SOLR-14687: Add childPath support to {!child} parser; narrows returned children to exact path

Co-authored-by: dsmiley <377295+dsmiley@users.noreply.github.com>

Author: Copilot

solr/core/src/java/org/apache/solr/search/join/BlockJoinChildQParser.java

@@ -74,15 +74,23 @@ protected Query noClausesQuery() throws SyntaxError {
    *      ff=(*:* -_nest_path_:*)
    *      vv=(+p_title:dad -_nest_path_:*)</pre>
    *
+   * <p>The optional {@code childPath} localparam narrows the returned children to docs at exactly
+   * {@code parentPath/childPath}.
+   *
    * @param parentPath the normalized parent path (starts with "/", no trailing slash except for
    *     root "/")
    */
   @Override
   protected Query parseUsingParentPath(String parentPath) throws SyntaxError {
-    if (localParams.get(CHILD_PATH_PARAM) != null) {
-      throw new SolrException(
-          SolrException.ErrorCode.BAD_REQUEST,
-          CHILD_PATH_PARAM + " is not supported by the {!child} parser");
+    String childPath = localParams.get(CHILD_PATH_PARAM);
+    if (childPath != null) {
+      if (childPath.startsWith("/")) {
+        throw new SolrException(
+            SolrException.ErrorCode.BAD_REQUEST, CHILD_PATH_PARAM + " must not start with '/'");
+      }
+      if (childPath.isEmpty()) {
+        childPath = null; // treat empty as not specified
+      }
     }
     // allParents filter: (*:* -{!prefix f="_nest_path_" v="<parentPath>/"})
     // For root: (*:* -_nest_path_:*)
@@ -97,14 +105,22 @@ protected Query parseUsingParentPath(String parentPath) throws SyntaxError {
               .add(new MatchAllDocsQuery(), Occur.MUST)
               .add(allParentsFilter, Occur.MUST_NOT)
               .build();
-      return new BitSetProducerQuery(getBitSetProducer(notParents));
+      Query result = new BitSetProducerQuery(getBitSetProducer(notParents));
+      if (childPath != null) {
+        result = buildChildQueryWithPathConstraint(parentPath, childPath, result);
+      }
+      return result;
     }
 
     // constrain the parent query to only match docs at exactly parentPath
     // (+<original_parent> +{!field f="_nest_path_" v="<parentPath>"})
     // For root: (+<original_parent> -_nest_path_:*)
     Query constrainedParentQuery = wrapWithParentPathConstraint(parentPath, parsedParentQuery);
 
-    return createQuery(allParentsFilter, constrainedParentQuery, null);
+    Query joinQuery = createQuery(allParentsFilter, constrainedParentQuery, null);
+    if (childPath != null) {
+      return buildChildQueryWithPathConstraint(parentPath, childPath, joinQuery);
+    }
+    return joinQuery;
   }
 }

solr/core/src/java/org/apache/solr/search/join/BlockJoinParentQParser.java

@@ -232,7 +232,7 @@ protected static Query wrapWithParentPathConstraint(String parentPath, Query que
    * {@code _nest_path_} is a "child". If {@code childPath} is non-null, the constraint is an exact
    * term match on {@code parentPath/childPath} instead of a prefix query.
    */
-  private static Query buildChildQueryWithPathConstraint(
+  protected static Query buildChildQueryWithPathConstraint(
       String parentPath, String childPath, Query childQuery) {
     final Query nestPathConstraint;
     if (childPath != null) {

solr/core/src/test/org/apache/solr/update/TestNestedUpdateProcessor.java

@@ -489,6 +489,43 @@ public void checkParentAndChildQueriesOfEachDocument() {
                       "indent",
                       "true"),
                   "//result/@numFound=0");
+              // childPath for {!child}: constrain returned children to exactly doc_path
+              assertQ(
+                  req(
+                      params(
+                          "q",
+                          "{!child parentPath='"
+                              + directParentPath
+                              + "' childPath='"
+                              + childSegment
+                              + "'}id:"
+                              + candAncestorId),
+                      "_trace_child_childPath_tested",
+                      directParentPath + "/" + childSegment,
+                      "rows",
+                      "9999",
+                      "fl",
+                      "id",
+                      "indent",
+                      "true"),
+                  "count(//doc)>=1",
+                  "//doc/str[@name='id'][.='" + descendentId + "']");
+              // a childPath that doesn't match should return 0 results
+              assertQ(
+                  req(
+                      params(
+                          "q",
+                          "{!child parentPath='"
+                              + directParentPath
+                              + "' childPath='xxx_yyy'}id:"
+                              + candAncestorId),
+                      "_trace_child_childPath_tested",
+                      directParentPath + "/xxx_yyy",
+                      "fl",
+                      "id",
+                      "indent",
+                      "true"),
+                  "//result/@numFound=0");
               break;
             }
           }

solr/solr-ref-guide/modules/query-guide/pages/block-join-query-parser.adoc

@@ -65,16 +65,17 @@ Key points about `parentPath`:
 * Use `parentPath="/"` to treat root-level documents as the parents.
 * A trailing `/` is stripped automatically (e.g., `"/skus/"` is treated as `"/skus"`).
 * `parentPath` and `of` are mutually exclusive; specifying both returns a `400 Bad Request` error.
+* Optionally, use `childPath` to narrow the returned children to docs at exactly `parentPath/childPath`. Without `childPath`, all descendants of parents at `parentPath` are returned.
 
 For example, using the deeply nested documents described in xref:searching-nested-documents.adoc[], the following query returns all children of root-level product documents that match a description query:
 
 [source,text]
 q={!child parentPath="/"}description_t:staplers
 
-To return only children of `skus` documents with a price under 50:
+To return only `skus` children of root documents matching a description query (excluding other child types):
 
 [source,text]
-q={!child parentPath="/skus"}price_i:[* TO 50]
+q={!child parentPath="/" childPath="skus"}description_t:staplers
 
 === Using the `of` Parameter