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[Bug] groupCount().by(missingProperty) creates an unserializable null map key #3446
Description
Bug Type (问题类型)
other exception / error (其他异常报错)
Before submit
- 我已经确认现有的 Issues 与 FAQ 中没有相同 / 重复问题 (I have confirmed and searched that there are no similar problems in the historical issue and documents)
Environment (环境信息)
- Server Version: Apache HugeGraph 1.7.0
- Gremlin Version: 3.5.1
- Backend: memory
- Data Size: 1 vertex and 0 edges
Expected & Actual behavior (期望与实际表现)
Create a vertex without property d1, then execute a grouping traversal
through the Gremlin REST endpoint:
g.V('v1').groupCount().by('d1')Expected: the endpoint should return a valid traversal result or a stable
validation error explaining that the grouping property is absent.
Actual: response serialization fails with:
org.apache.hugegraph.HugeException: Can't write json: Null key for a Map not allowed in JSON
The error identifies a HashMap["null"] in the response value. The server
therefore constructs an intermediate result that its REST serializer cannot
encode.
Vertex/Edge example (问题点 / 边数据举例)
Setup and failing traversal:
schema.vertexLabel('DocVertex').useCustomizeStringId().ifNotExist().create()
g.addV('DocVertex').property(T.id, 'v1').iterate()
g.V('v1').groupCount().by('d1')
The vertex is equivalent to:
{
"id": "v1",
"label": "DocVertex",
"properties": {}
}Schema [VertexLabel, EdgeLabel, IndexLabel] (元数据结构)
{
"vertex_label": {
"name": "DocVertex",
"id_strategy": "CUSTOMIZE_STRING",
"properties": []
},
"edge_labels": [],
"index_labels": []
}
Reference behavior:
https://hugegraph.apache.org/docs/clients/restful-api/gremlin/
https://tinkerpop.apache.org/docs/3.5.1/reference/#groupcount-step
Activity
dosubot commentedon Nov 1, 2026
A workaround to try is to give missing values an explicit, non-null bucket:
TinkerPop 3.5.1 documents that
coalesce()selects the first child traversal that emits a value, andgroupCount().by(...)uses that projection as the grouping key. For your example, this should produce{"__missing__":1}rather than a null-keyed map. Choose a sentinel that cannot collide with real property values. I haven't executed this against 1.7.0. [1]The exception text also pinpoints the serialization boundary: HugeGraph's
JsonUtil.toJson()calls Jackson'swriteValueAsString()and wraps serialization failures asHugeException("Can't write json: ..."). This supports your distinction between constructing the traversal result and failing to encode it, though it does not establish where the null grouping key originates. [2]To reply, just mention @dosu.
Docs are dead. Just use Dosu.