fix: registration of in-memory tables (letsql#232)

closes letsql#231

python/letsql/backends/let/__init__.py

@@ -53,10 +53,21 @@ def register(
         table_name: str | None = None,
         **kwargs: Any,
     ) -> ir.Table:
+        # FIXME: make sure all paths set the correct backend, table_or_expr pairs
         table_or_expr = None
         if isinstance(source, ir.Expr) and hasattr(source, "to_pyarrow_batches"):
             table_or_expr = source.op()
-            backend = source._find_backend(use_default=False)
+
+            backends, has_unbound = source._find_backends()
+            backend = None
+            if not backends:
+                if not has_unbound:
+                    source = super().execute(source)
+                    table_or_expr = None
+            elif len(backends) > 1:
+                raise ValueError("Multiple backends found for this expression")
+            else:
+                backend = backends[0]
 
             if isinstance(backend, Backend):
                 if backend is self and table_or_expr in self._sources:

python/letsql/tests/test_register.py

@@ -7,6 +7,8 @@
 import pyarrow.dataset as ds
 import pytest
 
+import letsql
+
 
 @pytest.fixture
 def gzip_csv(data_dir, tmp_path):
@@ -84,3 +86,9 @@ def test_register_dataset(con):
     dataset = ds.InMemoryDataset(tab)
     con.register(dataset, "my_table")
     assert con.table("my_table").x.sum().execute() == 6
+
+
+def test_register_memtable(con):
+    data = pd.DataFrame({"a": [1, 2, 3, 4, 5], "b": [2, 3, 4, 5, 6]})
+    t = letsql.memtable(data).pipe(con.register, "data")
+    assert t.a.sum().execute() == 15