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from json import dumps
from sqlalchemy.orm import class_mapper

def serialize(model):
  """Transforms a model into a dictionary which can be dumped to JSON."""
  # first we get the names of all the columns on your model
  columns = [c.key for c in class_mapper(model.__class__).columns]
  # then we return their values in a dict
  return dict((c, getattr(model, c)) for c in columns)

# we can then use this for your particular example
serialized_labels = [
  serialize(label)
  for label in session.query(LabelsData).filter(LabelsData.deleted == False)
]
your_json = dumps(serialized_labels)

class User:
   def as_dict(self):
       return {c.name: getattr(self, c.name) for c in self.__table__.columns}