fix: the unnest function lost needed type information

sqlalchemy_bigquery/base.py

@@ -35,6 +35,8 @@
 from google.api_core.exceptions import NotFound
 
 import sqlalchemy
+import sqlalchemy.sql.expression
+import sqlalchemy.sql.functions
 import sqlalchemy.sql.sqltypes
 import sqlalchemy.sql.type_api
 from sqlalchemy.exc import NoSuchTableError
@@ -1092,6 +1094,21 @@ def get_view_definition(self, connection, view_name, schema=None, **kw):
         return view.view_query
 
 
+class unnest(sqlalchemy.sql.functions.GenericFunction):
+    def __init__(self, *args, **kwargs):
+        expr = kwargs.pop("expr", None)
+        if expr is not None:
+            args = (expr,) + args
+        if len(args) != 1:
+            raise TypeError("The unnest function requires a single argument.")
+        arg = args[0]
+        if isinstance(arg, sqlalchemy.sql.expression.ColumnElement):
+            if not isinstance(arg.type, sqlalchemy.sql.sqltypes.ARRAY):
+                raise TypeError("The argument to unnest must have an ARRAY type.")
+            self.type = arg.type.item_type
+        super().__init__(*args, **kwargs)
+
+
 dialect = BigQueryDialect
 
 try:

tests/unit/test_sqlalchemy_bigquery.py

@@ -10,6 +10,7 @@
 from google.cloud import bigquery
 from google.cloud.bigquery.dataset import DatasetListItem
 from google.cloud.bigquery.table import TableListItem
+import packaging.version
 import pytest
 import sqlalchemy
 
@@ -178,3 +179,49 @@ def test_follow_dialect_attribute_convention():
 
     assert sqlalchemy_bigquery.dialect is sqlalchemy_bigquery.BigQueryDialect
     assert sqlalchemy_bigquery.base.dialect is sqlalchemy_bigquery.BigQueryDialect
+
+
+@pytest.mark.parametrize(
+    "args,kw,error",
+    [
+        ((), {}, "The unnest function requires a single argument."),
+        ((1, 1), {}, "The unnest function requires a single argument."),
+        ((1,), {"expr": 1}, "The unnest function requires a single argument."),
+        ((1, 1), {"expr": 1}, "The unnest function requires a single argument."),
+        (
+            (),
+            {"expr": sqlalchemy.Column("x", sqlalchemy.String)},
+            "The argument to unnest must have an ARRAY type.",
+        ),
+        (
+            (sqlalchemy.Column("x", sqlalchemy.String),),
+            {},
+            "The argument to unnest must have an ARRAY type.",
+        ),
+    ],
+)
+def test_unnest_function_errors(args, kw, error):
+    import sqlalchemy_bigquery  # noqa
+
+    with pytest.raises(TypeError, match=error):
+        sqlalchemy.func.unnest(*args, **kw)
+
+
+@pytest.mark.parametrize(
+    "args,kw",
+    [
+        ((), {"expr": sqlalchemy.Column("x", sqlalchemy.ARRAY(sqlalchemy.String))}),
+        ((sqlalchemy.Column("x", sqlalchemy.ARRAY(sqlalchemy.String)),), {}),
+    ],
+)
+def test_unnest_function(args, kw):
+    import sqlalchemy_bigquery  # noqa
+
+    f = sqlalchemy.func.unnest(*args, **kw)
+    assert isinstance(f.type, sqlalchemy.String)
+    if packaging.version.parse(sqlalchemy.__version__) >= packaging.version.parse(
+        "1.4"
+    ):
+        assert isinstance(
+            sqlalchemy.select([f]).subquery().c.unnest.type, sqlalchemy.String
+        )