SQLAlchemy: Use JSON type adapter for implementing CrateDB's OBJECT

SQLAlchemy's `sqltypes.JSON` provides a facade for vendor-specific JSON
types. Since it supports JSON SQL operations, it only works on backends
that have an actual JSON type, which are currently PostgreSQL, MySQL,
SQLite, and Microsoft SQL Server.

This patch starts leveraging the same infrastructure, thus bringing
corresponding interfaces to the CrateDB dialect. The major difference
is that it will not actually do any JSON marshalling, but propagate
corresponding data structures 1:1, because within CrateDB's SQL,
`OBJECT`s do not need to be serialized into JSON strings before
transfer.

CHANGES.txt

@@ -15,6 +15,8 @@ Unreleased
 
 - SQLAlchemy: Fix SQL statement caching for CrateDB's ``OBJECT`` type.
 
+- SQLAlchemy: Refactor ``OBJECT`` type to use SQLAlchemy's JSON type machinery.
+
 
 2023/04/18 0.31.1
 =================

src/crate/client/sqlalchemy/compiler.py

@@ -26,7 +26,7 @@
 from sqlalchemy.dialects.postgresql.base import PGCompiler
 from sqlalchemy.sql import compiler
 from sqlalchemy.types import String
-from .types import MutableDict, _Craty, Geopoint, Geoshape
+from .types import MutableDict, ObjectTypeImpl, Geopoint, Geoshape
 from .sa_version import SA_VERSION, SA_1_4
 
 
@@ -41,7 +41,7 @@ def rewrite_update(clauseelement, multiparams, params):
 
         "col['x'] = ?, col['y'] = ?", (1, 2)
 
-    by using the `Craty` (`MutableDict`) type.
+    by using the `ObjectType` (`MutableDict`) type.
     The update statement is only rewritten if an item of the MutableDict was
     changed.
     """
@@ -124,7 +124,7 @@ def get_column_specification(self, column, **kwargs):
             )
 
         if column.dialect_options['crate'].get('index') is False:
-            if isinstance(column.type, (Geopoint, Geoshape, _Craty)):
+            if isinstance(column.type, (Geopoint, Geoshape, ObjectTypeImpl)):
                 raise sa.exc.CompileError(
                     "Disabling indexing is not supported for column "
                     "types OBJECT, GEO_POINT, and GEO_SHAPE"
@@ -217,6 +217,9 @@ def visit_ARRAY(self, type_, **kw):
                 "CrateDB doesn't support multidimensional arrays")
         return 'ARRAY({0})'.format(self.process(type_.item_type))
 
+    def visit_OBJECT(self, type_, **kw):
+        return "OBJECT"
+
 
 class CrateCompiler(compiler.SQLCompiler):
 
@@ -226,6 +229,14 @@ def visit_getitem_binary(self, binary, operator, **kw):
             binary.right.value
         )
 
+    def visit_json_getitem_op_binary(
+        self, binary, operator, _cast_applied=False, **kw
+    ):
+        return "{0}['{1}']".format(
+            self.process(binary.left, **kw),
+            binary.right.value
+        )
+
     def visit_any(self, element, **kw):
         return "%s%sANY (%s)" % (
             self.process(element.left, **kw),

src/crate/client/sqlalchemy/dialect.py

@@ -183,10 +183,17 @@ class CrateDialect(default.DefaultDialect):
     insert_returning = True
     update_returning = True
 
-    def __init__(self, *args, **kwargs):
-        super(CrateDialect, self).__init__(*args, **kwargs)
-        # currently our sql parser doesn't support unquoted column names that
-        # start with _. Adding it here causes sqlalchemy to quote such columns
+    def __init__(self, **kwargs):
+        default.DefaultDialect.__init__(self, **kwargs)
+
+        # CrateDB does not need `OBJECT` types to be serialized as JSON.
+        # Corresponding data is forwarded 1:1, and will get marshalled
+        # by the low-level driver.
+        self._json_deserializer = lambda x: x
+        self._json_serializer = lambda x: x
+
+        # Currently, our SQL parser doesn't support unquoted column names that
+        # start with _. Adding it here causes sqlalchemy to quote such columns.
         self.identifier_preparer.illegal_initial_characters.add('_')
 
     def initialize(self, connection):

src/crate/client/sqlalchemy/tests/compiler_test.py

@@ -27,7 +27,7 @@
 from sqlalchemy.sql import text, Update
 
 from crate.client.sqlalchemy.sa_version import SA_VERSION, SA_1_4, SA_2_0
-from crate.client.sqlalchemy.types import Craty
+from crate.client.sqlalchemy.types import ObjectType
 
 
 class SqlAlchemyCompilerTest(TestCase):
@@ -38,7 +38,7 @@ def setUp(self):
         self.metadata = sa.MetaData()
         self.mytable = sa.Table('mytable', self.metadata,
                                 sa.Column('name', sa.String),
-                                sa.Column('data', Craty))
+                                sa.Column('data', ObjectType))
 
         self.update = Update(self.mytable).where(text('name=:name'))
         self.values = [{'name': 'crate'}]

src/crate/client/sqlalchemy/tests/dict_test.py

@@ -31,7 +31,7 @@
 except ImportError:
     from sqlalchemy.ext.declarative import declarative_base
 
-from crate.client.sqlalchemy.types import Craty, ObjectArray
+from crate.client.sqlalchemy.types import ObjectArray, ObjectType
 from crate.client.cursor import Cursor
 
 
@@ -47,7 +47,7 @@ def setUp(self):
         metadata = sa.MetaData()
         self.mytable = sa.Table('mytable', metadata,
                                 sa.Column('name', sa.String),
-                                sa.Column('data', Craty))
+                                sa.Column('data', ObjectType))
 
     def assertSQL(self, expected_str, selectable):
         actual_expr = selectable.compile(bind=self.engine)
@@ -124,7 +124,7 @@ class Character(Base):
             __tablename__ = 'characters'
             name = sa.Column(sa.String, primary_key=True)
             age = sa.Column(sa.Integer)
-            data = sa.Column(Craty)
+            data = sa.Column(ObjectType)
             data_list = sa.Column(ObjectArray)
 
         session = Session(bind=self.engine)
@@ -140,7 +140,7 @@ def test_assign_null_to_object_array(self):
         self.assertEqual(char_3.data_list, [None])
 
     @patch('crate.client.connection.Cursor', FakeCursor)
-    def test_assign_to_craty_type_after_commit(self):
+    def test_assign_to_object_type_after_commit(self):
         session, Character = self.set_up_character_and_cursor(
             return_value=[('Trillian', None)]
         )

src/crate/client/sqlalchemy/tests/match_test.py

@@ -30,7 +30,7 @@
 except ImportError:
     from sqlalchemy.ext.declarative import declarative_base
 
-from crate.client.sqlalchemy.types import Craty
+from crate.client.sqlalchemy.types import ObjectType
 from crate.client.sqlalchemy.predicates import match
 from crate.client.cursor import Cursor
 
@@ -60,7 +60,7 @@ def set_up_character_and_session(self):
         class Character(Base):
             __tablename__ = 'characters'
             name = sa.Column(sa.String, primary_key=True)
-            info = sa.Column(Craty)
+            info = sa.Column(ObjectType)
 
         session = Session(bind=self.engine)
         return session, Character

src/crate/client/sqlalchemy/tests/query_caching.py

@@ -76,7 +76,7 @@ def setup_data(self):
         self.session.execute(sa.text("REFRESH TABLE testdrive.characters;"))
 
     @skipIf(SA_VERSION < SA_1_4, "On SA13, the 'ResultProxy' object has no attribute 'scalar_one'")
-    def test_object_multiple_select(self):
+    def test_object_multiple_select_legacy(self):
         """
         The SQLAlchemy implementation of CrateDB's `OBJECT` type offers indexed
         access to the instance's content in form of a dictionary. Thus, it must
@@ -85,6 +85,8 @@ def test_object_multiple_select(self):
 
         This test verifies that two subsequent `SELECT` statements are translated
         well, and don't trip on incorrect SQL compiled statement caching.
+
+        This variant uses direct value matching on the `OBJECT`s attribute.
         """
         self.setup_data()
         Character = self.Character
@@ -97,6 +99,30 @@ def test_object_multiple_select(self):
         result = self.session.execute(selectable).scalar_one().data
         self.assertEqual({"y": 2}, result)
 
+    @skipIf(SA_VERSION < SA_1_4, "On SA13, the 'ResultProxy' object has no attribute 'scalar_one'")
+    def test_object_multiple_select_modern(self):
+        """
+        The SQLAlchemy implementation of CrateDB's `OBJECT` type offers indexed
+        access to the instance's content in form of a dictionary. Thus, it must
+        not use `cache_ok = True` on its implementation, i.e. this part of the
+        compiled SQL clause must not be cached.
+
+        This test verifies that two subsequent `SELECT` statements are translated
+        well, and don't trip on incorrect SQL compiled statement caching.
+
+        This variant uses comparator method matching on the `OBJECT`s attribute.
+        """
+        self.setup_data()
+        Character = self.Character
+
+        selectable = sa.select(Character).where(Character.data['x'].as_integer() == 1)
+        result = self.session.execute(selectable).scalar_one().data
+        self.assertEqual({"x": 1}, result)
+
+        selectable = sa.select(Character).where(Character.data['y'].as_integer() == 2)
+        result = self.session.execute(selectable).scalar_one().data
+        self.assertEqual({"y": 2}, result)
+
     @skipIf(SA_VERSION < SA_1_4, "On SA13, the 'ResultProxy' object has no attribute 'scalar_one'")
     def test_objectarray_multiple_select(self):
         """

src/crate/client/sqlalchemy/tests/warnings_test.py

@@ -8,14 +8,17 @@
 
 
 class SqlAlchemyWarningsTest(TestCase, ExtraAssertions):
+    """
+    Verify a few `DeprecationWarning` spots.
+
+    https://docs.python.org/3/library/warnings.html#testing-warnings
+    """
 
     @skipIf(SA_VERSION >= SA_1_4, "There is no deprecation warning for "
                                   "SQLAlchemy 1.3 on higher versions")
     def test_sa13_deprecation_warning(self):
         """
         Verify that a `DeprecationWarning` is issued when running SQLAlchemy 1.3.
-
-        https://docs.python.org/3/library/warnings.html#testing-warnings
         """
         with warnings.catch_warnings(record=True) as w:
 
@@ -31,3 +34,31 @@ def test_sa13_deprecation_warning(self):
             self.assertEqual(len(w), 1)
             self.assertIsSubclass(w[-1].category, DeprecationWarning)
             self.assertIn("SQLAlchemy 1.3 is effectively EOL.", str(w[-1].message))
+
+    def test_craty_object_deprecation_warning(self):
+        """
+        Verify that a `DeprecationWarning` is issued when accessing the deprecated
+        module variables `Craty`, and `Object`. The new type is called `ObjectType`.
+        """
+
+        with warnings.catch_warnings(record=True) as w:
+
+            # Import the deprecated symbol.
+            from crate.client.sqlalchemy.types import Craty  # noqa: F401
+
+            # Verify details of the deprecation warning.
+            self.assertEqual(len(w), 1)
+            self.assertIsSubclass(w[-1].category, DeprecationWarning)
+            self.assertIn("Craty is deprecated and will be removed in future releases. "
+                          "Please use ObjectType instead.", str(w[-1].message))
+
+        with warnings.catch_warnings(record=True) as w:
+
+            # Import the deprecated symbol.
+            from crate.client.sqlalchemy.types import Object  # noqa: F401
+
+            # Verify details of the deprecation warning.
+            self.assertEqual(len(w), 1)
+            self.assertIsSubclass(w[-1].category, DeprecationWarning)
+            self.assertIn("Object is deprecated and will be removed in future releases. "
+                          "Please use ObjectType instead.", str(w[-1].message))

src/crate/client/sqlalchemy/types.py

@@ -18,6 +18,7 @@
 # However, if you have executed another commercial license agreement
 # with Crate these terms will supersede the license and you may use the
 # software solely pursuant to the terms of the relevant commercial agreement.
+import warnings
 
 import sqlalchemy.types as sqltypes
 from sqlalchemy.sql import operators, expression
@@ -131,24 +132,31 @@ def __eq__(self, other):
         return dict.__eq__(self, other)
 
 
-class _Craty(sqltypes.UserDefinedType):
+class ObjectTypeImpl(sqltypes.UserDefinedType, sqltypes.JSON):
+
+    __visit_name__ = "OBJECT"
+
     cache_ok = False
+    none_as_null = False
 
-    class Comparator(sqltypes.TypeEngine.Comparator):
 
-        def __getitem__(self, key):
-            return default_comparator._binary_operate(self.expr,
-                                                      operators.getitem,
-                                                      key)
+# Designated name to refer to. `Object` is too ambiguous.
+ObjectType = MutableDict.as_mutable(ObjectTypeImpl)
 
-    def get_col_spec(self):
-        return 'OBJECT'
+# Backward-compatibility aliases.
+_deprecated_Craty = ObjectType
+_deprecated_Object = ObjectType
 
-    type = MutableDict
-    comparator_factory = Comparator
+# https://www.lesinskis.com/deprecating-module-scope-variables.html
+deprecated_names = ["Craty", "Object"]
 
 
-Object = Craty = MutableDict.as_mutable(_Craty)
+def __getattr__(name):
+    if name in deprecated_names:
+        warnings.warn(f"{name} is deprecated and will be removed in future releases. "
+                      f"Please use ObjectType instead.", DeprecationWarning)
+        return globals()[f"_deprecated_{name}"]
+    raise AttributeError(f"module {__name__} has no attribute {name}")
 
 
 class Any(expression.ColumnElement):