at least warn, if not error, when a RelationshipProperty is being thrown away

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

user_course = Table('user_course', Base.metadata,
    Column('user_id', Integer, ForeignKey('users.id')),
    Column('course_id', Integer, ForeignKey('courses.id'))
)

user_event = Table('user_event', Base.metadata,
    Column('user_id', Integer, ForeignKey('users.id')),
    Column('event_id', Integer, ForeignKey('events.id'))
)

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)

    courses = relationship("Course", secondary=user_course, backref="users")
    personal_events = relationship("Event", secondary=user_event, backref="users")

class Event(Base):
    __tablename__ = 'events'
    id = Column(Integer, primary_key=True)
    type = Column(String(45))

    __mapper_args__ = {
        'polymorphic_identity': 'event',
        'polymorphic_on': type
    }

class Course(Event):
    __tablename__ = 'courses'
    id = Column(Integer, ForeignKey('events.id'), primary_key=True)

    __mapper_args__ = {
        'polymorphic_identity': 'course',
    }

u1 = User()
c1 = Course()
u1.courses.append(c1)

amazingly, this is very easy to detect and all tests pass except one that is specifically testing if a relationship prop can be replaced (hence a warning):

diff -r b83dd4dc2200bece2896a125be6d4f0911669d15 lib/sqlalchemy/orm/mapper.py
--- a/lib/sqlalchemy/orm/mapper.py  Sat Mar 02 20:27:53 2013 -0500
+++ b/lib/sqlalchemy/orm/mapper.py  Sun Mar 03 12:20:47 2013 -0500
@@ -1117,6 +1117,8 @@
             for col in prop.columns + prop._orig_columns:
                 for col in col.proxy_set:
                     self._columntoproperty[col](col) = prop
+        elif key in self._props:
+            raise sa_exc.InvalidRequestError("Conflicting property %s" % key)

         prop.key = key

Additionally, when this runs we should try to repair the error message in attributes.py to either be more accurate, or more vague if the correct relationship can't be determined.

A warning here can be backported to 0.7.

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