How does SQLAlchemy handle checking if child classes have been inserted or not?

I'm testing out the classical style of mapping for SQL Alchemy. The problem I'm having is when I insert an object that has multiple database objects as properties. My model is like so:

class File():

    def __init__(self, fileName: str, modifiedDate: datetime) -> None:
        self.fileName = fileName
        self.modifiedDate = modifiedDate

    def __eq__(self, other: 'File') -> bool:
        return self.fileName == other.fileName

    def __hash__(self) -> int:
        return hash(self.fileName)

@dataclass(unsafe_hash=True)
class PackageID():
    packageRef: str

@dataclass
class LineItem():
    customerID: int
    file: File
    package: PackageID

My mappings are as follows:

Files = Table(
    "files", 
    mapper_registry.metadata, 
    Column('ID', Integer, primary_key=True, autoincrement=True),
    Column('fileName', String(255), nullable=False, unique=True),
    Column('modifiedDate', Date, nullable=False))

Packageids = Table(
    'packageids',
    mapper_registry.metadata,
    Column('ID', Integer, primary_key=True, autoincrement=True),
    Column('packageRef', String(255), nullable=False, unique=True)
)

Lineitems = Table(
    'lineitems',
    mapper_registry.metadata,
    Column('ID', Integer, primary_key=True),
    Column('customerID', Integer),
    Column('fileID', ForeignKey('files.ID'), nullable=False),
    Column('packageID', ForeignKey('packageids.ID'), nullable=False)
)

def startMappers():
    lineMapper = mapper_registry.map_imperatively(models.LineItem, Lineitems)
    fileMapper = mapper_registry.map_imperatively(models.File, Files, properties={'lineitem': relationship(models.LineItem, backref='file')})
    packageMapper = mapper_registry.map_imperatively(models.PackageID, Packageids, properties={'linteitem': relationship(models.LineItem, backref='package')})

And lastly the code the handles entering the objects to the database:

def testInsert():
    engine = startDB()

    file = models.File('TestFile12345.xlsx', datetime.now())
    package = models.PackageID(packageRef='TestID12345678')
    li = models.LineItem(11111, file=file, package=package)

    with Session(engine) as session:
        fp = FileRepository(session)
        pp = PackageRepository(session)
        lp = LineItemRepository(session)

        file = fp.get(file)
        package = pp.get(package)

        lp.add(li)

        session.commit()

Here the fp.get and pp.get just update the package to an existing model from the database if it exists. The first time I run this it works fine. If I try to enter another entry that has the same file I get a unique error constraint on the file table. I can't figure out the proper way of doing this. Is there a way to just add the line item instead of adding the file & package as well?