Question

In declarative approach, I want to exclude one property, its working properly when my column name and property name are same. But if I give different name then its not working.
Here is my sample code.

Base = declarative_base() 
class tblUser(Base):
    __tablename__ = 'tblUser'
    User_Id = Column('User_Id', String(100), primary_key=True)
    SequenceNo = Column('Sequence_No', Integer)
    FullName = Column('FullName', String(50))
    __mapper_args__ = {'exclude_properties' :['Sequence_No']}

user = tblUser()
user.User_Id = '1000001'
user.SequenceNo = 101
session.add(user)
session.commit()

In the above sample I don’t want the SequenceNo property to be updated in database even if I assign some value to it. So I used exclude_properties but still its updating the value in db. But if I change the property name from SequenceNo to Sequence_No (same as the column name) then its working as per the behaviour. Can anyone help me?

Thanks
Adhi

Answer 1

Unfortunately, __mapper_args__ is probably the wrong approach. It is intended to control the reflection of an existing database table into a mapper, not make a column ‘read-only’.

I think a better approach would be to use a hybrid property:

from sqlalchemy.ext.hybrid import hybrid_property

Base = declarative_base() 
class tblUser(Base):
    __tablename__ = 'tblUser'
    User_Id = Column('User_Id', String(100), primary_key=True)
    FullName = Column('FullName', String(50))

    _Sequence_No = Column('Sequence_No', Integer)
    _local_Sequence_No = None

    @hybrid_property
    @property
    def SequenceNo(self):
        if self._local_Sequence_No is not None:
            return self._local_Sequence_No
        return self._SequenceNo

    @SequenceNo.setter
    def SequenceNo(self, value):
        self._local_Sequence_No = value

The original Sequence_No column is available via a private attribute, and the SequenceNo property intercepts writes and stores them on the instance to be re-used later, but not written to the database.