func addRU(c *gc.C, svc *state.Service, rel *state.Relation, principal *state.Unit) (*state.Unit, *state.RelationUnit) {
	// Given the service svc in the relation rel, add a unit of svc and create
	// a RelationUnit with rel. If principal is supplied, svc is assumed to be
	// subordinate and the unit will be created by temporarily entering the
	// relation's scope as the principal.
	var u *state.Unit
	if principal == nil {
		unit, err := svc.AddUnit()
		c.Assert(err, jc.ErrorIsNil)
		u = unit
	} else {
		origUnits, err := svc.AllUnits()
		c.Assert(err, jc.ErrorIsNil)
		pru, err := rel.Unit(principal)
		c.Assert(err, jc.ErrorIsNil)
		err = pru.EnterScope(nil) // to create the subordinate
		c.Assert(err, jc.ErrorIsNil)
		err = pru.LeaveScope() // to reset to initial expected state
		c.Assert(err, jc.ErrorIsNil)
		newUnits, err := svc.AllUnits()
		c.Assert(err, jc.ErrorIsNil)
		for _, unit := range newUnits {
			found := false
			for _, old := range origUnits {
				if unit.Name() == old.Name() {
					found = true
					break
				}
			}
			if !found {
				u = unit
				break
			}
		}
		c.Assert(u, gc.NotNil)
	}
	preventUnitDestroyRemove(c, u)
	ru, err := rel.Unit(u)
	c.Assert(err, jc.ErrorIsNil)
	return u, ru
}

func (u *UniterAPIV3) prepareRelationResult(rel *state.Relation, unit *state.Unit) (params.RelationResult, error) {
	nothing := params.RelationResult{}
	ep, err := rel.Endpoint(unit.ApplicationName())
	if err != nil {
		// An error here means the unit's service is not part of the
		// relation.
		return nothing, err
	}
	return params.RelationResult{
		Id:   rel.Id(),
		Key:  rel.String(),
		Life: params.Life(rel.Life().String()),
		Endpoint: multiwatcher.Endpoint{
			ApplicationName: ep.ApplicationName,
			Relation:        multiwatcher.NewCharmRelation(ep.Relation),
		},
	}, nil
}

func (u *UniterAPI) prepareRelationResult(rel *state.Relation, unit *state.Unit) (params.RelationResult, error) {
	nothing := params.RelationResult{}
	ep, err := rel.Endpoint(unit.ServiceName())
	if err != nil {
		// An error here means the unit's service is not part of the
		// relation.
		return nothing, err
	}
	return params.RelationResult{
		Id:   rel.Id(),
		Key:  rel.String(),
		Life: params.Life(rel.Life().String()),
		Endpoint: params.Endpoint{
			ServiceName: ep.ServiceName,
			Relation:    ep.Relation,
		},
	}, nil
}