如何在扩展单独表模型的表模型上设置默认值，因为它是 SQL Alchemy 中的基础

Question

I've had a good dig around various questions and seem to be missing something that I feel must be fairly obvious.

In Postgres, using Python, SQLAlchemy and FactoryBoy to run (now failing) tests, I have two table models...

• File
• Letter

When Letter is instantiated, it extends File :

class Letter(File):

--

and File in turn extends SQL Alchemys DeclarativeBase :

class File(DeclarativeBase):

--

Within File , is a property called content_type .

content_type has now been set on File to be not nullable .

--

What I need to do feels very basic...

Within Letter , I need to set the default content_type to "application/pdf" ... that's it!

However, for all my trying, I cannot seem to get it to set this as the default content type for Letter

This means that tests we have running, fail on trying to create the File as it has no content_type to pass over to File , meaning I get a NotNullViolation .

I have tried setting this within the Letter model in a few different ways, but to no avail.

--

Approaches:

• I tried using __init__ in both the Custom Base model of File and as well as in Letter using kawgs and attempting to set it that way, though this did not work.
• I have thought about adding the column into the Letter model, but this defeats Third Normal Form so I assume there is a better way to do it.
• I have attempted to pass the value through the test we are running but without any luck (tests are using FactoryBoy )
• There is a LetterFactory (from FactoryBoy ) which is where the issue is coming from, I have attempted to add content_type = "application/pdf" to this without any luck.
• I have also attempt to set the content_type on the model return in the FactoryBoy - LetterFactory , again :

class Meta(SQLAlchemyOptions):
    model = Letter
    model.content_type = "application/pdf"

• Within the Letter model itself, I included the following, also without any luck:

@property
    def content_type(self):
        return "application/pdf"

--

Apologies on what I think might be a basic question.

Any pointers would be very welcomed.

It's perhaps just a case of me not being familiar with the right terminology in SQL Alchemy and missing the obvious.

Does anyone understand my issue and have any threads they could give me to pull on?

Answer 1

This worked for me, I must have got something wrong initially:

File(Base):

...
def __init__(self, **kwargs):
    super().__init__(**kwargs)
...

Letter(File):

...
def __init__(self, **kwargs):
    kwargs.setdefault("content_type", "application/pdf")
    super().__init__(**kwargs)
...