Add support for classical mapping of dataclasses

Added support for direct mapping of Python classes that are defined using
the Python ``dataclasses`` decorator.    See the section
:ref:`mapping_dataclasses` for background.  Pull request courtesy Václav
Klusák.

Fixes: #5027
Closes: #5516
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5516
Pull-request-sha: bb48c63d1561ca48c954ad9f84a3eb2646571115

Change-Id: Ie33db2aae4adeeb5d99633fe926b9c30bab0b885

3 files changed, 25 insertions, 5 deletions

diff --git a/lib/sqlalchemy/orm/descriptor_props.py b/lib/sqlalchemy/orm/descriptor_props.py
index 39cf86e34..c2efa24a1 100644
--- a/lib/sqlalchemy/orm/descriptor_props.py
+++ b/lib/sqlalchemy/orm/descriptor_props.py
@@ -56,7 +56,7 @@ class DescriptorProperty(MapperProperty):
 
         if self.descriptor is None:
             desc = getattr(mapper.class_, self.key, None)
-            if mapper._is_userland_descriptor(desc):
+            if mapper._is_userland_descriptor(self.key, desc):
                 self.descriptor = desc
 
         if self.descriptor is None:
diff --git a/lib/sqlalchemy/orm/mapper.py b/lib/sqlalchemy/orm/mapper.py
index 446f6790e..755d4afc7 100644
--- a/lib/sqlalchemy/orm/mapper.py
+++ b/lib/sqlalchemy/orm/mapper.py
@@ -56,6 +56,12 @@ from ..sql import util as sql_util
 from ..sql import visitors
 from ..util import HasMemoized
 
+try:
+    import dataclasses
+except ImportError:
+    # The dataclasses module was added in Python 3.7
+    dataclasses = None
+
 
 _mapper_registry = weakref.WeakKeyDictionary()
 _already_compiling = False
@@ -2632,7 +2638,7 @@ class Mapper(
 
         return result
 
-    def _is_userland_descriptor(self, obj):
+    def _is_userland_descriptor(self, assigned_name, obj):
         if isinstance(
             obj,
             (
@@ -2643,7 +2649,14 @@ class Mapper(
         ):
             return False
         else:
-            return True
+            return assigned_name not in self._dataclass_fields
+
+    @HasMemoized.memoized_attribute
+    def _dataclass_fields(self):
+        if dataclasses is None or not dataclasses.is_dataclass(self.class_):
+            return frozenset()
+
+        return {field.name for field in dataclasses.fields(self.class_)}
 
     def _should_exclude(self, name, assigned_name, local, column):
         """determine whether a particular property should be implicitly
@@ -2656,16 +2669,19 @@ class Mapper(
 
         # check for class-bound attributes and/or descriptors,
         # either local or from an inherited class
+        # ignore dataclass field default values
         if local:
             if self.class_.__dict__.get(
                 assigned_name, None
             ) is not None and self._is_userland_descriptor(
-                self.class_.__dict__[assigned_name]
+                assigned_name, self.class_.__dict__[assigned_name]
             ):
                 return True
         else:
             attr = self.class_manager._get_class_attr_mro(assigned_name, None)
-            if attr is not None and self._is_userland_descriptor(attr):
+            if attr is not None and self._is_userland_descriptor(
+                assigned_name, attr
+            ):
                 return True
 
         if (
diff --git a/lib/sqlalchemy/testing/requirements.py b/lib/sqlalchemy/testing/requirements.py
index 9b8caac2e..4114137d4 100644
--- a/lib/sqlalchemy/testing/requirements.py
+++ b/lib/sqlalchemy/testing/requirements.py
@@ -1143,6 +1143,10 @@ class SuiteRequirements(Requirements):
         )
 
     @property
+    def dataclasses(self):
+        return self.python37
+
+    @property
     def cpython(self):
         return exclusions.only_if(
             lambda: util.cpython, "cPython interpreter needed"