allow StrSQLCompiler to generate columns w/o names

e.g. for this:

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()


class B(Base):
    __tablename__ = 'b'
    id = Column(Integer, primary_key=True)
    a_id = Column(Integer)


class AbstractThing(Base):
    __abstract__ = True
    id = Column(Integer, primary_key=True)


class A(AbstractThing):
    __tablename__ = 'a'
    bs = relationship("B", primaryjoin=B.a_id == AbstractThing.id)

configure_mappers()

patch:

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index 85d5ff6..38013fb 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -660,12 +660,15 @@ class SQLCompiler(Compiled):
             return label.element._compiler_dispatch(
                 self, within_columns_clause=False, **kw)

+    def _fallback_column_name(self, column):
+        raise exc.CompileError("Cannot compile Column object until "
+                               "its 'name' is assigned.")
+
     def visit_column(self, column, add_to_result_map=None,
                      include_table=True, **kwargs):
         name = orig_name = column.name
         if name is None:
-            raise exc.CompileError("Cannot compile Column object until "
-                                   "its 'name' is assigned.")
+            name = self._fallback_column_name(column)

         is_literal = column.is_literal
         if not is_literal and isinstance(name, elements._truncated_label):
@@ -2203,6 +2206,9 @@ class StrSQLCompiler(SQLCompiler):

     """

+    def _fallback_column_name(self, column):
+        return "<name unknown>"
+
     def visit_getitem_binary(self, binary, operator, **kw):
         return "%s[%s]" % (
             self.process(binary.left, **kw),

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