title: org.apache.solr.common.SolrException: TransactionLog doesn't know how to serialize class org.bson.types.ObjectId; try implementing ObjectResolver?
body: <p>When performing a data import from mongodb, Solr throws the following error:</p>
  
  <pre><code>org.apache.solr.common.SolrException: TransactionLog doesn't know how to serialize class org.bson.types.ObjectId; try implementing ObjectResolver?
  at org.apache.solr.update.TransactionLog$1.resolve(TransactionLog.java:100)
  at org.apache.solr.common.util.JavaBinCodec.writeVal(JavaBinCodec.java:234)
  at org.apache.solr.common.util.JavaBinCodec.writeSolrInputDocument(JavaBinCodec.java:589)
  at org.apache.solr.update.TransactionLog.write(TransactionLog.java:395)
  at org.apache.solr.update.UpdateLog.add(UpdateLog.java:532)
  at org.apache.solr.update.UpdateLog.add(UpdateLog.java:516)
  at org.apache.solr.update.DirectUpdateHandler2.doNormalUpdate(DirectUpdateHandler2.java:320)
  at org.apache.solr.update.DirectUpdateHandler2.addDoc0(DirectUpdateHandler2.java:239)
  at org.apache.solr.update.DirectUpdateHandler2.addDoc(DirectUpdateHandler2.java:194)
  at org.apache.solr.update.processor.RunUpdateProcessor.processAdd(RunUpdateProcessorFactory.java:67)
  at org.apache.solr.update.processor.UpdateRequestProcessor.processAdd(UpdateRequestProcessor.java:55)
  at org.apache.solr.update.processor.DistributedUpdateProcessor.doLocalAdd(DistributedUpdateProcessor.java:979)
  at org.apache.solr.update.processor.DistributedUpdateProcessor.versionAdd(DistributedUpdateProcessor.java:1192)
  at org.apache.solr.update.processor.DistributedUpdateProcessor.processAdd(DistributedUpdateProcessor.java:748)
  at org.apache.solr.update.processor.LogUpdateProcessorFactory$LogUpdateProcessor.processAdd(LogUpdateProcessorFactory.java:103)
  at org.apache.solr.handler.dataimport.SolrWriter.upload(SolrWriter.java:80)
  at org.apache.solr.handler.dataimport.DataImportHandler$1.upload(DataImportHandler.java:254)
  at org.apache.solr.handler.dataimport.DocBuilder.buildDocument(DocBuilder.java:526)
  at org.apache.solr.handler.dataimport.DocBuilder.buildDocument(DocBuilder.java:414)
  at org.apache.solr.handler.dataimport.DocBuilder.doFullDump(DocBuilder.java:329)
  at org.apache.solr.handler.dataimport.DocBuilder.execute(DocBuilder.java:232)
  at org.apache.solr.handler.dataimport.DataImporter.doFullImport(DataImporter.java:415)
  at org.apache.solr.handler.dataimport.DataImporter.runCmd(DataImporter.java:474)
  at org.apache.solr.handler.dataimport.DataImporter.lambda$runAsync$0(DataImporter.java:457)
  at java.lang.Thread.run(Thread.java:748)
  </code></pre>
  
  <p>My Solr version is 6.6.0. What could be the reason for the error and how can it be resolved?</p>