title: How to index multi dimension array in solr field
body: <p>I am indexing Mysql database with solr, I have one-many relation between users table and order table:one user can have many orders.</p>
  
  <p>order table have many columns (id, orderDate, caseNumber). </p>
  
  <p>My goal is to index these tables in solr and have <code>USR_ID</code> field to store the user id, <code>ORDERS</code> feild  type= multidimensional array to store each order for that user as an associative array.</p>
  
  <p>the desired result is: </p>
  
  <pre><code>{
          "USR_ID":"10",
          "ORDERS":[
               {"ID":"1" ,"ORDER_DATE":"12-03-2018", "CASE_NUMBER":"554"}, //FIRST FIELD
               {"ID":"9","ORDER_DATE":"15-03-2018", "CASE_NUMBER":"569"} //SECOND FIELD
           ]
  }
  </code></pre>
  
  <p>what i am getting is one dimensional array with all orders columns</p>
  
  <pre><code>{
          "USR_ID":"10",
          "ORDERS":[
               "1", "12-03-2018", "554", //FIRST FIELD
               "9", "15-03-2018", "569" //SECOND FIELD
           ]
  }
  </code></pre>
  
  <p>Here is what I tried.
  entities config in data-config.xml </p>
  
  <pre><code>&lt;dataConfig&gt;
      &lt;dataSource type="JdbcDataSource" 
                  driver="com.mysql.jdbc.Driver"
                  url="jdbc:mysql://localhost/mydb1" 
                  user="" 
                  password=""/&gt;
      &lt;document&gt;
          &lt;entity name="USERS"  
              pk="USR_ID"
              query="SELECT USR_UID, FROM USERS"
              deltaImportQuery="SELECT USR_UID, FROM USERS WHERE USR_UID='${dih.delta.USR_UID}'"
              deltaQuery="SELECT USR_UID FROM USERS WHERE USERS.USR_UPDATE_DATE &gt; '${dih.last_index_time}'"&gt;
              &lt;entity name="ORDER"  pk="ID"
                  query="SELECT ID AS ORDERID, ORDER_DATE, CASE_NUMBER FROM ORDER WHERE USR_ID = '${USERS.USR_UID}'"
                  deltaQuery="select ID from ORDER where UPDATED_AT &gt; '${dih.last_index_time}'"
                  parentDeltaQuery="SELECT USR_UID FROM USERS WHERE USR_UID = ${ORDER.USR_UID}"&gt;
                      &lt;field column="ORDERID" name="ORDERS" /&gt;
                      &lt;field column="CREATION_DATE" name="ORDERS" /&gt;
                      &lt;field column="CASE_NUMBER" name="ORDERS" /&gt;     
              &lt;/entity&gt;
          &lt;/entity&gt;
      &lt;/document&gt;
  &lt;/dataConfig&gt;
  
  </code></pre>
  
  <p>Here is fields definition in schema.xml file</p>
  
  <pre><code>
    &lt;field name="USR_ID" type="string" indexed="true" stored="true" required="true" multiValued="false" /&gt;
    &lt;field name="ORDERS" type="text_general" indexed="true" stored="true" required="false" multiValued="true"/&gt;
  
  </code></pre>