一、问题

如果select中的联合主键组合成的key不唯一（当只select部分联合主键时可能发生），那么就会把相同的key合并成一条数据。

例如KEY_A，KEY_B，KEY_C是联合主键

如果只返回KEY_A，KEY_B，那么SQL查询结果是三条，但是MyBatis返回方结果只有2条，数据3的key和数据2的key是一样的，所以不会返回数据3，MyBatis返回的结果为

如果是单主键则没有问题

二、问题分析

1.联合主键时MyBatis的处理方式

MyBatis调用query(Statement statement, ResultHandler resultHandler)方法查询

package org.apache.ibatis.executor.statement;
...
public class PreparedStatementHandler extends BaseStatementHandler {...@Overridepublic <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {PreparedStatement ps = (PreparedStatement) statement;ps.execute();//处理SQL的查询结果return resultSetHandler.<E> handleResultSets(ps);}...
}

类所在JAR包如下图：

其中resultSetHandler.<E> handleResultSets(ps);方法的实现如下

package org.apache.ibatis.executor.resultset;
...
public class DefaultResultSetHandler implements ResultSetHandler {...@Overridepublic List<Object> handleResultSets(Statement stmt) throws SQLException {ErrorContext.instance().activity("handling results").object(mappedStatement.getId());final List<Object> multipleResults = new ArrayList<Object>();int resultSetCount = 0;ResultSetWrapper rsw = getFirstResultSet(stmt);List<ResultMap> resultMaps = mappedStatement.getResultMaps();int resultMapCount = resultMaps.size();validateResultMapsCount(rsw, resultMapCount);while (rsw != null && resultMapCount > resultSetCount) {ResultMap resultMap = resultMaps.get(resultSetCount);//处理结果集handleResultSet(rsw, resultMap, multipleResults, null);rsw = getNextResultSet(stmt);cleanUpAfterHandlingResultSet();resultSetCount++;}String[] resultSets = mappedStatement.getResulSets();if (resultSets != null) {while (rsw != null && resultSetCount < resultSets.length) {ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);if (parentMapping != null) {String nestedResultMapId = parentMapping.getNestedResultMapId();ResultMap resultMap = configuration.getResultMap(nestedResultMapId);handleResultSet(rsw, resultMap, null, parentMapping);}rsw = getNextResultSet(stmt);cleanUpAfterHandlingResultSet();resultSetCount++;}}return collapseSingleResultList(multipleResults);}...
}

其中调用handleResultSet(rsw, resultMap, multipleResults, null);处理结果集

package org.apache.ibatis.executor.resultset;
...
public class DefaultResultSetHandler implements ResultSetHandler {...private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {try {if (parentMapping != null) {handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);} else {if (resultHandler == null) {DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);//处理结果集handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);multipleResults.add(defaultResultHandler.getResultList());} else {handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);}}} finally {// issue #228 (close resultsets)closeResultSet(rsw.getResultSet());}}...
}

其中handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);处理结果集。

package org.apache.ibatis.executor.resultset;
...
public class DefaultResultSetHandler implements ResultSetHandler {...private void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {if (resultMap.hasNestedResultMaps()) {ensureNoRowBounds();checkResultHandler();//处理结果集handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);} else {//【#单主键时调用】handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);}}...
}

其中handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);处理结果集

package org.apache.ibatis.executor.resultset;
...
public class DefaultResultSetHandler implements ResultSetHandler {...private void handleRowValuesForNestedResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {final DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();skipRows(rsw.getResultSet(), rowBounds);Object rowValue = null;//循环每条记录while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {final ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);//生成rowKeyfinal CacheKey rowKey = createRowKey(discriminatedResultMap, rsw, null);//通过rowKey去nestedResultObjects取partialObjectObject partialObject = nestedResultObjects.get(rowKey);// issue #577 && #542//【#resultOrdered为true时】resultOrdered官方解释：这个设置仅针对嵌套结果 select 语句适用：如果为 true，就是假设包含了嵌套结果集或是分组了，这样的话当返回一个主结果行的时候，就不会发生有对前面结果集的引用的情况。这就使得在获取嵌套的结果集的时候不至于导致内存不够用。默认值：false。if (mappedStatement.isResultOrdered()) {if (partialObject == null && rowValue != null) {//会clear，这样下个循环中partialObject就为null了nestedResultObjects.clear();//加的是上一行的数据rowValuestoreObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());}//保存当前行的数据，给下一行用rowValue = getRowValue(rsw, discriminatedResultMap, rowKey, rowKey, null, partialObject);} else {rowValue = getRowValue(rsw, discriminatedResultMap, rowKey, rowKey, null, partialObject);//如果partialObject为null，即在nestedResultObjects中通过rowKey查询不到结果；正常是null的，除非生成的rowKey与其它行的数据的rowKey重复了，这样就不会调用storeObject方法，不会把数据加到resultHandler中if (partialObject == null) {//在resultHandler的list中增加当前行数据storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());}}}//【#resultOrdered为true时】isResultOrdered()为true，且是最后一行，增加当前行数据到resultHandlerif (rowValue != null && mappedStatement.isResultOrdered() && shouldProcessMoreRows(resultContext, rowBounds)) {storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());}}...
}

其中调用createRowKey(discriminatedResultMap, rsw, null);生成rowKey

package org.apache.ibatis.executor.resultset;
...
public class DefaultResultSetHandler implements ResultSetHandler {...  private CacheKey createRowKey(ResultMap resultMap, ResultSetWrapper rsw, String columnPrefix) throws SQLException {final CacheKey cacheKey = new CacheKey();cacheKey.update(resultMap.getId());//主键字段组成的listList<ResultMapping> resultMappings = getResultMappingsForRowKey(resultMap);if (resultMappings.size() == 0) {if (Map.class.isAssignableFrom(resultMap.getType())) {createRowKeyForMap(rsw, cacheKey);} else {createRowKeyForUnmappedProperties(resultMap, rsw, cacheKey, columnPrefix);}} else {//如果表设置了主键//修改cacheKey属性createRowKeyForMappedProperties(resultMap, rsw, cacheKey, resultMappings, columnPrefix);}return cacheKey;}...
}

其中createRowKeyForMappedProperties(resultMap, rsw, cacheKey, resultMappings, columnPrefix);修改cacheKey属性

package org.apache.ibatis.executor.resultset;
...
public class DefaultResultSetHandler implements ResultSetHandler {...  private void createRowKeyForMappedProperties(ResultMap resultMap, ResultSetWrapper rsw, CacheKey cacheKey, List<ResultMapping> resultMappings, String columnPrefix) throws SQLException {//循环主键包含的字段for (ResultMapping resultMapping : resultMappings) {if (resultMapping.getNestedResultMapId() != null && resultMapping.getResultSet() == null) {// Issue #392final ResultMap nestedResultMap = configuration.getResultMap(resultMapping.getNestedResultMapId());createRowKeyForMappedProperties(nestedResultMap, rsw, cacheKey, nestedResultMap.getConstructorResultMappings(),prependPrefix(resultMapping.getColumnPrefix(), columnPrefix));} else if (resultMapping.getNestedQueryId() == null) {//主键字段之一final String column = prependPrefix(resultMapping.getColumn(), columnPrefix);final TypeHandler<?> th = resultMapping.getTypeHandler();//查询出来的结果集中的字段的名称组成的listList<String> mappedColumnNames = rsw.getMappedColumnNames(resultMap, columnPrefix);// Issue #114//查询出来的结果集中是否包含当前主键字段之一if (column != null && mappedColumnNames.contains(column.toUpperCase(Locale.ENGLISH))) {final Object value = th.getResult(rsw.getResultSet(), column);if (value != null) {//如果是，且字段中有值，把column和value更新到cacheKey中cacheKey.update(column);cacheKey.update(value);}}}}}...
}

所以，如果查询出的主键字段组合后不唯一，那么生成的cacheKey就不唯一，那么在handleRowValuesForNestedResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)中Object partialObject = nestedResultObjects.get(rowKey);的partialObject就不为空。

2.单主键时MyBatis的处理方式

在上述handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)方法中会调用handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);方法

package org.apache.ibatis.executor.resultset;
...
public class DefaultResultSetHandler implements ResultSetHandler { ...private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)throws SQLException {DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();skipRows(rsw.getResultSet(), rowBounds);//循环增加行while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);Object rowValue = getRowValue(rsw, discriminatedResultMap);storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());}}...
}

三、解决方式

1.SQL中把select中的主键字段写全

<resultMap id="BaseResultMap" type="com.test.model.Entity"><id column="KEY_A" jdbcType="DECIMAL" property="keyA" /><id column="KEY_B" jdbcType="DECIMAL" property="keyB" /><id column="KEY_C" jdbcType="DECIMAL" property="keyC" /><result column="DES" jdbcType="DECIMAL" property="des" />
</resultMap>
<select id="findEntity" parameterType="com.test.model.Entity" resultMap="ResultMap">select KEY_A,KEY_B,KEY_C,DESfrom ENTITY
</select>

2.设置resultOrdered为true

<resultMap id="BaseResultMap" type="com.test.model.Entity"><id column="KEY_A" jdbcType="DECIMAL" property="keyA" /><id column="KEY_B" jdbcType="DECIMAL" property="keyB" /><id column="KEY_C" jdbcType="DECIMAL" property="keyC" /><result column="DES" jdbcType="DECIMAL" property="des" />
</resultMap>
<select id="findEntity" parameterType="com.test.model.Entity" resultMap="ResultMap"resultOrdered="true">select KEY_A,KEY_B,DESfrom ENTITY
</select>