport', kwargs={'pk': self.pk})

    def to_csv(self):
        return (
            self.device.identifier,
            self.name,
            self.label,
            self.get_type_display(),
            self.positions,
            self.description,
        )


#
# Device bays
#

@extras_features('webhooks')
class DeviceBay(ComponentModel):
    """
    An empty space within a Device which can house a child device
    """
    installed_device = models.OneToOneField(
        to='dcim.Device',
        on_delete=models.SET_NULL,
        related_name='parent_bay',
        blank=True,
        null=True
    )
    tags = TaggableManager(through=TaggedItem)

    csv_headers = ['device', 'name', 'label', 'installed_device', 'description']

    class Meta:
        ordering = ('device', '_name')
        unique_together = ('device', 'name')

    def get_absolute_url(self):
        return reverse('dcim:devicebay', kwargs={'pk': self.pk})

    def to_csv(self):
        return (
            self.device.identifier,
            self.name,
            self.label,
            self.installed_device.identifier if self.installed_device else None,
            self.description,
        )

    def clean(self):

        # Validate that the parent Device can have DeviceBays
        if not self.device.device_type.is_parent_device:
            raise ValidationError("This type of device ({}) does not support device bays.".format(
                self.device.device_type
            ))

        # Cannot install a device into itself, obviously
        if self.device == self.installed_device:
            raise ValidationError("Cannot install a device into itself.")

        # Check that the installed device is not already installed elsewhere
        if self.installed_device:
            current_bay = DeviceBay.objects.filter(installed_device=self.installed_device).first()
            if current_bay and current_bay != self:
                raise ValidationError({
                    'installed_device': "Cannot install the specified device; device is already installed in {}".format(
                        current_bay
                    )
                })


#
# Inventory items
#

@extras_features('export_templates', 'webhooks')
class InventoryItem(ComponentModel):
    """
    An InventoryItem represents a serialized piece of hardware within a Device, such as a line card or power supply.
    InventoryItems are used only for inventory purposes.
    """
    parent = models.ForeignKey(
        to='self',
        on_delete=models.CASCADE,
        related_name='child_items',
        blank=True,
        null=True
    )
    manufacturer = models.ForeignKey(
        to='dcim.Manufacturer',
        on_delete=models.PROTECT,
        related_name='inventory_items',
        blank=True,
        null=True
    )
    part_id = models.CharField(
        max_length=50,
        verbose_name='Part ID',
        blank=True,
        help_text='Manufacturer-assigned part identifier'
    )
    serial = models.CharField(
        max_length=50,
        verbose_name='Serial number',
        blank=True
    )
    asset_tag = models.CharField(
        max_length=50,
        unique=True,
        blank=True,
        null=True,
        verbose_name='Asset tag',
        help_text='A unique tag used to identify this item'
    )
    discovered = models.BooleanField(
        default=False,
        help_text='This item was automatically discovered'
    )

    tags = TaggableManager(through=TaggedItem)

    csv_headers = [
        'device', 'name', 'label', 'manufacturer', 'part_id', 'serial', 'asset_tag', 'discovered', 'description',
    ]

    class Meta:
        ordering = ('device__id', 'parent__id', '_name')
        unique_together = ('device', 'parent', 'name')

    def get_absolute_url(self):
        return reverse('dcim:inventoryitem', kwargs={'pk': self.pk})

    def to_csv(self):
        return (
            self.device.name or '{{{}}}'.format(self.device.pk),
            self.name,
            self.label,
            self.manufacturer.name if self.manufacturer else None,
            self.part_id,
            self.serial,
            self.asset_tag,
            self.discovered,
            self.description,
        )