write to multiple contexts in a single session

[zuotian]

Hi,

I have been using rdflib for a while, and just recently switched to
surfrdf. I really like the project.

My current script needs to write triples to a sesame store in multiple
contexts (named graphs). For example

store = Store(reader='sesame2', ..., default_context='http://
context1')
session.Session(store)
Person = session.get_class(ns.FOAF.Person)

person1 = Person()
person.save()

# now I need to write another person to a different context.
person2 = Person()
person2.foaf_knows = person1
person2.save()

How can I save person2 to context2? Do I need to create a different
store and recreate Person class? The script will generate 1 million
triples in 20k named graphs.

Thanks,
Zuotian

[Cosmin Basca]

Hi Zuotian,

I haven't tried this solution, but it should work. You need to change
the value of the *context* attribute before saving. For example in you
code:

# this is save din the default context you declared for the store
person1 = Person()
person1.save()

# to save the second person in a diff context do this:
person2 = Person(context='http://mysecondcontext.com')
person2.foaf_knows = person1
person2.save()

# another way to do it is like this:
person2 = Person()
person2.context = 'http://mysecondcontext.com'
person2.save()

Hope this helps,
Cheers,
Cosmin

[zuotian]

Great! That's exactly what I am looking for.

Thanks,
Zuotian

[zuotian]

Hi Cosmin,

One more scenario.

# Suppose I have person1 in default context
person1 = Person()
person1.context = context1
person1.foaf_name = "me"
person1.save()

# now I want to add more triples about person1 but save them in a
different context.
person1.foaf_knows = "you"
person1.context = context2
person1.save()

When the second save is called, not surprisingly, person1's foaf_name
is also saved to context2. So I reinitialized person1 for context2, so
that foaf_name is no longer in context2. However, the triple "person1
rdf:type Person" still exists in both contexts.

My question is, is there a way to save an object and only write out
the properties that has not been save to the store?

Thanks,
Zuotian

[Cosmin Basca]

Hi Zuotian,

Surf is not intended to be a generic RDF management framework, it's
main purpose is to bridge the gap between the object-oriented paradigm
and that of RDF. The short answer to your scenario is no. Let me
detail: when an instance of a class is created it will always have a
triple of type <instance_subject, rdf:type, MyClassType> associated,
that triple will go into the current context when saved. As with most
things in python, you can "hack" a solution for your problem but it
won't be pretty. You can either remove the rdf:type triple from the
store after you save your instances or re-implement the graph() method
of your particular resource either for all instances (via subclassing)
or a specific instance by replacing the method dynamically
(person1.graph = my_graph_method). Inside this method you should
remove that triple before you return the graph. This is however a hack
and I would advise you not to do it as it can introduce hard to spot
bugs, potentially breaking down Surf. Hope this clarifies your
question,

Cheers,
Cosmin