# Buggy code
def _get_referrers(
    self,
    scls: so.Object,
    *,
    scls_type: Optional[Type[so.Object_T]] = None,
    field_name: Optional[str] = None,
) -> FrozenSet[so.Object_T]:
    try:
        refs = self._refs_to[scls.id]
    except KeyError:
        return frozenset()
    else:
        referrers: Set[so.Object] = set()

        if scls_type is not None:
            if field_name is not None:
                for (st, fn), ids in refs.items():
                    if st is scls_type and fn == field_name:
                        referrers.update(self.get_by_id(objid) for objid in ids)
            else:
                for (st, _), ids in refs.items():
                    if st is scls_type:
                        referrers.update(self.get_by_id(objid) for objid in ids)
        elif field_name is not None:
            raise ValueError(
                "get_referrers: field_name cannot be used without scls_type"
            )
        else:
            refids = itertools.chain.from_iterable(refs.values())
            referrers.update(self.get_by_id(objid) for objid in refids)

        return frozenset(referrers)  # type: ignore


# Traceback (AttributeError)
ERROR: InternalServerError: 'UnionType' object has no attribute 'get_name'
Hint: This is most likely a bug in EdgeDB. Please consider opening an issue ticket at https://github.com/edgedb/edgedb/issues/new?template=bug_report.md
Server traceback:
Traceback (most recent call last):
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/site-packages/edb/server/procpool/worker.py", line 75, in worker
res = await meth(*args)
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/site-packages/edb/server/compiler/compiler.py", line 1808, in compile_eql_tokens
return self._compile(ctx=ctx, tokens=eql_tokens)
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/site-packages/edb/server/compiler/compiler.py", line 1442, in _compile
comp: dbstate.BaseQuery = self._compile_dispatch_ql(ctx, stmt)
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/site-packages/edb/server/compiler/compiler.py", line 1365, in _compile_dispatch_ql
return self._compile_ql_migration(ctx, ql)
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/site-packages/edb/server/compiler/compiler.py", line 794, in _compile_ql_migration
target_schema = s_ddl.apply_sdl(
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/site-packages/edb/schema/ddl.py", line 371, in apply_sdl
ddl_stmts = s_decl.sdl_to_ddl(current_schema, documents)
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/site-packages/edb/edgeql/declarative.py", line 230, in sdl_to_ddl
trace_dependencies(decl_ast, ctx=ctx)
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/functools.py", line 875, in wrapper
return dispatch(args[0].__class__)(*args, **kw)
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/site-packages/edb/edgeql/declarative.py", line 499, in trace_default
_register_item(node, ctx=ctx)
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/site-packages/edb/edgeql/declarative.py", line 623, in _register_item
trace_dependencies(cmd, ctx=ctx)
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/functools.py", line 875, in wrapper
return dispatch(args[0].__class__)(*args, **kw)
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/site-packages/edb/edgeql/declarative.py", line 443, in trace_ConcretePointer
_register_item(
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/site-packages/edb/edgeql/declarative.py", line 638, in _register_item
tdeps = qltracer.trace_refs(
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/site-packages/edb/edgeql/tracer.py", line 162, in trace_refs
trace(qltree, ctx=ctx)
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/functools.py", line 875, in wrapper
return dispatch(args[0].__class__)(*args, **kw)
File "/usr/lib/x86_64-linux-gnu/edgedb-server-1-alpha6/lib/python3.8/site-packages/edb/edgeql/tracer.py", line 653, in trace_Select
ctx.path_prefix = tip.get_name(ctx.schema)
AttributeError: 'UnionType' object has no attribute 'get_name'

# Fixed code
def _get_referrers(
    self,
    scls: so.Object,
    *,
    scls_type: Optional[Type[so.Object_T]] = None,
    field_name: Optional[str] = None,
) -> FrozenSet[so.Object_T]:
    try:
        refs = self._refs_to[scls.id]
    except KeyError:
        return frozenset()
    else:
        referrers: Set[so.Object] = set()

        if scls_type is not None:
            if field_name is not None:
                for (st, fn), ids in refs.items():
                    if issubclass(st, scls_type) and fn == field_name:
                        referrers.update(self.get_by_id(objid) for objid in ids)
            else:
                for (st, _), ids in refs.items():
                    if issubclass(st, scls_type):
                        referrers.update(self.get_by_id(objid) for objid in ids)
        elif field_name is not None:
            for (_, fn), ids in refs.items():
                if fn == field_name:
                    referrers.update(self.get_by_id(objid) for objid in ids)
        else:
            refids = itertools.chain.from_iterable(refs.values())
            referrers.update(self.get_by_id(objid) for objid in refids)

        return frozenset(referrers)  # type: ignore