Improved error message on append schema mismatch (#241)

pantab/src/pantab.cpp

@@ -380,6 +380,41 @@ static auto makeInsertHelper(std::shared_ptr<hyperapi::Inserter> inserter,
   }
 }
 
+///
+/// If a table already exists, ensure the structure is the same as what we
+/// append
+///
+void assertColumnsEqual(
+    const std::vector<hyperapi::TableDefinition::Column> &new_columns,
+    const std::vector<hyperapi::TableDefinition::Column> &old_columns) {
+  const size_t new_size = new_columns.size();
+  const size_t old_size = old_columns.size();
+  if (new_size != old_size) {
+    throw std::invalid_argument(
+        "Number of columns in new table definition does not match existing");
+  }
+
+  for (size_t i = 0; i < new_size; i++) {
+    const auto new_col = new_columns[i];
+    const auto old_col = old_columns[i];
+    const auto new_name = new_col.getName();
+    const auto old_name = old_col.getName();
+    if (new_name != old_name) {
+      throw std::invalid_argument(
+          "Column name mismatch at index " + std::to_string(i) +
+          "; new: " + new_name.toString() + " old: " + old_name.toString());
+    }
+
+    const auto new_type = new_col.getType();
+    const auto old_type = old_col.getType();
+    if (new_type != old_type) {
+      throw std::invalid_argument(
+          "Column type mismatch at index " + std::to_string(i) +
+          "; new: " + new_type.toString() + " old: " + old_type.toString());
+    }
+  }
+};
+
 using SchemaAndTableName = std::tuple<std::string, std::string>;
 
 void write_to_hyper(
@@ -433,10 +468,12 @@ void write_to_hyper(
     const hyperapi::TableName table_name{hyper_schema, hyper_table};
     const hyperapi::TableDefinition tableDef{table_name, hyper_columns};
     catalog.createSchemaIfNotExists(*table_name.getSchemaName());
-    if (table_mode == "w") {
+
+    if ((table_mode == "a") && (catalog.hasTable(table_name))) {
+      const auto existing_def = catalog.getTableDefinition(table_name);
+      assertColumnsEqual(hyper_columns, std::move(existing_def.getColumns()));
+    } else {
       catalog.createTable(tableDef);
-    } else if (table_mode == "a") {
-      catalog.createTableIfNotExists(tableDef);
     }
     const auto inserter =
         std::make_shared<hyperapi::Inserter>(connection, tableDef);

pantab/tests/test_writer.py

@@ -22,15 +22,27 @@ def test_bad_table_mode_raises(df, tmp_hyper):
         pantab.frames_to_hyper({"a": df}, tmp_hyper, table_mode="x")
 
 
-@pytest.mark.parametrize("new_dtype", ["int64", float])
-def test_append_mode_raises_column_dtype_mismatch(new_dtype, df, tmp_hyper, table_name):
+@pytest.mark.parametrize(
+    "new_dtype,hyper_type_name", [("int64", "BIGINT"), (float, "DOUBLE PRECISION")]
+)
+def test_append_mode_raises_column_dtype_mismatch(
+    new_dtype, hyper_type_name, df, tmp_hyper, table_name
+):
     pantab.frame_to_hyper(df, tmp_hyper, table=table_name)
 
     df["int16"] = df["int16"].astype(new_dtype)
-    # TODO: a better error message from hyper would be nice here
-    # seems like a limitation of hyper api
-    msg = ""
-    with pytest.raises(RuntimeError, match=msg):
+
+    msg = f"Column type mismatch at index 0; new: {hyper_type_name} old: SMALLINT"
+    with pytest.raises(ValueError, match=msg):
+        pantab.frame_to_hyper(df, tmp_hyper, table=table_name, table_mode="a")
+
+
+def test_append_mode_raises_ncolumns_mismatch(df, tmp_hyper, table_name):
+    pantab.frame_to_hyper(df, tmp_hyper, table=table_name)
+
+    df = df.drop(columns=["int16"])
+    msg = "Number of columns"
+    with pytest.raises(ValueError, match=msg):
         pantab.frame_to_hyper(df, tmp_hyper, table=table_name, table_mode="a")