What type is on the other end of relation in sqlalchemy without creating objects?

Question

How to obtain a name of the class that should be on the other end of the relation? It was declared when creating relationship. I guess that information should be somwhere in sqlalchemy.orm.util.class_mapper

Let's say we have these three classes and a relations between them. Book * --- 1 Shelf and Book * --- * Author

class Shelf(Base):
    __tablename__ = 'shelves'
    id = Column(Integer, primary_key = True)
    name = Column(String)
    #one-to-many books by backref in Book.shelf

    def __init__(self, name=""):
        self.name = name

class Book(Base):
    __tablename__ = 'books'
    id = Column(Integer, primary_key = True)
    title = Column(String)
    #many-to-one shelf
    shelf_id = Column(Integer, ForeignKey('shelves.id'))
    shelf = relationship(Shelf, backref=backref('books', order_by=id))
    def __init__(self, title=""):
        self.title = title

author_book = Table('author_book', metadata, 
                    Column('author_id', Integer, ForeignKey('authors.id')),
                    Column('book_id', Integer, ForeignKey('books.id'))
                    )

class Author(Base):
    __tablename__ = 'authors'
    id = Column(Integer, primary_key = True)
    name = Column(String)
    #many-to-many books
    books = relationship('Book', secondary=author_book, backref='authors')

    def __init__(self, name=""):
        self.name = name

From class_mapper(Class).iterate_properties we can easily get different properties: ColumnProperty and RelationshipProperty. There should be a way to get at leat the name of the class that is in relation. Any ideas?

Answer 1

I found a sort of solution, but I'm not 100% sure if it works always.To find a class (type ) expected on the other end of relation

class_mapper(Shelf)._props['books'].mapper.class_

We may not know the names of attributes (like books in this case), but it isn't much trouble to go through dictionary *_props* with looking for values witch are instances of sqlalchemy.orm.properties.RelationshipProperty

Answer 2

Maybe a little bit neater solution.

for prop in class_mapper(Shelf).iterate_properties:
    if isinstance(prop, sqlalchemy.orm.RelationshipProperty):
       print prop.mapper.class_

works with one-to-many and many-to-many in both directions