more actionable federation errors for nil key field queries

plugin/federation/federation.gotpl

@@ -294,6 +294,9 @@ func (ec *executionContext) resolveManyEntities(
 	{{- if .Resolvers }}
 
 		func entityResolverNameFor{{$entity.Name}}(ctx context.Context, rep EntityRepresentation) (string, error) {
+			// we collect errors because a later entity resolver may work fine
+			// when an entity has multiple keys
+			entityResolverErrs := []error{}
 			{{- range .Resolvers }}
 				for {
 					var (
@@ -310,10 +313,15 @@ func (ec *executionContext) resolveManyEntities(
 						{{- range $i, $field := .Field }}
 							val, ok = m["{{.}}"]
 							if !ok {
+								entityResolverErrs = append(entityResolverErrs,
+									fmt.Errorf("%w due to missing Key Field \"{{.}}\" for {{$entity.Name}}", ErrTypeNotFound))
 								break
 							}
 							{{- if (ne $i $keyField.Field.LastIndex ) }}
 								if m, ok = val.(map[string]interface{}); !ok {
+									// nested field value is not a map[string]interface so don't use it
+									entityResolverErrs = append(entityResolverErrs,
+										fmt.Errorf("%w due to nested Key Field \"{{.}}\" value not matching map[string]any for {{$entity.Name}}", ErrTypeNotFound))
 									break
 								}
 							{{- else}}
@@ -324,12 +332,15 @@ func (ec *executionContext) resolveManyEntities(
 						{{- end}}
 					{{- end }}
 					if allNull {
+						entityResolverErrs = append(entityResolverErrs,
+							fmt.Errorf("%w due to all null value KeyFields for {{$entity.Name}}", ErrTypeNotFound))
 						break
 					}
 					return "{{.ResolverName}}", nil
 				}
 			{{- end }}
-			return "", fmt.Errorf("%w for {{$entity.Name}}", ErrTypeNotFound)
+			return "", fmt.Errorf("%w for {{$entity.Name}} due to %v", ErrTypeNotFound,
+				errors.Join(entityResolverErrs...).Error())
 		}
 	{{- end }}
 {{- end }}

plugin/federation/federation_entityresolver_test.go

@@ -173,12 +173,12 @@ func TestEntityResolver(t *testing.T) {
 				Bar int `json:"bar"`
 			} `json:"_entities"`
 		}
-
+		eq := entityQuery([]string{
+			"WorldWithMultipleKeys {foo hello {name}}",
+			"WorldWithMultipleKeys {bar}",
+		})
 		err := c.Post(
-			entityQuery([]string{
-				"WorldWithMultipleKeys {foo hello {name}}",
-				"WorldWithMultipleKeys {bar}",
-			}),
+			eq,
 			&resp,
 			client.Var("representations", representations),
 		)