Refuse a path and sources together

`sources` was ignored beside a database path, so `sources=["nope"]` opened the
path and read as though the selection had been honoured. A path and a name
already conflict inside `DatabaseScope.resolve`; this is the same rule where a
caller can reach it.

docs/python.md

@@ -261,8 +261,9 @@ their documents.
 none: `search` returns no results, and `ask` and `analyze` run with no evidence
 from any database.
 
-On the constructor, `sources=[]` raises `ValueError` instead. A selection of
-nothing to search is a legitimate question; a client over no database is not.
+On the constructor, `sources` is rejected alongside a database path, and `[]`
+raises `ValueError`. A selection of nothing to search is a legitimate question;
+a client over no database is not.
 
 #### Inspecting the client scope
 

haiku_rag_slim/haiku/rag/client/__init__.py

@@ -142,10 +142,11 @@ class HaikuRAG:
             read_only: Whether to open the database in read-only mode.
             sources: Names from ``config.lancedb.databases`` this client covers,
                 None for all of them. Only that setting names databases, so a
-                name raises when ``lancedb.uri`` placed the database. Ignored
-                when ``db_path`` says which database to open. ``[]`` raises: a
-                client over no database can do nothing, unlike ``sources=[]`` on
-                a search, which is a selection of nothing to search.
+                name raises when ``lancedb.uri`` placed the database, and is
+                rejected alongside ``db_path``, which says the same thing
+                another way. ``[]`` raises too: a client over no database can do
+                nothing, unlike ``sources=[]`` on a search, which is a selection
+                of nothing to search.
         """
         self._configured = config if config is not None else get_config()
         # What the caller configured, kept intact: entering derives a
@@ -153,6 +154,11 @@ class HaikuRAG:
         # same set rather than the answer from last time.
         self._config = self._configured
         self._requested_db_path = Path(db_path) if db_path is not None else None
+        if self._requested_db_path is not None and sources is not None:
+            raise AmbiguousDatabaseError(
+                "a path and `sources` both say which databases to open; pass "
+                "one of them"
+            )
         self._skip_validation = skip_validation
         self._create = create
         self._read_only = read_only

tests/multi_db/test_scope.py

@@ -6,6 +6,7 @@ from pydantic import ValidationError
 from haiku.rag.client import HaikuRAG
 from haiku.rag.client.scope import DatabaseScope
 from haiku.rag.config.models import AppConfig, LanceDBConfig
+from haiku.rag.store.exceptions import AmbiguousDatabaseError
 from haiku.rag.utils import locate_database
 from tests.multi_db.helpers import (
     _config,
@@ -237,6 +238,15 @@ class TestPlacingADatabase:
         async with HaikuRAG(temp_db_path, create=True) as rag:
             assert await rag.reader_for(None) is rag
 
+    def test_a_path_and_sources_cannot_both_choose(self, tmp_path):
+        """`sources` used to be ignored beside a path, so selecting a database
+        that is not the one at the path opened the path anyway."""
+        config = _config(tmp_path, ["alpha", "beta"])
+
+        for sources in ([], ["alpha"], ["nope"]):
+            with pytest.raises(AmbiguousDatabaseError, match="pass one of them"):
+                HaikuRAG(tmp_path / "alpha.lancedb", config=config, sources=sources)
+
     @pytest.mark.asyncio
     async def test_one_database_refuses_a_name_it_does_not_cover(self, tmp_path):
         """Answering with itself would hand back the wrong database's reader for