Fixed bug where the expression system relied upon the ``str()``

form of a some expressions when referring to the ``.c`` collection
on a ``select()`` construct, but the ``str()`` form isn't available
since the element relies on dialect-specific compilation constructs,
notably the ``__getitem__()`` operator as used with a Postgresql
``ARRAY`` element.  The fix also adds a new exception class
:class:`.UnsupportedCompilationError` which is raised in those cases
where a compiler is asked to compile something it doesn't know
how to.  Also in 0.8.3.
[ticket:2780]

4 files changed, 35 insertions, 5 deletions

diff --git a/lib/sqlalchemy/exc.py b/lib/sqlalchemy/exc.py
index f5dc1119d..cfd1e2bc7 100644
--- a/lib/sqlalchemy/exc.py
+++ b/lib/sqlalchemy/exc.py
@@ -72,6 +72,18 @@ class CircularDependencyError(SQLAlchemyError):
 class CompileError(SQLAlchemyError):
     """Raised when an error occurs during SQL compilation"""
 
+class UnsupportedCompilationError(CompileError):
+    """Raised when an operation is not supported by the given compiler.
+
+
+    .. versionadded:: 0.8.3
+
+    """
+
+    def __init__(self, compiler, element_type):
+        super(UnsupportedCompilationError, self).__init__(
+                    "Compiler %r can't render element of type %s" %
+                                (compiler, element_type))
 
 class IdentifierError(SQLAlchemyError):
     """Raised when a schema name is beyond the max character limit"""
diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index 93dc3fc4d..a5f545de9 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -696,8 +696,12 @@ class SQLCompiler(engine.Compiled):
         if disp:
             return disp(binary, operator, **kw)
         else:
-            return self._generate_generic_binary(binary,
-                                OPERATORS[operator], **kw)
+            try:
+                opstring = OPERATORS[operator]
+            except KeyError:
+                raise exc.UnsupportedCompilationError(self, operator)
+            else:
+                return self._generate_generic_binary(binary, opstring, **kw)
 
     def visit_custom_op_binary(self, element, operator, **kw):
         return self._generate_generic_binary(element,
diff --git a/lib/sqlalchemy/sql/expression.py b/lib/sqlalchemy/sql/expression.py
index 6ee110e9c..9e5c4cfcb 100644
--- a/lib/sqlalchemy/sql/expression.py
+++ b/lib/sqlalchemy/sql/expression.py
@@ -2360,7 +2360,10 @@ class ColumnElement(ClauseElement, ColumnOperators):
         """
         if name is None:
             name = self.anon_label
-            key = str(self)
+            try:
+                key = str(self)
+            except exc.UnsupportedCompilationError:
+                key = self.anon_label
         else:
             key = name
         co = ColumnClause(_as_truncated(name) if name_is_truncatable else name,
diff --git a/lib/sqlalchemy/sql/visitors.py b/lib/sqlalchemy/sql/visitors.py
index 24ada8d9f..7b729bf7f 100644
--- a/lib/sqlalchemy/sql/visitors.py
+++ b/lib/sqlalchemy/sql/visitors.py
@@ -26,6 +26,7 @@ http://techspot.zzzeek.org/2008/01/23/expression-transformations/
 from collections import deque
 from .. import util
 import operator
+from .. import exc
 
 __all__ = ['VisitableType', 'Visitable', 'ClauseVisitor',
     'CloningVisitor', 'ReplacingCloningVisitor', 'iterate',
@@ -70,14 +71,24 @@ def _generate_dispatch(cls):
             getter = operator.attrgetter("visit_%s" % visit_name)
 
             def _compiler_dispatch(self, visitor, **kw):
-                return getter(visitor)(self, **kw)
+                try:
+                    meth = getter(visitor)
+                except AttributeError:
+                    raise exc.UnsupportedCompilationError(visitor, cls)
+                else:
+                    return meth(self, **kw)
         else:
             # The optimization opportunity is lost for this case because the
             # __visit_name__ is not yet a string. As a result, the visit
             # string has to be recalculated with each compilation.
             def _compiler_dispatch(self, visitor, **kw):
                 visit_attr = 'visit_%s' % self.__visit_name__
-                return getattr(visitor, visit_attr)(self, **kw)
+                try:
+                    meth = getattr(visitor, visit_attr)
+                except AttributeError:
+                    raise exc.UnsupportedCompilationError(visitor, cls)
+                else:
+                    return meth(self, **kw)
 
         _compiler_dispatch.__doc__ = \
           """Look for an attribute named "visit_" + self.__visit_name__