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How to mark a model field as "non-persisted", i.e. to not be pushed to the DB? #147
Description
First Check
- I added a very descriptive title to this issue.
- I used the GitHub search to find a similar issue and didn't find it.
- I searched the SQLModel documentation, with the integrated search.
- I already searched in Google "How to X in SQLModel" and didn't find any information.
- I already read and followed all the tutorial in the docs and didn't find an answer.
- I already checked if it is not related to SQLModel but to Pydantic.
- I already checked if it is not related to SQLModel but to SQLAlchemy.
Commit to Help
- I commit to help with one of those options 👆
Example Code
from typing import Optional
from pydantic.fields import PrivateAttr
from sqlmodel import Field, Session, SQLModel, create_engine, select
class Hero(SQLModel, table=True):
id: Optional[int] = Field(default=None, primary_key=True)
_name: str = PrivateAttr() # This field is not committed to the db
secret_name: str
sqlite_file_name = "database.db"
sqlite_url = f"sqlite:///{sqlite_file_name}"
engine = create_engine(sqlite_url, echo=True)
if __name__ == "__main__":
SQLModel.metadata.create_all(engine)
hero_1 = Hero(secret_name="Dive Wilson")
hero_1._name = "hello"
print(hero_1) # Hero will have a _hello attribute
with Session(engine) as session:
session.add(hero_1)
session.commit()
with Session(engine) as session:
statement = select(Hero)
results = session.exec(statement)
for hero in results:
print(hero) # Hero will not have a _hello attribute (when read from the db)Description
It is unclear how to add fields to a model that should not be written to the DB. I have achieved this by prepending the attribute name with an underscore as shown in the code sample above. I decided to achieve it that way based on how Pydantic handles these private attributes (https://pydantic-docs.helpmanual.io/usage/models/#automatically-excluded-attributes).
If there is a better way to do this I'd love to know. Alternatively I'd love to update the documentation if this is the correct way. I took a look at the file structure a bit but wasn't entirely sure where the best place to add it was, so I decided to open an issue instead of a PR.
Operating System
macOS
Operating System Details
No response
SQLModel Version
0.0.4
Python Version
Python 3.9.5
Additional Context
No response
Activity
adjaunzemis commentedon Oct 27, 2021
It looks like you're working through the tutorials. I think the HeroBase vs Hero vs HeroRead vs... construct using inheritance in the later sections on "FastAPI and Pydantic" might be what you're looking for:
https://sqlmodel.tiangolo.com/tutorial/fastapi/multiple-models/
Basically, only the class with table=true will define the table columns. You can inherit from this class to add other fields that won't show up in the table.
JLHasson commentedon Oct 28, 2021
@adjaunzemis thanks for the response. Looking through that page again it looks like I can get to what I want, but IMO it's messy. Here's what I've done:
This works but IMO it's messy. From the docs:
It would be much nicer if I could just do
And then just use
Heroeverywhere, and only have the values inHeroBase(the class withtable=True) committed to the DB.Unless there is a better way to do the casting that I've done above I think I will just keep using
PrivateAttralong with@propertydecorators to keep values on the same class and just know that private attributes aren't written to the DB.Of course, this comes with it's own set of problems 🙄 pydantic/pydantic#1577
Also, as far as I can tell
SQLModeldoesn't properly set thedefault=Nonevalue frompydantic.PrivateAttron table classes, may open a separate issue for that one.JLHasson commentedon Oct 28, 2021
Closing because the comment above technically solves the issue, but IMO it could be improved
JLHasson commentedon Oct 28, 2021
Opened new issue for the
PrivateAttrissue: #149