def get(self, id, pk='id', deleted=False):
        try:
            if pk == 'id':
                id = int(id)
            if deleted:
                spec = {pk: id}
            else:
                spec = {pk: id, 'data': {'$exists': 1}}
            fields = {'_id': 0}
            row = self.db.docs.find_one(spec, fields)
        except Exception as e:
            raise DatabaseException(e)
        if row:
            return Document(**row)
        raise NotFoundError('%s %s not found' % (pk, id))
def loss_density(isa, subreg=[], ax=0):
    """plot loss density of NC/I7/ISA7 model
    Args:
      isa: Isa7/NC object
    """
    if subreg:
        if isinstance(subreg, list):
            sr = subreg
        else:
            sr = [subreg]
        elements = [e for s in sr for sre in isa.get_subregion(s).elements()
                    for e in sre]
    else:
        elements = [e for e in isa.elements]

    lossd = np.array([e.loss_density*1e-3 for e in elements])
    _contour(ax, 'Loss Density kW/m³', elements, lossd)