Query including fragments return null

[xsoufiane]

Describe the bug
We have a query using fragments

{
  ...ShoppingListFragmentTest
}

fragment ShoppingListFragmentTest on ShoppingListQueryInterface {
shoppingLists {
  results {
    id
  }
  total
}

the router returns data:null for it, although the sub-service returns a valid non-null response. We are assuming that there is an issue in assembling the response. The query plan for the query is:

query plan is: QueryPlan {
   usage_reporting: UsageReporting { 
 	stats_report_key: "# -\nfragment ShoppingListFragmentTest on ShoppingListQueryInterface{shoppingLists{results{id}total {...ShoppingListFragmentTest}", 
        referenced_fields_by_type: {
           "ShoppingListQueryInterface": ReferencedFieldsForType { field_names: ["shoppingLists"], is_interface: true }, 
           "ShoppingListQueryResult": ReferencedFieldsForType { field_names: ["results", "total"], is_interface: false },            
           "ShoppingList": ReferencedFieldsForType { field_names: ["id"], is_interface: false }
        } 
    },
     root: Fetch(FetchNode { service_name: "sphere", requires: [], variable_usages: [], operation: "{shoppingLists{results{id}total}}", operation_name: None, operation_kind: Query }) }

Expected behavior
router should be able to assemble the response and return non-null data.

[xsoufiane]

for info we are using router-v0.9.3

[xsoufiane]

for info we are using router-v0.9.3

same issue in v0.9.5

[Geal]

thanks! That looks like #1286, no?

[Geal]

could you give more information on the types and the data returned by the subgraph?

[Geal]

@xsoufiane does this look like the schema you are working with?
https://github.com/apollographql/router/pull/1296/files#diff-ca21c6c2d8f5eacbc8fd08524c17571552b1ec649c2911b908b03b94b2c7c99fR4578-R4603
Maybe with the object field set to non null?

[xsoufiane]

@xsoufiane does this look like the schema you are working with?
https://github.com/apollographql/router/pull/1296/files#diff-ca21c6c2d8f5eacbc8fd08524c17571552b1ec649c2911b908b03b94b2c7c99fR4578-R4603
Maybe with the object field set to non null?

thank you @Geal yes it's the same 🤩

[Geal]

Great, then I have a good idea of the cause. May I ask why you go through a fragment then an interface on the query object? This feels a bit weird to me so there's probably a use case I don't know about

[xsoufiane]

Great, then I have a good idea of the cause. May I ask why you go through a fragment then an interface on the query object? This feels a bit weird to me so there's probably a use case I don't know about

also not sure why, those interfaces are implemented by other types, and the fragments maybe to not repeat the same code. Probably @yanns can tell more about this.

thanks! That looks like #1286, no?

seems related 🤔

[yanns]

Yes we use interfaces on query so that one can re-use the same queries on the top field, or on another sub-field:

{
  orders { // all orders
     ....
  }
}

{
  store(key: "store-DE") {
    orders { // orders in the store in Germany
       ....
    }
  }
}

[Mithras]

While this is an open issue. Is there a reason router silently returns null instead of propagating an exception or something? I have another case where router returns null (not related to fragments) and the lack of details hurts my soul

[Geal]

@yanns thanks, that's interesting :)
@Mithras yes, that's not great for now, we're planning to add error messages along with the nullability rules. Could you post another issue for your other case? I'd be happy to take a look

[Geal]

@Mithras making a note of the error message for nullified fields in #1304

[xsoufiane]

thank you @Geal