How to find one-to-many relations of a class?

[AlexVhr]

Hi all.

I need to find out if a certain class has one-to-many relations to other classes, and what exactly this classes are. Something like this:

class Parent(Base):

    __tablename__ = 'parent'

    id = Column(Integer, primary_key=True)

    children = relationship("Child")

class Child(Base):

    __tablename__ = 'child'

    id = Column(Integer, primary_key=True)

    parent_id = Column(Integer, ForeignKey('parent.id'))

I'm kind of half way there:

def findRelatedClasses(cls):

    #iterate cls.__dict__, find a class member with

    #property.direction.name == ‘ONETOMANY’.

    #Now, to find out which class it links to…

 

findRelatedClasses(Parent) should return a list of type objects ([Child] in this instance). Any ideas how to do that? Thanks!

[Juan Hernandez]

I'm not sure I get your question...

if you have this already

children = relationship("Child")

what is it that you need to know?? 

By using MyClass.children you'll have access to it's children and they will be, one2many relationships

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[AlexVhr]

I have "this allready" at compile-time. But at run-time I will have to deal with instances of different classes dinamically. So I need to inspect the instances to find out if they are linked to others or not, and what is that they are linked to. Something like this:

def print_related_records(instance):

   related_classes = find_related_classes(instance.__class__) #returns  {'children':Child}

   #iterate the related_classes dict, query all the related records, then print them

   ...

john = Parent('John Smith')

betty = Child('Betty')

john.children.append(betty)

...

print_related_records(john) #prints 'Betty'

print_related_records(betty) #prints nothing

[Michael Bayer]

On Dec 12, 2012, at 4:27 AM, AlexVhr wrote:

Hi all.

I need to find out if a certain class has one-to-many relations to other classes, and what exactly this classes are. Something like this:

I'm kind of half way there:

def findRelatedClasses(cls):

    #iterate cls.__dict__, find a class member with

    #property.direction.name == ‘ONETOMANY’.

    #Now, to find out which class it links to…

 

findRelatedClasses(Parent) should return a list of type objects ([Child] in this instance). Any ideas how to do that? Thanks!

property.mapper is the target Mapper and mapper.class_ is the class which is mapped.

in 0.7 you want to use class_mapper(cls).iterate_properties() to get at MapperProperty, don't bother with __dict__ (and even then I'd be using dir()/getattr() to deal with descriptors).

There's an improved introspection interface in 0.8, see: http://docs.sqlalchemy.org/en/rel_0_8/core/inspection.html , you'd use inspect(cls).relationships.

[AlexVhr]

Thanks a lot, it works perfectly. On a matter of using 0.8 - is it allready stable enough to switch to? (My projects are nothing critical, but still...)

[Michael Bayer]

yeah 0.8 is fine, was almost ready to put 0.8.0b2 and then final the other day, just little things coming in....

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[AlexVhr]

Glad to hear it, I'll give it a try.