Support nested cursor

By specifying a special name NESTED_CURSOR to `resultSet` attribute,

Should fix mybatis#566

src/main/java/org/apache/ibatis/executor/resultset/DefaultResultSetHandler.java

@@ -569,8 +569,20 @@ private Object getPropertyMappingValue(ResultSet rs, MetaObject metaResultObject
       return getNestedQueryMappingValue(rs, metaResultObject, propertyMapping, lazyLoader, columnPrefix);
     }
     if (propertyMapping.getResultSet() != null) {
-      addPendingChildRelation(rs, metaResultObject, propertyMapping); // TODO is that OK?
-      return DEFERRED;
+      if (ResultMapping.NESTED_CURSOR.equals(propertyMapping.getResultSet())) {
+        final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);
+        ResultMap nestedResultMap = resolveDiscriminatedResultMap(rs,
+            configuration.getResultMap(propertyMapping.getNestedResultMapId()),
+            getColumnPrefix(columnPrefix, propertyMapping));
+        ResultSetWrapper rsw = new ResultSetWrapper(rs.getObject(column, ResultSet.class), configuration);
+        List<Object> results = new ArrayList<>();
+        handleResultSet(rsw, nestedResultMap, results, null);
+        linkObjects(metaResultObject, propertyMapping, results.get(0), true);
+        return metaResultObject.getValue(propertyMapping.getProperty());
+      } else {
+        addPendingChildRelation(rs, metaResultObject, propertyMapping); // TODO is that OK?
+        return DEFERRED;
+      }
     } else {
       final TypeHandler<?> typeHandler = propertyMapping.getTypeHandler();
       final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);
@@ -1527,10 +1539,19 @@ private void createRowKeyForMap(ResultSetWrapper rsw, CacheKey cacheKey) throws
   }
 
   private void linkObjects(MetaObject metaObject, ResultMapping resultMapping, Object rowValue) {
+    linkObjects(metaObject, resultMapping, rowValue, false);
+  }
+
+  private void linkObjects(MetaObject metaObject, ResultMapping resultMapping, Object rowValue,
+      boolean isNestedCursorResult) {
     final Object collectionProperty = instantiateCollectionPropertyIfAppropriate(resultMapping, metaObject);
     if (collectionProperty != null) {
       final MetaObject targetMetaObject = configuration.newMetaObject(collectionProperty);
-      targetMetaObject.add(rowValue);
+      if (isNestedCursorResult) {
+        targetMetaObject.addAll((List<?>) rowValue);
+      } else {
+        targetMetaObject.add(rowValue);
+      }
 
       // it is possible for pending creations to get set via property mappings,
       // keep track of these, so we can rebuild them.
@@ -1543,10 +1564,16 @@ private void linkObjects(MetaObject metaObject, ResultMapping resultMapping, Obj
         pendingPccRelations.put(originalObject, pendingRelation);
       }
     } else {
-      metaObject.setValue(resultMapping.getProperty(), rowValue);
+      metaObject.setValue(resultMapping.getProperty(),
+          isNestedCursorResult ? toSingleObj((List<?>) rowValue) : rowValue);
     }
   }
 
+  private Object toSingleObj(List<?> list) {
+    // Even if there are multiple elements, silently returns the first one.
+    return list.isEmpty() ? null : list.get(0);
+  }
+
   private Object instantiateCollectionPropertyIfAppropriate(ResultMapping resultMapping, MetaObject metaObject) {
     final String propertyName = resultMapping.getProperty();
     Object propertyValue = metaObject.getValue(propertyName);

src/main/java/org/apache/ibatis/mapping/MappedStatement.java

@@ -1,5 +1,5 @@
 /*
- *    Copyright 2009-2024 the original author or authors.
+ *    Copyright 2009-2025 the original author or authors.
  *
  *    Licensed under the Apache License, Version 2.0 (the "License");
  *    you may not use this file except in compliance with the License.
@@ -16,6 +16,7 @@
 package org.apache.ibatis.mapping;
 
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 
@@ -203,6 +204,11 @@ public MappedStatement build() {
       assert mappedStatement.id != null;
       assert mappedStatement.sqlSource != null;
       assert mappedStatement.lang != null;
+      if (mappedStatement.resultSets != null
+          && Arrays.asList(mappedStatement.resultSets).contains(ResultMapping.NESTED_CURSOR)) {
+        throw new IllegalStateException(
+            "Result set name '" + ResultMapping.NESTED_CURSOR + "' is reserved, please assign another name.");
+      }
       mappedStatement.resultMaps = Collections.unmodifiableList(mappedStatement.resultMaps);
       return mappedStatement;
     }

src/main/java/org/apache/ibatis/mapping/ResultMapping.java

@@ -1,5 +1,5 @@
 /*
- *    Copyright 2009-2024 the original author or authors.
+ *    Copyright 2009-2025 the original author or authors.
  *
  *    Licensed under the Apache License, Version 2.0 (the "License");
  *    you may not use this file except in compliance with the License.
@@ -30,6 +30,11 @@
  */
 public class ResultMapping {
 
+  /**
+   * Reserved result set name indicating a nested cursor is mapped to this property.
+   */
+  public static final String NESTED_CURSOR = "NESTED_CURSOR";
+
   private Configuration configuration;
   private String property;
   private String column;
@@ -166,7 +171,8 @@ private void validate() {
         if (resultMapping.foreignColumn != null) {
           numForeignColumns = resultMapping.foreignColumn.split(",").length;
         }
-        if (numColumns != numForeignColumns) {
+        if (numColumns != numForeignColumns && !NESTED_CURSOR.equals(resultMapping.resultSet)) {
+          // Nested cursor does not use 'foreignKey'
           throw new IllegalStateException(
               "There should be the same number of columns and foreignColumns in property " + resultMapping.property);
         }

src/site/es/xdoc/sqlmap-xml.xml

@@ -1428,6 +1428,31 @@ When using this functionality, it is preferable for the entire mapping hierarchy
     columnPrefix="co_" />
 </resultMap>]]></source>
 
+
+<h4>Nested Cursor for Association</h4>
+
+<p>Some databases can return <code>java.sql.ResultSet</code> as a column value.<br />
+Here is the statement and result map.</p>
+
+<source><![CDATA[<resultMap id="blogResult" type="Blog">
+  <id property="id" column="id" />
+  <result property="title" column="title" />
+  <association property="author" column="author" resultSet="NESTED_CURSOR">
+    <id property="id" column="id" />
+    <result property="username" column="username" />
+  </association>
+</resultMap>
+
+<select id="selectBlog" resultMap="blogResult">
+  select b.id, b.name, cursor(
+    select a.id, a.username from author a where b.author_id = a.id
+  ) author from blog b
+</select>]]></source>
+
+<p>Compared to the examples in the previous section, the key difference is the <code>resultSet</code> attribute in the <code>&lt;association&gt;</code> element.<br />
+Its value <code>NESTED_CURSOR</code> indicates that the value of the column <code>author</code> is nested cursor.</p>
+
+
         <h4>ResultSets múltiples en Association</h4>
 
         <table>
@@ -1601,6 +1626,29 @@ SELECT * FROM AUTHOR WHERE ID = #{id}]]></source>
   <result property="body" column="body"/>
 </resultMap>]]></source>
 
+
+<h4>Nested Cursor for Collection</h4>
+
+<p>It might be obvious, but nested cursor can return multiple rows.<br />
+Just like <code>&lt;association&gt;</code>, you just need to specify <code>resultSet="NESTED_CURSOR"</code> in the <code>&lt;collection&gt;</code> element.</p>
+
+<source><![CDATA[<resultMap id="blogResult" type="Blog">
+  <id property="id" column="id" />
+  <result property="title" column="title" />
+  <collection property="posts" column="posts" resultSet="NESTED_CURSOR">
+    <id property="id" column="id" />
+    <result property="subject" column="subject" />
+    <result property="body" column="body" />
+  </collection>
+</resultMap>
+
+<select id="selectBlog" resultMap="blogResult">
+  select b.id, b.name, cursor(
+    select p.id, p.subject, p.body from post p where p.blog_id = b.id
+  ) posts from blog b
+</select>]]></source>
+
+
         <h4>ResultSets múltiples en Collection</h4>
 
         <p>

src/site/ja/xdoc/sqlmap-xml.xml

@@ -1608,6 +1608,31 @@ User{username=Peter, roles=[Users, Maintainers, Approvers]}
     columnPrefix="co_" />
 </resultMap>]]></source>
 
+
+<h4>ネストされたカーソルを association にマッピングする</h4>
+
+<p>データベースによっては列の値として <code>java.sql.ResultSet</code> 返すことができます。<br />
+このような結果をマッピングする例を説明します。</p>
+
+<source><![CDATA[<resultMap id="blogResult" type="Blog">
+  <id property="id" column="id" />
+  <result property="title" column="title" />
+  <association property="author" column="author" resultSet="NESTED_CURSOR">
+    <id property="id" column="id" />
+    <result property="username" column="username" />
+  </association>
+</resultMap>
+
+<select id="selectBlog" resultMap="blogResult">
+  select b.id, b.name, cursor(
+    select a.id, a.username from author a where b.author_id = a.id
+  ) author from blog b
+</select>]]></source>
+
+<p>上の章の例との重要な違いは、<code>&lt;association&gt;</code> 要素の <code>resultSet</code> 属性に特別な値 <code>NESTED_CURSOR</code> を指定している点です。<br />
+これによって、<code>author</code> 列の値をネストされたカーソルとしてマッピングすることができます。</p>
+
+
         <h4>複数の ResultSet を association にマッピングする</h4>
 
         <table>
@@ -1789,6 +1814,29 @@ SELECT * FROM AUTHOR WHERE ID = #{id}]]></source>
   <result property="body" column="body"/>
 </resultMap>]]></source>
 
+
+<h4>ネストされたカーソルを collection にマッピングする</h4>
+
+<p>当然ですが、ネストされたカーソルが複数の値を返す場合もあります。<br />
+先に説明した <code>&lt;association&gt;</code> の場合と同様、 <code>&lt;collection&gt;</code> 要素に <code>resultSet="NESTED_CURSOR"</code> を指定してください。</p>
+
+<source><![CDATA[<resultMap id="blogResult" type="Blog">
+  <id property="id" column="id" />
+  <result property="title" column="title" />
+  <collection property="posts" column="posts" resultSet="NESTED_CURSOR">
+    <id property="id" column="id" />
+    <result property="subject" column="subject" />
+    <result property="body" column="body" />
+  </collection>
+</resultMap>
+
+<select id="selectBlog" resultMap="blogResult">
+  select b.id, b.name, cursor(
+    select p.id, p.subject, p.body from post p where p.blog_id = b.id
+  ) posts from blog b
+</select>]]></source>
+
+
         <h4>複数の ResultSets を collection にマッピングする</h4>
 
         <p>