Updates to JSON field don't persist to DB

Question

We have a model with a JSON field where user flags get inserted. Inserting does work as expected, but when removing certain flags, they stay in the field and changes don't get persisted to the DB.

We have the following method in our model:

def del_flag(self, key):
    if self.user_flags is None or not key in self.user_flags:
        return False
    else:
        del self.user_flags[key]
        db.session.commit()        
        return True

The databasse is postgres and we use the SQLalchemy JSON field dialect for the field type. Any advice on this?

Answer 1

If you are using Postgres < 9.4 you can't update JSON field directly. You need flag_modified function to report the change to SQLAlchemy:

from sqlalchemy.orm.attributes import flag_modified
model.data['key'] = 'New value'
flag_modified(model, "data")
session.add(model)
session.commit()

Answer 2

My issue was referencing the row object returned from SQLAlchemy when creating the new row. e.g. this does not work:

row = db.session.query(SomeTable).filter_by(id=someId).first()
print(row.details)
newDetails = row.details
newDetails['key'] = 'new data'
row.details = newDetails
db.session.commit()

but creating a new dict does work

row = db.session.query(SomeTable).filter_by(id=someId).first()
print(row.details)
newDetails = dict(row.details)
newDetails['key'] = 'new data'
row.details = newDetails
db.session.commit()

notice dict(row.details)

Answer 3

I'm using JSON field and I referred below document.

https://docs.sqlalchemy.org/en/13/core/type_basics.html?highlight=json#sqlalchemy.types.JSON

It shows how to make JSON-dict field mutable. (Default is immutable)

like this..

from sqlalchemy.ext.mutable import MutableDict
from sqlalchemy import Column, Integer, JSON

class TableABC(Base):
    __tablename__ = 'table_abc'
    id = Column(Integer, primary_key=True)
    info = Column(MutableDict.as_mutable(JSON))

Then I could update json field as ORM.