func (s *AssignSuite) assertAssignedUnit(c *gc.C, unit *state.Unit) string {
// Get service networks.
service, err := unit.Service()
c.Assert(err, gc.IsNil)
includeNetworks, excludeNetworks, err := service.Networks()
c.Assert(err, gc.IsNil)
// Check the machine on the unit is set.
machineId, err := unit.AssignedMachineId()
c.Assert(err, gc.IsNil)
// Check that the principal is set on the machine.
machine, err := s.State.Machine(machineId)
c.Assert(err, gc.IsNil)
include, exclude, err := machine.RequestedNetworks()
c.Assert(err, gc.IsNil)
c.Assert(include, gc.DeepEquals, includeNetworks)
c.Assert(exclude, gc.DeepEquals, excludeNetworks)
machineUnits, err := machine.Units()
c.Assert(err, gc.IsNil)
c.Assert(machineUnits, gc.HasLen, 1)
// Make sure it is the right unit.
c.Assert(machineUnits[0].Name(), gc.Equals, unit.Name())
return machineId
}