can Mybatis support mapping the dynamic columns to an map field of a bean?

Question

I have a requirement which may be little complicated. Think about I have bean class like:

public class A {
   private String column1;
   private String column2;
   private Map<String,String> dynamicColumns = Maps.newHashMap();
   ....
   getter&setter of column1 and column2
   ....
   public void addExtraColumnValue(String column, String value) {
     dynamicColumns.put(column, value);
   }

}

The dynamicColumns is determined by the mapper sql like this:

<select id="queryDynamicColumns" parameterType="java.util.Map"
    resultMap="aResultMap">
         select colum1,colum2, ${dynamicColumns} from table_1
</select>

How do I write this resultMap?

I wonder do the mybatis can support this mapping in mapper configuration?( the dynamic columns value will put into the map. ) I do not get a good answer from the official website, and also I try the plugin which intercept the resultSetHandler and it seems it do not give a help for my case by looking the source code of DefaultResultSetHandler.

Answer 1

Have you tried ResultHandler?

question MyBatis - ResultHandler is not invoked could be useful.

In mybatis 3, you could see method SqlSession#select(String statement, ResultHandler handler).

You could do what ever you want in your custom ResultHandler.

Please let me know, if any problem.

---

It comes to my mind, if you derived from DefaultResultSetHandler and look at this:

public List<Object> handleResultSets(Statement stmt) throws SQLException {
  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);
    handleResultSet(rsw, resultMap, multipleResults, null);
    rsw = getNextResultSet(stmt);
    cleanUpAfterHandlingResultSet();
    resultSetCount++;
  }

  while (rsw != null && resultSetCount < mappedStatement.getResulSets().length) {
    ResultMapping parentMapping = nextResultMaps.get(mappedStatement.getResulSets()[  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);
}

You can update the resultMaps runtime information, such as update its ResultMapping#column and then do dynamic binding. That's should work!

Answer 2

Yes it can be done.

1.Whatever you are doing to select dynamic columns is ok.

2.To map the result: Since you are selecting the columns(dynamic) from table_1(Which will not change),you can have a POJO class with the fields present in table_1 and put that in resultMap.

<select id="queryDynamicColumns" parameterType="java.util.Map"
    resultMap="**Pojo Class With Fields As Columns of table_1**">
         select colum1,colum2, ${dynamicColumns} from table_1
</select>

Update:

If your database table columns changes than try using JAVA REFLECTION to dynamic columns