Question

I’m having a great time learning Python, but I’ve just gotten a bit stuck on trying to incorporate a recursive function into SQLAlchemy.

Essentially, have a function which creates a instance of a class to be put in the database. Inside this function, I get user input on whether the instance has a parent class (defined using a self-referential adjacency table). If it does, the function is then called again, recursively. This function seems to work if no parent class is needed, but whenever the recursive element is activated it crashes.

My code is like so:

engine = create_engine('sqlite:///recDB.db')
Session = sessionmaker(bind=engine)
session = Session()
Base = declarative_base()

class IngList(Base):
    __tablename__ = "ingList"

    id = Column(Integer, primary_key = True)
    ingredient = Column(String, nullable=False)
    parentIng = Column(Integer, ForeignKey('ingList.id'))
    children = relationship("IngList",backref=backref('parent', remote_side=[id]))

    def __init__(self, ingredient):
        self.ingredient = ingredient

def addIngredient(ingredient):
    global newIngList
    newIng = IngList(ingName) #create new class instance
    parentIng = raw_input("To add parent ingredient, type it.  Otherwise press enter")
    if parentIng != '':
        parentIngObj = addIngredient(parentIng) # Recursion!
        newIng.parentIng = parentIngObj
    newIngList.append(newIng)

if __name__ == '__main__':
    newIngList = []
    ingredient = raw_input("Enter new ingredient")
    addIngredient(ingredient)
    for ing in newIngList
        session.add(ing)
    session.commit()

I’ve kept this sample simple to keep it readable, but if I’m missing some important information, please let me know.

I assumed that my problem was that the class instances were losing scope as I recursed, but adding the global to the list variable did not seem to fix that. I also thought that the fact that a session acts as sort of a buffer would handle any scope problems.

Does this have something to do with eager loading? I saw that in the documentation, but don’t really understand it.

The error I’m getting is:

Traceback (most recent call last):
  File "C:\workspace\recipes\langProc.py", line 102, in <module>
    session.commit()
  File "c:\Python27\lib\site-packages\sqlalchemy\orm\session.py", line 645, in commit
    self.transaction.commit()
  File "c:\Python27\lib\site-packages\sqlalchemy\orm\session.py", line 313, in commit
    self._prepare_impl()
  File "c:\Python27\lib\site-packages\sqlalchemy\orm\session.py", line 297, in _prepare_impl
    self.session.flush()
  File "c:\Python27\lib\site-packages\sqlalchemy\orm\session.py", line 1547, in flush
    self._flush(objects)
  File "c:\Python27\lib\site-packages\sqlalchemy\orm\session.py", line 1616, in _flush
    flush_context.execute()
  File "c:\Python27\lib\site-packages\sqlalchemy\orm\unitofwork.py", line 328, in execute
    rec.execute(self)
  File "c:\Python27\lib\site-packages\sqlalchemy\orm\unitofwork.py", line 472, in execute
    uow
  File "c:\Python27\lib\site-packages\sqlalchemy\orm\mapper.py", line 2153, in _save_obj
    execute(statement, params)
  File "c:\Python27\lib\site-packages\sqlalchemy\engine\base.py", line 1399, in execute
    params)
  File "c:\Python27\lib\site-packages\sqlalchemy\engine\base.py", line 1532, in _execute_clauseelement
    compiled_sql, distilled_params
  File "c:\Python27\lib\site-packages\sqlalchemy\engine\base.py", line 1640, in _execute_context
    context)
  File "c:\Python27\lib\site-packages\sqlalchemy\engine\base.py", line 1633, in _execute_context
    context)
  File "c:\Python27\lib\site-packages\sqlalchemy\engine\default.py", line 330, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.InterfaceError: (InterfaceError) Error binding parameter 0 
- probably unsupported type.
u'UPDATE "ingList" SET "parentIng"=? WHERE "ingList".id = ?' 
(<assignDB.IngList object at 0x00000000096969E8>, 4)

Answer 1

I think you have couple of bugs/typos which make your code not work. I assume that you created a small code sample to show the issue, and I hope that once you fix those in the original code, you problem will be solved as well:

• instead of newIng.parentIng = parentIngObj you should have newIng.parent = parentIngObj. I believe this should solve the issue.
  So you have to assign the parent instance to the relationship object, and not to its key. Using J.F. Sebastian‘s suggestions could work too if the objects were already stored in the database, but new instances do not have the id assigned yet
• addIngredient(...) has two issues:
  • typo: the parameter ingredient should be renamed to ingName or vice-versa
  • bigger issue: the addIngredient(...) does not return any value, so in fact you assign None to the parent side of the relationship.
    Again, given this is just a sample code, you might not have these issues in your real code.