
Traceback (most recent call last):


 "/Users/adammb/anaconda3/envs/urbandatascience/lib/python3.8/site-packages/tornado/web.py", line 1702, in _execute

result = method(*self.path_args, **self.path_kwargs)


 "/Users/adammb/anaconda3/envs/urbandatascience/lib/python3.8/site-packages/tornado/web.py", line 3173, in wrapper

return method(self, *args, **kwargs)


 "/Users/adammb/anaconda3/envs/urbandatascience/lib/python3.8/site-packages/nbgrader/server_extensions/formgrader/base.py", line 108, in wrapper

return f(self, *args, **kwargs)


 "/Users/adammb/anaconda3/envs/urbandatascience/lib/python3.8/site-packages/nbgrader/server_extensions/formgrader/base.py", line 117, in wrapper

return f(self, *args, **kwargs)


 "/Users/adammb/anaconda3/envs/urbandatascience/lib/python3.8/site-packages/nbgrader/server_extensions/formgrader/apihandlers.py", line 167, in get

submissions = self.api.get_submissions(assignment_id)


 "/Users/adammb/anaconda3/envs/urbandatascience/lib/python3.8/site-packages/nbgrader/apps/api.py", line 587, in get_submissions

db_submissions = gb.submission_dicts(assignment_id)


 "/Users/adammb/anaconda3/envs/urbandatascience/lib/python3.8/site-packages/nbgrader/api.py", line 3053, in submission_dicts

all_scores = aliased(union_all(


 "/Users/adammb/anaconda3/envs/urbandatascience/lib/python3.8/site-packages/sqlalchemy/orm/util.py", line 1201, in aliased

return AliasedClass(


 "/Users/adammb/anaconda3/envs/urbandatascience/lib/python3.8/site-packages/sqlalchemy/orm/util.py", line 495, in __init__

mapper = insp.mapper

AttributeError: 'CompoundSelect' object has no attribute 'mapper'

```Traceback (most recent call last):


 "temp.py", line 16, in <module>

Base.metadata.create_all(engine)


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/sql/schema.py", line 4287, in create_all

ddl.SchemaGenerator, self, checkfirst=checkfirst, tables=tables


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 2036, in _run_visitor

conn._run_visitor(visitorcallable, element, **kwargs)


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1607, in _run_visitor

visitorcallable(self.dialect, self, **kwargs).traverse_single(element)


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/sql/visitors.py", line 131, in traverse_single

return meth(obj, **kw)


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/sql/ddl.py", line 781, in visit_metadata

_is_metadata_operation=True,


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/sql/visitors.py", line 131, in traverse_single

return meth(obj, **kw)


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/sql/ddl.py", line 833, in visit_table

self.traverse_single(index)


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/sql/visitors.py", line 131, in traverse_single

return meth(obj, **kw)


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/sql/ddl.py", line 862, in visit_index

self.connection.execute(CreateIndex(index))


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 988, in execute

return meth(self, multiparams, params)


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/sql/ddl.py", line 72, in _execute_on_connection

return connection._execute_ddl(self, multiparams, params)


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1043, in _execute_ddl

else None,


 "<string>", line 1, in <lambda>


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/sql/elements.py", line 462, in compile

return self._compiler(dialect, bind=bind, **kw)


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/sql/ddl.py", line 29, in _compiler

return dialect.ddl_compiler(dialect, self, **kw)


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/sql/compiler.py", line 319, in __init__

self.string = self.process(self.statement, **compile_kwargs)


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/sql/compiler.py", line 350, in process

return obj._compiler_dispatch(self, **kwargs)


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/sql/visitors.py", line 91, in _compiler_dispatch

return meth(self, **kw)


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/dialects/sqlite/base.py", line 1191, in visit_create_index

for expr in index.expressions


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/dialects/sqlite/base.py", line 1191, in <genexpr>

for expr in index.expressions


 "/home/jmabey/Code/bug/env/lib/python3.7/site-packages/sqlalchemy/sql/compiler.py", line 350, in process

return obj._compiler_dispatch(self, **kwargs)
AttributeError: 'NoneType' object has no attribute '_compiler_dispatch'
