SelectBase no longer a FromClause

As part of the SQLAlchemy 2.0 migration project, a conceptual change has
been made to the role of the :class:`.SelectBase` class hierarchy,
which is the root of all "SELECT" statement constructs, in that they no
longer serve directly as FROM clauses, that is, they no longer subclass
:class:`.FromClause`.  For end users, the change mostly means that any
placement of a :func:`.select` construct in the FROM clause of another
:func:`.select` requires first that it be wrapped in a subquery first,
which historically is through the use of the :meth:`.SelectBase.alias`
method, and is now also available through the use of
:meth:`.SelectBase.subquery`.    This was usually a requirement in any
case since several databases don't accept unnamed SELECT subqueries
in their FROM clause in any case.

See the documentation in this change for lots more detail.

Fixes: #4617
Change-Id: I0f6174ee24b9a1a4529168e52e855e12abd60667

3 files changed, 22 insertions, 17 deletions

diff --git a/lib/sqlalchemy/orm/query.py b/lib/sqlalchemy/orm/query.py
index c73a8147c..13402e7f4 100644
--- a/lib/sqlalchemy/orm/query.py
+++ b/lib/sqlalchemy/orm/query.py
@@ -253,11 +253,6 @@ class Query(object):
                         "expected when the base alias is being set."
                     )
                 fa.append(info.selectable)
-            elif not info.is_clause_element or not info._is_from_clause:
-                raise sa_exc.ArgumentError(
-                    "argument is not a mapped class, mapper, "
-                    "aliased(), or FromClause instance."
-                )
             else:
                 from_obj = coercions.expect(
                     roles.StrictFromClauseRole, from_obj, allow_select=True
@@ -271,7 +266,9 @@ class Query(object):
         if (
             set_base_alias
             and len(self._from_obj) == 1
-            and isinstance(select_from_alias, expression.Alias)
+            and isinstance(
+                select_from_alias, sql.selectable.AliasedReturnsRows
+            )
         ):
             equivs = self.__all_equivs()
             self._from_obj_alias = sql_util.ColumnAdapter(
@@ -2302,7 +2299,14 @@ class Query(object):
         if (
             len(keys) == 2
             and isinstance(
-                keys[0], (expression.FromClause, type, AliasedClass)
+                keys[0],
+                (
+                    # note this would be FromClause once
+                    # coercion of SELECT is removed
+                    expression.Selectable,
+                    type,
+                    AliasedClass,
+                ),
             )
             and isinstance(
                 keys[1],
@@ -2761,7 +2765,9 @@ class Query(object):
                 # if the destination selectable is a plain select(),
                 # turn it into an alias().
                 if isinstance(right_selectable, expression.SelectBase):
-                    right_selectable = right_selectable.alias()
+                    right_selectable = coercions.expect(
+                        roles.FromClauseRole, right_selectable
+                    )
                     need_adapter = True
 
                 # make the right hand side target into an ORM entity
@@ -2781,7 +2787,8 @@ class Query(object):
             and (
                 right_mapper.with_polymorphic
                 and isinstance(
-                    right_mapper._with_polymorphic_selectable, expression.Alias
+                    right_mapper._with_polymorphic_selectable,
+                    expression.AliasedReturnsRows,
                 )
                 or overlap
                 # test for overlap:
@@ -3201,13 +3208,10 @@ class Query(object):
         """
         statement = coercions.expect(roles.SelectStatementRole, statement)
 
-        if not isinstance(
+        # TODO: coercions above should have this handled
+        assert isinstance(
             statement, (expression.TextClause, expression.SelectBase)
-        ):
-            raise sa_exc.ArgumentError(
-                "from_statement accepts text(), select(), "
-                "and union() objects only."
-            )
+        )
 
         self._statement = statement
 
diff --git a/lib/sqlalchemy/orm/strategies.py b/lib/sqlalchemy/orm/strategies.py
index b0dffe5dd..7e150414b 100644
--- a/lib/sqlalchemy/orm/strategies.py
+++ b/lib/sqlalchemy/orm/strategies.py
@@ -1629,7 +1629,6 @@ class JoinedLoader(AbstractRelationshipLoader):
             # the object becomes shared among threads.  this prevents
             # races for column identities.
             inspect(to_adapt).selectable.c
-
             self._aliased_class_pool.append(to_adapt)
 
         return self._aliased_class_pool[idx]
diff --git a/lib/sqlalchemy/orm/util.py b/lib/sqlalchemy/orm/util.py
index 80d4b26b2..4b4fa4052 100644
--- a/lib/sqlalchemy/orm/util.py
+++ b/lib/sqlalchemy/orm/util.py
@@ -825,7 +825,9 @@ def aliased(element, alias=None, name=None, flat=False, adapt_on_names=False):
             raise sa_exc.ArgumentError(
                 "adapt_on_names only applies to ORM elements"
             )
-        return element._anonymous_fromclause(name=name, flat=flat)
+        return coercions.expect(
+            roles.AnonymizedFromClauseRole, element, name=name, flat=flat
+        )
     else:
         return AliasedClass(
             element,