Sitecore Solr Error: Processing Field Name. Resolving Multiple Field found on Solr Field Map. No matching template field on index field name, return type ‘String’ and field type ”

After an upgrade to Sitecore 9, our Sitecore search logs were filled with thousands of warnings, like the below:

WARN Processing Field Name : Overview Text. Resolving Multiple Field found on Solr Field Map. No matching template field on index field name 'overview_text', return type 'String' and field type ''

What’s the fix?

You need to add field mappings for each of the fields in your Solr index. In our case, we had no mapping for ‘overview_text’, so Sitecore / Solr didn’t know how to treat the field. Add a config patch and specify a returnType for the fields you see as warnings in the log:

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/" xmlns:role="http://www.sitecore.net/xmlconfig/role/" xmlns:search="http://www.sitecore.net/xmlconfig/search/">
  <sitecore>
    <contentSearch>
      <indexConfigurations>
        <defaultSolrIndexConfiguration>
          <fieldMap>
            <fieldNames hint="raw:AddFieldByFieldName">
              <field fieldName="overview_text" returnType="text" />
            </fieldNames>
          </fieldMap>
        </defaultSolrIndexConfiguration>
      </indexConfigurations>
    </contentSearch>
  </sitecore>
</configuration>

** UPDATE 25/01/2018 **

While the above is suitable for adding a few fields, having hundreds or thousands of fields in your Solr index will lead to having to maintain lots of the above configuration entries. I raised a ticket with Sitecore, and was told “We registered this behavior as a bug with the reference #‚Äč195567”. Sitecore’s suggested workaround is to add a log4Net filter which will stop the problematic entries from reaching the log. For example:

<!-- Filter out Solr log warnings-->
    <log4net>
      <appender name="SearchLogFileAppender">
        <filter type="log4net.Filter.StringMatchFilter">
          <regexToMatch  value="Resolving Multiple Field found on Solr Field Map. No matching solr search field configuration on index field name|Search field name in Solr with Template Resolver is returning no entry|Resolving Multiple Field found on Solr Field Map. No matching template field on index field name|Solr with Template Resolver is returning multiple entry|is being skipped. Reason: No Field Type Name" />
          <acceptOnMatch value="false" />
        </filter>
      </appender>
    </log4net>

Hopefully a proper fix or configuration guidance will be released at some point.