func (e *exporter) setUnitPayloads(exUnit description.Unit, payloads []payload.FullPayloadInfo) error {
	unitID := exUnit.Tag().Id()
	machineID := exUnit.Machine().Id()
	for _, payload := range payloads {
		if payload.Machine != machineID {
			return errors.NotValidf("payload for unit %q reports wrong machine %q (should be %q)", unitID, payload.Machine, machineID)
		}
		args := description.PayloadArgs{
			Name:   payload.Name,
			Type:   payload.Type,
			RawID:  payload.ID,
			State:  payload.Status,
			Labels: payload.Labels,
		}
		exUnit.AddPayload(args)
	}
	return nil
}

func (i *importer) makeUnitDoc(s description.Service, u description.Unit) (*unitDoc, error) {
	// NOTE: if we want to support units having different charms deployed
	// than the service recomments and migrate that, then we should serialize
	// the charm url for each unit rather than grabbing the services charm url.
	// Currently the units charm url matching the service is a precondiation
	// to migration.
	charmUrl, err := charm.ParseURL(s.CharmURL())
	if err != nil {
		return nil, errors.Trace(err)
	}

	var subordinates []string
	if subs := u.Subordinates(); len(subs) > 0 {
		for _, s := range subs {
			subordinates = append(subordinates, s.Id())
		}
	}

	return &unitDoc{
		Name:         u.Name(),
		Service:      s.Name(),
		Series:       s.Series(),
		CharmURL:     charmUrl,
		Principal:    u.Principal().Id(),
		Subordinates: subordinates,
		// StorageAttachmentCount int `bson:"storageattachmentcount"`
		MachineId:    u.Machine().Id(),
		Tools:        i.makeTools(u.Tools()),
		Life:         Alive,
		PasswordHash: u.PasswordHash(),
	}, nil
}

func (i *importer) unit(s description.Service, u description.Unit) error {
	i.logger.Debugf("importing unit %s", u.Name())

	// 1. construct a unitDoc
	udoc, err := i.makeUnitDoc(s, u)
	if err != nil {
		return errors.Trace(err)
	}

	// 2. construct a statusDoc for the workload status and agent status
	agentStatus := u.AgentStatus()
	if agentStatus == nil {
		return errors.NotValidf("missing agent status")
	}
	agentStatusDoc := i.makeStatusDoc(agentStatus)

	workloadStatus := u.WorkloadStatus()
	if workloadStatus == nil {
		return errors.NotValidf("missing workload status")
	}
	workloadStatusDoc := i.makeStatusDoc(workloadStatus)

	ops := addUnitOps(i.st, addUnitOpsArgs{
		unitDoc:           udoc,
		agentStatusDoc:    agentStatusDoc,
		workloadStatusDoc: workloadStatusDoc,
		meterStatusDoc: &meterStatusDoc{
			Code: u.MeterStatusCode(),
			Info: u.MeterStatusInfo(),
		},
	})
	// We should only have constraints for principal agents.
	// We don't encode that business logic here, if there are constraints
	// in the imported model, we put them in the database.
	if cons := u.Constraints(); cons != nil {
		agentGlobalKey := unitAgentGlobalKey(u.Name())
		ops = append(ops, createConstraintsOp(i.st, agentGlobalKey, i.constraints(cons)))
	}

	if err := i.st.runTransaction(ops); err != nil {
		return errors.Trace(err)
	}

	unit := newUnit(i.st, udoc)
	if annotations := u.Annotations(); len(annotations) > 0 {
		if err := i.st.SetAnnotations(unit, annotations); err != nil {
			return errors.Trace(err)
		}
	}
	if err := i.importStatusHistory(unit.globalKey(), u.WorkloadStatusHistory()); err != nil {
		return errors.Trace(err)
	}
	if err := i.importStatusHistory(unit.globalAgentKey(), u.AgentStatusHistory()); err != nil {
		return errors.Trace(err)
	}

	return nil
}

func (i *importer) unit(s description.Application, u description.Unit) error {
	i.logger.Debugf("importing unit %s", u.Name())

	// 1. construct a unitDoc
	udoc, err := i.makeUnitDoc(s, u)
	if err != nil {
		return errors.Trace(err)
	}

	// 2. construct a statusDoc for the workload status and agent status
	agentStatus := u.AgentStatus()
	if agentStatus == nil {
		return errors.NotValidf("missing agent status")
	}
	agentStatusDoc := i.makeStatusDoc(agentStatus)

	workloadStatus := u.WorkloadStatus()
	if workloadStatus == nil {
		return errors.NotValidf("missing workload status")
	}
	workloadStatusDoc := i.makeStatusDoc(workloadStatus)

	workloadVersion := u.WorkloadVersion()
	versionStatus := status.Active
	if workloadVersion == "" {
		versionStatus = status.Unknown
	}
	workloadVersionDoc := statusDoc{
		Status:     versionStatus,
		StatusInfo: workloadVersion,
	}

	ops, err := addUnitOps(i.st, addUnitOpsArgs{
		unitDoc:            udoc,
		agentStatusDoc:     agentStatusDoc,
		workloadStatusDoc:  workloadStatusDoc,
		workloadVersionDoc: workloadVersionDoc,
		meterStatusDoc: &meterStatusDoc{
			Code: u.MeterStatusCode(),
			Info: u.MeterStatusInfo(),
		},
	})
	if err != nil {
		return errors.Trace(err)
	}

	// If the unit is a principal, add it to its machine.
	if u.Principal().Id() == "" {
		ops = append(ops, txn.Op{
			C:      machinesC,
			Id:     u.Machine().Id(),
			Assert: txn.DocExists,
			Update: bson.M{"$addToSet": bson.M{"principals": u.Name()}},
		})
	}

	// We should only have constraints for principal agents.
	// We don't encode that business logic here, if there are constraints
	// in the imported model, we put them in the database.
	if cons := u.Constraints(); cons != nil {
		agentGlobalKey := unitAgentGlobalKey(u.Name())
		ops = append(ops, createConstraintsOp(i.st, agentGlobalKey, i.constraints(cons)))
	}

	if err := i.st.runTransaction(ops); err != nil {
		i.logger.Debugf("failed ops: %#v", ops)
		return errors.Trace(err)
	}

	unit := newUnit(i.st, udoc)
	if annotations := u.Annotations(); len(annotations) > 0 {
		if err := i.st.SetAnnotations(unit, annotations); err != nil {
			return errors.Trace(err)
		}
	}
	if err := i.importStatusHistory(unit.globalKey(), u.WorkloadStatusHistory()); err != nil {
		return errors.Trace(err)
	}
	if err := i.importStatusHistory(unit.globalAgentKey(), u.AgentStatusHistory()); err != nil {
		return errors.Trace(err)
	}
	if err := i.importStatusHistory(unit.globalWorkloadVersionKey(), u.WorkloadVersionHistory()); err != nil {
		return errors.Trace(err)
	}
	if err := i.importUnitPayloads(unit, u.Payloads()); err != nil {
		return errors.Trace(err)
	}

	return nil
}