query: Add ContextMeta function.

Deduplicate metadata lookup code.

conservancy_beancount/reports/query.py

@@ -108,7 +108,6 @@ from .. import data
 from .. import rtutil
 
 PROGNAME = 'query-report'
-SENTINEL = object()
 logger = logging.getLogger('conservancy_beancount.reports.query')
 
 CellFunc = Callable[[Any], odf.table.TableCell]
@@ -154,6 +153,14 @@ class PostingContext:
     store: Store
 
 
+def ContextMeta(context: PostingContext) -> data.PostingMeta:
+    """Build a read-only PostingMeta object from the query context"""
+    # We use sys.maxsize as the index because using a constant is fast, and
+    # that helps keep the object read-only: if it ever tries to manipulate
+    # the transaction, it'll get an IndexError.
+    return data.PostingMeta(context.entry, sys.maxsize, context.posting).detached()
+
+
 class MetaDocs(bc_query_env.AnyMeta):
     """Return a list of document links from metadata."""
     def __init__(self, operands: List[bc_query_compile.EvalNode]) -> None:
@@ -266,16 +273,10 @@ class RTTicket(bc_query_compile.EvalFunction):
         rt_key, meta_key, limit = self.eval_args(context)
         rt_field = self._rt_key(rt_key)
         meta_key = self._meta_key(meta_key)
-        if context.posting.meta is None:
-            meta_value: Any = SENTINEL
-        else:
-            meta_value = context.posting.meta.get(meta_key, SENTINEL)
-        if meta_value is SENTINEL:
-            meta_value = context.entry.meta.get(meta_key)
-        if not isinstance(meta_value, str) or limit < 1:
-            meta_value = ''
+        if limit < 1:
+            return set()
         ticket_ids: Set[str] = set()
-        for link_s in meta_value.split():
+        for link_s in ContextMeta(context).report_links(meta_key):
             rt_id = rtutil.RT.parse(link_s)
             if rt_id is not None:
                 ticket_ids.add(rt_id[0])