PaginatedResults: add props for before/after IDs

This adds two new properties to PaginatedResults - .next_page_after_id
and .prev_page_before_id. These can be used when creating the links to
the before/after pages, instead of needing to access the right
element/property "manually".

This will be most useful in listings that contain items of multiple
types (such as comments and topics), since we won't necessarily know
which type the first/last elements are.

tildes/tildes/models/pagination.py

@@ -3,7 +3,7 @@
 from typing import Any, Iterator, List, Optional, TypeVar
 
 from pyramid.request import Request
-from sqlalchemy import Column, func
+from sqlalchemy import Column, func, inspect
 
 from tildes.lib.id import id36_to_id
 from .model_query import ModelQuery
@@ -204,3 +204,19 @@ class PaginatedResults:
     def __len__(self) -> int:
         """Return the number of results."""
         return len(self.results)
+
+    @property
+    def next_page_after_id(self) -> int:
+        """Return "after" ID that should be used to fetch the next page."""
+        if not self.has_next_page:
+            raise AttributeError
+
+        return inspect(self.results[-1]).identity[0]
+
+    @property
+    def prev_page_before_id(self) -> int:
+        """Return "before" ID that should be used to fetch the prev page."""
+        if not self.has_prev_page:
+            raise AttributeError
+
+        return inspect(self.results[0]).identity[0]

tildes/tildes/templates/topic_listing.jinja2

@@ -154,13 +154,13 @@
   <div class="pagination">
     {% if topics.has_prev_page %}
       <a class="page-item btn" id="prev-page"
-        href="{{ request.current_listing_base_url({'before': topics[0].topic_id36}) }}"
+        href="{{ request.current_listing_base_url({'before': topics.prev_page_before_id}) }}"
       >Prev</a>
     {% endif %}
 
     {% if topics.has_next_page %}
       <a class="page-item btn" id="next-page"
-        href="{{ request.current_listing_base_url({'after': topics[-1].topic_id36}) }}"
+        href="{{ request.current_listing_base_url({'after': topics.next_page_after_id}) }}"
       >Next</a>
     {% endif %}
   </div>