reports: Add RelatedPostings.first_meta_links() method.

Basically moving this from AccrualPostings into the superclass.

conservancy_beancount/reports/accrual.py

@@ -193,7 +193,7 @@ class AccrualPostings(core.RelatedPostings):
             norm_func = self.accrual_type.normalize_amount
             entity_pred = lambda post: norm_func(post.units).number > 0
         self.entity = self._single_item(self.entities(entity_pred))
-        self.invoice = self._single_item(self.first_links('invoice'))
+        self.invoice = self._single_item(self.first_meta_links('invoice', None))
         self.end_balance = norm_func(self.balance_at_cost())
 
     def _single_item(self, seq: Iterable[T]) -> Union[T, Sentinel]:
@@ -213,9 +213,6 @@ class AccrualPostings(core.RelatedPostings):
             if pred(post) and 'entity' in post.meta
         )
 
-    def first_links(self, key: MetaKey, default: Optional[str]=None) -> Iterator[Optional[str]]:
-        return (post.meta.first_link(key, default) for post in self)
-
     def make_consistent(self) -> Iterator[Tuple[MetaValue, 'AccrualPostings']]:
         account_ok = isinstance(self.account, str)
         entity_ok = isinstance(self.entity, str)
@@ -506,7 +503,7 @@ class OutgoingReport(BaseReport):
         self.rt_wrapper = rtutil.RT(rt_client)
 
     def _primary_rt_id(self, posts: AccrualPostings) -> rtutil.TicketAttachmentIds:
-        rt_ids = {url for url in posts.first_links('rt-id') if url is not None}
+        rt_ids = list(posts.first_meta_links('rt-id'))
         rt_ids_count = len(rt_ids)
         if rt_ids_count != 1:
             raise ValueError(f"{rt_ids_count} rt-id links found")

conservancy_beancount/reports/core.py

@@ -304,6 +304,23 @@ class RelatedPostings(Sequence[data.Posting]):
     def all_meta_links(self, key: MetaKey) -> Iterator[str]:
         return filters.iter_unique(self._all_meta_links(key))
 
+    @overload
+    def first_meta_links(self, key: MetaKey, default: str='') -> Iterator[str]: ...
+
+    @overload
+    def first_meta_links(self, key: MetaKey, default: None) -> Iterator[Optional[str]]: ...
+
+    def first_meta_links(self,
+                         key: MetaKey,
+                         default: Optional[str]='',
+    ) -> Iterator[Optional[str]]:
+        retval = filters.iter_unique(
+            post.meta.first_link(key, default) for post in self
+        )
+        if default == '':
+            retval = (s for s in retval if s)
+        return retval
+
     def iter_with_balance(self) -> Iterator[Tuple[data.Posting, Balance]]:
         balance = MutableBalance()
         for post in self:

tests/test_reports_related_postings.py

@@ -275,6 +275,20 @@ def test_all_meta_links_preserves_order():
     ) for c in '121323')
     assert list(related.all_meta_links('approval')) == list('123')
 
+def test_first_meta_links():
+    related = core.RelatedPostings(testutil.Posting(
+        'Assets:Cash', 10, contract=value, _meta_type=data.Metadata,
+    ) for value in ['1 2', '', '1 3', testutil.PAST_DATE, '2 3', None])
+    del related[-1].meta['contract']
+    assert list(related.first_meta_links('contract')) == list('12')
+
+def test_first_meta_links_fallback():
+    related = core.RelatedPostings(testutil.Posting(
+        'Assets:Cash', 10, contract=value, _meta_type=data.Metadata,
+    ) for value in ['1 2', testutil.PAST_DATE, '1 3', None, '2 3'])
+    del related[-2].meta['contract']
+    assert list(related.first_meta_links('contract', None)) == ['1', None, '2']
+
 def test_group_by_meta_zero():
     assert not list(core.RelatedPostings.group_by_meta([], 'metacurrency'))