fix: Group by field dependency without selection

internal/planner/mapper/mapper.go

@@ -210,31 +210,12 @@
 		}
 	}
 
-	// Resolve groupBy mappings i.e. alias remapping and handle missed inner group.
-	if selectRequest.GroupBy.HasValue() {
-		groupByFields := selectRequest.GroupBy.Value().Fields
-		// Remap all alias field names to use their internal field name mappings.
-		for index, groupByField := range groupByFields {
-			fieldDesc, ok := definition.GetFieldByName(groupByField)
-			if ok && fieldDesc.Kind.IsObject() {
-				if fieldDesc.Kind.IsArray() {
-					return nil, NewErrInvalidFieldToGroupBy(groupByField)
-				}
-				groupByFields[index] = request.ToFieldID(groupByField)
-			}
-		}
-
-		selectRequest.GroupBy = immutable.Some(
-			request.GroupBy{
-				Fields: groupByFields,
-			},
-		)
-
-		// If there is a groupBy, and no inner group has been requested, we need to map the property here
-		if _, isGroupFieldMapped := mapping.IndexesByName[request.GroupFieldName]; !isGroupFieldMapped {
-			index := mapping.GetNextIndex()
-			mapping.Add(index, request.GroupFieldName)
-		}
+	// Resolve groupBy dependencies: alias/relation remapping, map the missed inner
+	// group field, and ensure groupBy fields are fetched even when not part of the
+	// selection set.
+	fields, err = resolveGroupByDependencies(definition, selectRequest, mapping, fields)
+	if err != nil {
+		return nil, err
 	}
 
 	targetable, err := toTargetable(thisIndex, selectRequest, mapping)
@@ -391,6 +372,78 @@
 	return nil, ErrMissingSelect
 }
 
+// resolveGroupByDependencies remaps the groupBy fields to their internal field names,
+// maps the synthetic GROUP field if no inner group was requested, and ensures that
+// every groupBy field is fetched even when it is not part of the selection set.
+//
+// This mirrors how [resolveOrderDependencies] adds order fields that were missed due
+// to not being rendered.
+func resolveGroupByDependencies(
+	definition client.CollectionVersion,
+	selectRequest *request.Select,
+	mapping *core.DocumentMapping,
+	fields []Requestable,
+) ([]Requestable, error) {
+	if !selectRequest.GroupBy.HasValue() {
+		return fields, nil
+	}
+
+	groupByFields := selectRequest.GroupBy.Value().Fields
+	// Remap all object (relation) field names to use their internal foreign-key field
+	// id, as that is the scalar value actually stored on - and fetched from - the document.
+	for index, groupByField := range groupByFields {
+		fieldDesc, ok := definition.GetFieldByName(groupByField)
+		if ok && fieldDesc.Kind.IsObject() {
+			if fieldDesc.Kind.IsArray() {
+				return nil, NewErrInvalidFieldToGroupBy(groupByField)
+			}
+			groupByFields[index] = request.ToFieldID(groupByField)
+		}
+	}
+
+	selectRequest.GroupBy = immutable.Some(
+		request.GroupBy{
+			Fields: groupByFields,
+		},
+	)
+
+	// If there is a groupBy, and no inner group has been requested, we need to map the property here
+	if _, isGroupFieldMapped := mapping.IndexesByName[request.GroupFieldName]; !isGroupFieldMapped {
+		index := mapping.GetNextIndex()
+		mapping.Add(index, request.GroupFieldName)
+	}
+
+	// Ensure every groupBy field is fetched, even when it is not part of the selection
+	// set. The field is added as a hidden dependency (it is given no render key) so that
+	// its value is available for group-key computation without being returned in the response.
+	for _, groupByField := range groupByFields {
+		alreadyRequested := false
+		for _, existingField := range fields {
+			if existingField.GetName() == groupByField {
+				alreadyRequested = true
+				break
+			}
+		}
+		if alreadyRequested {
+			continue
+		}
+
+		fieldIndexes := mapping.IndexesByName[groupByField]
+		if len(fieldIndexes) == 0 {
+			// Should be unreachable for a valid groupBy field as all base fields are
+			// mapped by getTopLevelInfo, but guard against an out-of-range panic.
+			continue
+		}
+
+		fields = append(fields, &Field{
+			Index: fieldIndexes[0],
+			Name:  groupByField,
+		})
+	}
+
+	return fields, nil
+}
+
 // resolveAggregates figures out which fields the given aggregates are targeting
 // and converts the aggregateRequest into an Aggregate, appending it onto the given
 // fields slice.

tests/integration/query/one_to_many/with_group_unselected_field_test.go

@@ -0,0 +1,181 @@
+// Copyright 2026 Democratized Data Foundation
+//
+// This file is part of the DefraDB test suite.
+//
+// The DefraDB test suite is licensed under either:
+//
+//   (1) GNU Affero General Public License v3
+//   (2) Business Source License 1.1
+//
+// See tests/LICENSE for details.
+
+package one_to_many
+
+import (
+	"testing"
+
+	"github.com/sourcenetwork/defradb/tests/action"
+	testUtils "github.com/sourcenetwork/defradb/tests/integration"
+)
+
+var booksByThreeAuthors = []any{
+	&action.AddDoc{
+		CollectionID: 0,
+		Doc: `{
+			"name": "Painted House",
+			"rating": 4.9,
+			"_authorID": "bae-9d52c335-c8e3-5782-8daa-e359c106e0ab"
+		}`,
+	},
+	&action.AddDoc{
+		CollectionID: 0,
+		Doc: `{
+			"name": "A Time for Mercy",
+			"rating": 4.5,
+			"_authorID": "bae-9d52c335-c8e3-5782-8daa-e359c106e0ab"
+		}`,
+	},
+	&action.AddDoc{
+		CollectionID: 0,
+		Doc: `{
+			"name": "Candide",
+			"rating": 4.95,
+			"_authorID": "bae-b9c6cd5a-a931-5984-994d-7c435baa9f32"
+		}`,
+	},
+	&action.AddDoc{
+		CollectionID: 0,
+		Doc: `{
+			"name": "Zadig",
+			"rating": 4.91,
+			"_authorID": "bae-b9c6cd5a-a931-5984-994d-7c435baa9f32"
+		}`,
+	},
+	&action.AddDoc{
+		CollectionID: 1,
+		Doc: `{
+			"name": "John Grisham",
+			"age": 65,
+			"verified": true
+		}`,
+	},
+	&action.AddDoc{
+		CollectionID: 1,
+		Doc: `{
+			"name": "Voltaire",
+			"age": 327,
+			"verified": true
+		}`,
+	},
+}
+
+func TestQueryOneToMany_WithGroupByRelatedIDWithoutRenderedGroupField(t *testing.T) {
+	test := testUtils.TestCase{
+		Actions: append(
+			append([]any{}, booksByThreeAuthors...),
+			&action.Request{
+				Request: `query {
+					Book(groupBy: [_authorID]) {
+						GROUP {
+							name
+						}
+					}
+				}`,
+				Results: map[string]any{
+					"Book": []map[string]any{
+						{
+							"GROUP": []map[string]any{
+								{"name": "Painted House"},
+								{"name": "A Time for Mercy"},
+							},
+						},
+						{
+							"GROUP": []map[string]any{
+								{"name": "Candide"},
+								{"name": "Zadig"},
+							},
+						},
+					},
+				},
+				NonOrderedResults: true,
+			},
+		),
+	}
+
+	executeTestCase(t, test)
+}
+
+func TestQueryOneToMany_WithGroupByRelationObjectRenderingRelatedID(t *testing.T) {
+	test := testUtils.TestCase{
+		Actions: append(
+			append([]any{}, booksByThreeAuthors...),
+			&action.Request{
+				Request: `query {
+					Book(groupBy: [author]) {
+						_authorID
+						GROUP {
+							name
+						}
+					}
+				}`,
+				Results: map[string]any{
+					"Book": []map[string]any{
+						{
+							"_authorID": "bae-9d52c335-c8e3-5782-8daa-e359c106e0ab",
+							"GROUP": []map[string]any{
+								{"name": "Painted House"},
+								{"name": "A Time for Mercy"},
+							},
+						},
+						{
+							"_authorID": "bae-b9c6cd5a-a931-5984-994d-7c435baa9f32",
+							"GROUP": []map[string]any{
+								{"name": "Candide"},
+								{"name": "Zadig"},
+							},
+						},
+					},
+				},
+				NonOrderedResults: true,
+			},
+		),
+	}
+
+	executeTestCase(t, test)
+}
+
+func TestQueryOneToMany_WithGroupByRelationObjectWithoutRenderedGroupField(t *testing.T) {
+	test := testUtils.TestCase{
+		Actions: append(
+			append([]any{}, booksByThreeAuthors...),
+			&action.Request{
+				Request: `query {
+					Book(groupBy: [author]) {
+						GROUP {
+							name
+						}
+					}
+				}`,
+				Results: map[string]any{
+					"Book": []map[string]any{
+						{
+							"GROUP": []map[string]any{
+								{"name": "Painted House"},
+								{"name": "A Time for Mercy"},
+							},
+						},
+						{
+							"GROUP": []map[string]any{
+								{"name": "Candide"},
+								{"name": "Zadig"},
+							},
+						},
+					},
+				},
+				NonOrderedResults: true,
+			},
+		),
+	}
+
+	executeTestCase(t, test)
+}