Hi everyone,
I m trying to get to grips with SQLAlchemy by parsing a file, extracting
certain fields and then storing them in a database. I am currently using the
object relational and trying to get a sense of how it all works. I have two
objects Paper and Author, which is a many to many relationship. So currently
I am parsing the file and then creating a Paper object
names = [ list of author names ]
authors = [ Author(a) for a in names ]
p = Paper(title, authors)
session.add(p)
session.commit(p)
However, when I get to a new paper with an already existing author I get the
following error,
sqlalchemy.exceptions.IntegrityError: (IntegrityError) (1062, "Duplicate
entry 'Sole R' for key 'name'") u'INSERT INTO authors (name) VALUES (%s)'
['Sole R']
which makes sense, however I d prefer not to query the database over and
over to check if "Sole R" has already been added to the database. So I try a
different approach without building Author objects and using the name string
instead as an arg to Paper:
and get the error:
AttributeError: 'str' object has no attribute '_state'
So I'm out of ideas, I want to make it fast as possible and without firing
off lots of queries to the database to look for identity. Is there a way
supported by the ORM that I ve missed completely, so is it more normal to
create a cache whereby you add things to a dict and look for identity in the
dict otherwise query the database? The problem is that some of the files I
am going to be parsing are quite large (Gb) and I dont want to saturate my
database server with requests and likewise I only have limited memory on my
machines.
Have I missed something simple in the documentation?
My code is below:
Many thanks in advance,
Nathan
document_table = Table('documents', metadata,
Column('id', Integer, primary_key=True),
Column('title', String, nullable=False),
)
authors_table = Table('authors', metadata,
Column('id', Integer, primary_key=True),
Column('name', String(40), unique=True),
)
papers_to_authors_table = Table('p2a_assocation', metadata,
Column('document', Integer, ForeignKey('
documents.id')),
Column('author', Integer, ForeignKey('
authors.id'))
)
class Author(object):
def __init__(self, name):
self.name = name
class Paper(object):
def __init__(self, title, authors):
self.title = title
self.authors = author
mapper(Paper, document_table, properties={'authors':relation(Author,
secondary=papers_to_authors_table, backref='publications')})
mapper(Author, authors_table)
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