def METHOD_NAME(self, element_id=None):
    if element_id is None:
        element_id = self.element_id
    n = len(element_id)
    rho = zeros(n, dtype='float64')
    i = where(element_id == self.element_id)[0]
    for pid in self.property_id[i]:
        j = where(pid == self.property_id[i])[0]
        rhoi = self.model.properties_solid.psolid.get_density_by_property_id(pid)
        rho[j] = rhoi
    assert rho.shape == (n, ), rho.shape
    return rho