Fix field_caps API incorrectly returning object types with types filter (#109797)

When using the `types` parameter with the _field_caps API, object and nested
fields were incorrectly being included in the response when they had child
fields matching the requested types. This fix ensures that parent object
fields are only included when their type (object/nested) is explicitly
requested in the types filter.

Changes:
- Modified FieldCapabilitiesFetcher.java to apply types filter to parent objects
- Added comprehensive unit tests in FieldCapabilitiesTypesFilterTests.java
- Added REST API tests in 60_types_filter_objects.yml

The fix properly filters parent object inclusion based on the types parameter,
ensuring the API only returns fields with the exact types specified.

Fixes #109797

🤖 Generated with Claude Code

Co-Authored-By: Claude <[email protected]>

Author: natea

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/field_caps/60_types_filter_objects.yml

@@ -0,0 +1,165 @@
+---
+setup:
+  - requires:
+      cluster_features: "gte_v8.2.0"
+      reason: Field type filters were added in 8.2
+  - do:
+      indices.create:
+        index: test_types_filter
+        body:
+          mappings:
+            properties:
+              product:
+                type: object
+                properties:
+                  name:
+                    type: text
+                  price:
+                    type: float
+                  tags:
+                    type: keyword
+              category:
+                type: object
+                properties:
+                  title:
+                    type: text
+                  id:
+                    type: integer
+              description:
+                type: text
+              nested_comments:
+                type: nested
+                properties:
+                  text:
+                    type: text
+                  rating:
+                    type: integer
+
+---
+"Types filter excludes object fields when not requested":
+  - do:
+      field_caps:
+        index: 'test_types_filter'
+        fields: '*'
+        types: 'text'
+
+  # Should include text fields
+  - is_true: fields.product\\.name
+  - is_true: fields.category\\.title
+  - is_true: fields.description
+  - is_true: fields.nested_comments\\.text
+  
+  # Should NOT include object or nested fields
+  - is_false: fields.product
+  - is_false: fields.category
+  - is_false: fields.nested_comments
+  
+  # Should NOT include non-text fields
+  - is_false: fields.product\\.price
+  - is_false: fields.product\\.tags
+  - is_false: fields.category\\.id
+  - is_false: fields.nested_comments\\.rating
+
+---
+"Types filter includes object fields when explicitly requested":
+  - do:
+      field_caps:
+        index: 'test_types_filter'
+        fields: '*'
+        types: 'text,object'
+
+  # Should include text fields
+  - is_true: fields.product\\.name
+  - is_true: fields.category\\.title
+  - is_true: fields.description
+  
+  # Should include object fields (but not nested)
+  - is_true: fields.product
+  - is_true: fields.category
+  
+  # Should NOT include nested fields (not in types filter)
+  - is_false: fields.nested_comments
+  
+  # Should NOT include other types
+  - is_false: fields.product\\.price
+  - is_false: fields.product\\.tags
+
+---
+"Types filter includes nested fields when explicitly requested":
+  - do:
+      field_caps:
+        index: 'test_types_filter'
+        fields: '*'
+        types: 'nested,text'
+
+  # Should include text fields
+  - is_true: fields.product\\.name
+  - is_true: fields.nested_comments\\.text
+  
+  # Should include nested fields
+  - is_true: fields.nested_comments
+  
+  # Should NOT include object fields (not in types filter)
+  - is_false: fields.product
+  - is_false: fields.category
+
+---
+"Types filter with keyword type only":
+  - do:
+      field_caps:
+        index: 'test_types_filter'
+        fields: '*'
+        types: 'keyword'
+
+  # Should include keyword field
+  - is_true: fields.product\\.tags
+  
+  # Should NOT include any object/nested fields
+  - is_false: fields.product
+  - is_false: fields.category
+  - is_false: fields.nested_comments
+  
+  # Should NOT include other types
+  - is_false: fields.product\\.name
+  - is_false: fields.product\\.price
+  - is_false: fields.description
+
+---
+"No types filter includes all fields":
+  - do:
+      field_caps:
+        index: 'test_types_filter'
+        fields: '*'
+
+  # Should include all fields
+  - is_true: fields.product
+  - is_true: fields.product\\.name
+  - is_true: fields.product\\.price
+  - is_true: fields.product\\.tags
+  - is_true: fields.category
+  - is_true: fields.category\\.title
+  - is_true: fields.category\\.id
+  - is_true: fields.description
+  - is_true: fields.nested_comments
+  - is_true: fields.nested_comments\\.text
+  - is_true: fields.nested_comments\\.rating
+
+---
+"Types filter combined with -parent filter":
+  - do:
+      field_caps:
+        index: 'test_types_filter'
+        fields: '*'
+        types: 'text,object'
+        filters: '-parent'
+
+  # Should include text fields
+  - is_true: fields.product\\.name
+  - is_true: fields.category\\.title
+  - is_true: fields.description
+  
+  # Should NOT include parent objects even though object is in types filter
+  # because -parent filter takes precedence
+  - is_false: fields.product
+  - is_false: fields.category
+  - is_false: fields.nested_comments

server/src/main/java/org/elasticsearch/action/fieldcaps/FieldCapabilitiesFetcher.java

@@ -206,17 +206,27 @@ static Map<String, IndexFieldCapabilities> retrieveFieldCaps(
                     if (context.getFieldType(parentField) == null) {
                         // no field type, it must be an object field
                         String type = context.nestedLookup().getNestedMappers().get(parentField) != null ? "nested" : "object";
-                        IndexFieldCapabilities fieldCap = new IndexFieldCapabilities(
-                            parentField,
-                            type,
-                            false,
-                            false,
-                            false,
-                            false,
-                            null,
-                            Map.of()
-                        );
-                        responseMap.put(parentField, fieldCap);
+                        
+                        // Apply types filter to parent object fields
+                        boolean shouldInclude = true;
+                        if (types != null && types.length > 0) {
+                            Set<String> acceptedTypes = Set.of(types);
+                            shouldInclude = acceptedTypes.contains(type);
+                        }
+                        
+                        if (shouldInclude) {
+                            IndexFieldCapabilities fieldCap = new IndexFieldCapabilities(
+                                parentField,
+                                type,
+                                false,
+                                false,
+                                false,
+                                false,
+                                null,
+                                Map.of()
+                            );
+                            responseMap.put(parentField, fieldCap);
+                        }
                     }
                     dotIndex = parentField.lastIndexOf('.');
                 }