Remove internal use of string attr in loader option

Fixed issue where a "removed in 2.0" warning were generated internally by
the relationship loader mechanics.

This changeset started the effort of converting all string usage
in the test suite, however this is a much longer job as the
use of strings in loader options is widespread.  In particular
I'm not totally comfortable with strings not being accepted
in obvious spots like Load(User).load_only("x", "y", "z"), which
points to a new string expecting functionality that's not
what's there now.  However at the moment it seems like we need
to continue removing all support for strings and then figure out
"immediate strings from an explicit class" later.

Fixes: #6115
Change-Id: I6b314d135d2bc049fd66500914b772c1fe60b5b3

4 files changed, 87 insertions, 69 deletions

diff --git a/lib/sqlalchemy/orm/interfaces.py b/lib/sqlalchemy/orm/interfaces.py
index 7aae9ec37..a660d7e1a 100644
--- a/lib/sqlalchemy/orm/interfaces.py
+++ b/lib/sqlalchemy/orm/interfaces.py
@@ -83,6 +83,7 @@ class ORMFromClauseRole(roles.StrictFromClauseRole):
     _role_name = "ORM mapped entity, aliased entity, or FROM expression"
 
 
+@inspection._self_inspects
 class MapperProperty(
     HasCacheKey, _MappedAttribute, InspectionAttr, util.MemoizedSlots
 ):
diff --git a/lib/sqlalchemy/orm/strategies.py b/lib/sqlalchemy/orm/strategies.py
index 339a5bcf1..b11758090 100644
--- a/lib/sqlalchemy/orm/strategies.py
+++ b/lib/sqlalchemy/orm/strategies.py
@@ -992,7 +992,7 @@ class LazyLoader(AbstractRelationshipLoader, util.MemoizedSlots):
                         compile_context.compile_options._current_path[
                             rev.parent
                         ]
-                    ).lazyload(rev.key).process_compile_state(compile_context)
+                    ).lazyload(rev).process_compile_state(compile_context)
 
         stmt = stmt.add_criteria(
             lambda stmt: stmt._add_context_option(
diff --git a/lib/sqlalchemy/orm/strategy_options.py b/lib/sqlalchemy/orm/strategy_options.py
index 932c4a37d..ba4e5c466 100644
--- a/lib/sqlalchemy/orm/strategy_options.py
+++ b/lib/sqlalchemy/orm/strategy_options.py
@@ -234,84 +234,90 @@ class Load(Generative, LoaderOption):
                         raise
 
             path = path[attr]
-        elif _is_mapped_class(attr):
-            # TODO: this does not appear to be a valid codepath.  "attr"
-            # would never be a mapper.  This block is present in 1.2
-            # as well however does not seem to be accessed in any tests.
-            if not orm_util._entity_corresponds_to_use_path_impl(
-                attr.parent, path[-1]
-            ):
-                if raiseerr:
-                    raise sa_exc.ArgumentError(
-                        "Attribute '%s' does not "
-                        "link from element '%s'" % (attr, path.entity)
-                    )
-                else:
-                    return None
         else:
-            prop = found_property = attr.property
-
-            if not orm_util._entity_corresponds_to_use_path_impl(
-                attr.parent, path[-1]
-            ):
-                if raiseerr:
-                    raise sa_exc.ArgumentError(
-                        'Attribute "%s" does not '
-                        'link from element "%s".%s'
-                        % (
-                            attr,
-                            path.entity,
-                            (
-                                "  Did you mean to use "
-                                "%s.of_type(%s)?"
-                                % (path[-2], attr.class_.__name__)
-                                if len(path) > 1
-                                and path.entity.is_mapper
-                                and attr.parent.is_aliased_class
-                                else ""
-                            ),
+            insp = inspect(attr)
+
+            if insp.is_mapper or insp.is_aliased_class:
+                # TODO: this does not appear to be a valid codepath.  "attr"
+                # would never be a mapper.  This block is present in 1.2
+                # as well however does not seem to be accessed in any tests.
+                if not orm_util._entity_corresponds_to_use_path_impl(
+                    attr.parent, path[-1]
+                ):
+                    if raiseerr:
+                        raise sa_exc.ArgumentError(
+                            "Attribute '%s' does not "
+                            "link from element '%s'" % (attr, path.entity)
                         )
-                    )
-                else:
-                    return None
+                    else:
+                        return None
+            elif insp.is_property:
+                prop = found_property = attr
+                path = path[prop]
+            elif insp.is_attribute:
+                prop = found_property = attr.property
 
-            if attr._extra_criteria:
-                self._extra_criteria = attr._extra_criteria
+                if not orm_util._entity_corresponds_to_use_path_impl(
+                    attr.parent, path[-1]
+                ):
+                    if raiseerr:
+                        raise sa_exc.ArgumentError(
+                            'Attribute "%s" does not '
+                            'link from element "%s".%s'
+                            % (
+                                attr,
+                                path.entity,
+                                (
+                                    "  Did you mean to use "
+                                    "%s.of_type(%s)?"
+                                    % (path[-2], attr.class_.__name__)
+                                    if len(path) > 1
+                                    and path.entity.is_mapper
+                                    and attr.parent.is_aliased_class
+                                    else ""
+                                ),
+                            )
+                        )
+                    else:
+                        return None
 
-            if getattr(attr, "_of_type", None):
-                ac = attr._of_type
-                ext_info = of_type_info = inspect(ac)
+                if attr._extra_criteria:
+                    self._extra_criteria = attr._extra_criteria
 
-                if polymorphic_entity_context is None:
-                    polymorphic_entity_context = self.context
+                if getattr(attr, "_of_type", None):
+                    ac = attr._of_type
+                    ext_info = of_type_info = inspect(ac)
 
-                existing = path.entity_path[prop].get(
-                    polymorphic_entity_context, "path_with_polymorphic"
-                )
+                    if polymorphic_entity_context is None:
+                        polymorphic_entity_context = self.context
 
-                if not ext_info.is_aliased_class:
-                    ac = orm_util.with_polymorphic(
-                        ext_info.mapper.base_mapper,
-                        ext_info.mapper,
-                        aliased=True,
-                        _use_mapper_path=True,
-                        _existing_alias=inspect(existing)
-                        if existing is not None
-                        else None,
+                    existing = path.entity_path[prop].get(
+                        polymorphic_entity_context, "path_with_polymorphic"
                     )
 
-                    ext_info = inspect(ac)
+                    if not ext_info.is_aliased_class:
+                        ac = orm_util.with_polymorphic(
+                            ext_info.mapper.base_mapper,
+                            ext_info.mapper,
+                            aliased=True,
+                            _use_mapper_path=True,
+                            _existing_alias=inspect(existing)
+                            if existing is not None
+                            else None,
+                        )
 
-                path.entity_path[prop].set(
-                    polymorphic_entity_context, "path_with_polymorphic", ac
-                )
+                        ext_info = inspect(ac)
 
-                path = path[prop][ext_info]
+                    path.entity_path[prop].set(
+                        polymorphic_entity_context, "path_with_polymorphic", ac
+                    )
 
-                self._of_type = of_type_info
+                    path = path[prop][ext_info]
 
-            else:
-                path = path[prop]
+                    self._of_type = of_type_info
+
+                else:
+                    path = path[prop]
 
         if for_strategy is not None:
             found_property._get_strategy(for_strategy)
diff --git a/lib/sqlalchemy/testing/assertions.py b/lib/sqlalchemy/testing/assertions.py
index 289ac9a0a..02137474b 100644
--- a/lib/sqlalchemy/testing/assertions.py
+++ b/lib/sqlalchemy/testing/assertions.py
@@ -135,7 +135,12 @@ def uses_deprecated(*messages):
 
 @contextlib.contextmanager
 def _expect_warnings(
-    exc_cls, messages, regex=True, assert_=True, py2konly=False
+    exc_cls,
+    messages,
+    regex=True,
+    assert_=True,
+    py2konly=False,
+    raise_on_any_unexpected=False,
 ):
 
     if regex:
@@ -145,7 +150,13 @@ def _expect_warnings(
 
     seen = set(filters)
 
-    real_warn = warnings.warn
+    if raise_on_any_unexpected:
+
+        def real_warn(msg, *arg, **kw):
+            raise AssertionError("Got unexpected warning: %r" % msg)
+
+    else:
+        real_warn = warnings.warn
 
     def our_warn(msg, *arg, **kw):
         if isinstance(msg, exc_cls):
@@ -159,7 +170,7 @@ def _expect_warnings(
         if not exception or not issubclass(exception, exc_cls):
             return real_warn(msg, *arg, **kw)
 
-        if not filters:
+        if not filters and not raise_on_any_unexpected:
             return
 
         for filter_ in filters: