Fix master_waypoint relationship.

Thanks to @mirabilos!

librestash/lib/model.py

@@ -62,7 +62,10 @@ class Waypoint(Base):
     longitude = Column(Float)
 
     listing_id = Column(Integer, ForeignKey('listings.id'))
-    listing = relationship('Listing', back_populates='waypoints')
+    listing = relationship('Listing', back_populates='waypoints', primaryjoin='Waypoint.listing_id == Listing.id')
+
+    master_of_id = Column(Integer, ForeignKey('listings.id'))
+    master_of = relationship('Listing', back_populates='master_waypoint', primaryjoin='Waypoint.master_of_id == Listing.id')
 
     @property
     def platform_prefix(self):
@@ -83,8 +86,8 @@ class Listing(Base):
     description = Column(String)
     hints = Column(String)
 
-    master_waypoint_code = Column(String)
-    waypoints = relationship('Waypoint', back_populates='listing')
+    master_waypoint = relationship('Waypoint', uselist=False, primaryjoin='Waypoint.master_of_id == Listing.id')
+    waypoints = relationship('Waypoint', back_populates='listing', primaryjoin='Waypoint.listing_id == Listing.id')
 
     logs = relationship('Log', back_populates='listing')
 
@@ -94,16 +97,6 @@ class Listing(Base):
     owner_account_id = Column(Integer, ForeignKey('geocacher_accounts.id'))
     owner_account = relationship('GeocacherAccount', back_populates='listings')
 
-    # FIXME use proper relationship
-    @property
-    def master_waypoint(self):
-        for wp in self.waypoints:
-            if wp.code == self.master_waypoint_code:
-                return wp
-    @master_waypoint.setter
-    def master_waypoint(self, wp):
-        self.master_waypoint_code = wp.code
-
     @property
     def code(self):
         return self.master_waypoint.code