Fix drag_info in citationtreeview, so that it doesn't break merge citation. Also add some comments about drag and drop.

svn: r18443

src/ScratchPad.py

@@ -142,6 +142,15 @@ def obj2target(target):
          }
     return d[target] if target in d else None
 
+# FIXME: CITATION: Dropping a citation onto an embedded tab should open 
+# CitationEdit. However, note should behave similarly, and although there is
+# code to call EditNote, EditNote does not appear to be called. I don't
+# understand how to get the editor called.
+
+# FIXME: CITATION: It should be possible to drop a source onto an embedded tab,
+# with CitationEdit being opened, However, It is not possible to do the drop,
+# and as described above I do not understand how to get the editor called.
+
 def model_contains(model, data):
     """
     Returns True if data is a row in model.

src/plugins/view/citationtreeview.py

@@ -196,18 +196,21 @@ class CitationTreeView(ListView):
         return self.dbstate.db.get_citation_bookmarks()
 
     def drag_info(self):
+        # Since drag only needs to work when just one row is selected, ideally,
+        # this should just return SOURCE_LINK if one source is selected and
+        # CITATION_LINK if one citation is selected, and probably None
+        # otherwise. However, this doesn't work. Drag and drop failed to work at
+        # all for citationtree view, and I think this was because None is
+        # returned during initialisation. There is also a problem where it seems
+        # at some point during a citation merge, neither a Source nor a Citation
+        # is selected. Hence the simplistic solution implemented below, where
+        # CITATION_LINK is always returned except when it is obviously correct
+        # to return SOURCE_LINK.
+        
         selection = self.selected_handles()
-        if len(selection) == 1:
-            handle = selection[0]
-            # The handle will either be a Source handle or a Citation handle
-            source = self.dbstate.db.get_source_from_handle(handle)
-            citation = self.dbstate.db.get_citation_from_handle(handle)
-            if (not source and not citation) or (source and citation):
-                raise ValueError("selection must be either source or citation")
-            if source:
+        if len(selection) == 1 and \
+            self.dbstate.db.get_source_from_handle(selection[0]):
                 return DdTargets.SOURCE_LINK
-            else:
-                return DdTargets.CITATION_LINK
         else:
             return DdTargets.CITATION_LINK