Solr index field are indexed with ItemFieldName_sm & not as ItemField_s

Question

Recently we have upgraded our project to Sitecore 10 & SOLR 8.4 , after the upgrade we set up the solr & rebuilt indexes for all indexes. But we are seeing a lot of index fields are stored/updated in SOLR as string collection with _sm([""]) , and it should be string with _s(""). These are not the computed fields. Can anyone let me know how to update the fields to string index fields.

Answer 1

We faced this issue while upgrading as well, so we noticed that some configurations need to be changed like in above @Marek answer you can see droplink|droptree returnType is stringCollection instead it should be string, if you change it index again then you will notice now droplink field will come as _s in solr indexing. So changed these as below -

<defaultSolrIndexConfiguration type="Sitecore.ContentSearch.SolrProvider.SolrIndexConfiguration, Sitecore.ContentSearch.SolrProvider">
<fieldMap type="Sitecore.ContentSearch.SolrProvider.SolrFieldMap, Sitecore.ContentSearch.SolrProvider">
    <!--  FIELD TYPE MAPPING
             This allows you to map a field type in Sitecore to a type in the index.
             USAGE: When you add new field types to Sitecore, add the mappings here so they work through the Linq Layer 
           -->
    <fieldTypes hint="raw:AddFieldByFieldTypeName">
        <fieldType fieldTypeName="checkbox" returnType="bool"/>
        <fieldType fieldTypeName="date|datetime" returnType="datetime"/>
        <fieldType fieldTypeName="html|rich text|single-line text|multi-line text|text|memo|image|reference" returnType="text"/>
        <fieldType fieldTypeName="word document" returnType="text"/>
        <fieldType fieldTypeName="integer" returnType="long"/>
        <fieldType fieldTypeName="number" returnType="float"/>
        <fieldType fieldTypeName="icon|droplist|grouped droplist" returnType="string"/>
        <fieldType fieldTypeName="checklist|multilist|treelist|tree list|treelistex|tree list|multilist with search|treelist with search" returnType="stringCollection"/>
        <fieldType fieldTypeName="name lookup value list|name value list" returnType="stringCollection"/>
        <fieldType fieldTypeName="grouped droplink|tree" returnType="stringCollection"/>
        <fieldType fieldTypeName="droplink|droptree" returnType="string"/>
    </fieldTypes>
</fieldMap>

Or instead of string you can use shortId as well, because droplink and droptree both store type is Guid ID

<fieldType fieldTypeName="droplink|droptree" returnType="shortid" />

And you can find typeMatch for shortID that also adds _s

<typeMatch typeName="shortid" type="Sitecore.Data.ShortID, Sitecore.Kernel" fieldNameFormat="{0}_s" settingType="Sitecore.ContentSearch.SolrProvider.SolrSearchFieldConfiguration, Sitecore.ContentSearch.SolrProvider" />

Update 1 : You can add patch like below -

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/" 
xmlns:set="http://www.sitecore.net/xmlconfig/set/">
   <sitecore>
    <contentSearch>
      <indexConfigurations>
        <defaultSolrIndexConfiguration>
          <fieldMap>
            <fieldTypes hint="raw:AddFieldByFieldTypeName">
              <fieldType fieldTypeName="droplink|droptree">
                <patch:attribute name="returnType">string</patch:attribute>
              </fieldType>
            </fieldTypes>
          </fieldMap>
        </defaultSolrIndexConfiguration>
      </indexConfigurations>
    </contentSearch> 
   </sitecore>
</configuration>

Answer 2

You should check your SolrIndexConfiguration in /sitecore/admin/showconfig.aspx.

There should be entries for every type of the field and their corresponding type in Solr:

<defaultSolrIndexConfiguration type="Sitecore.ContentSearch.SolrProvider.SolrIndexConfiguration, Sitecore.ContentSearch.SolrProvider">
    <fieldMap type="Sitecore.ContentSearch.SolrProvider.SolrFieldMap, Sitecore.ContentSearch.SolrProvider">
        <!--  FIELD TYPE MAPPING
                 This allows you to map a field type in Sitecore to a type in the index.
                 USAGE: When you add new field types to Sitecore, add the mappings here so they work through the Linq Layer 
               -->
        <fieldTypes hint="raw:AddFieldByFieldTypeName">
            <fieldType fieldTypeName="checkbox" returnType="bool"/>
            <fieldType fieldTypeName="date|datetime" returnType="datetime"/>
            <fieldType fieldTypeName="html|rich text|single-line text|multi-line text|text|memo|image|reference" returnType="text"/>
            <fieldType fieldTypeName="word document" returnType="text"/>
            <fieldType fieldTypeName="integer" returnType="long"/>
            <fieldType fieldTypeName="number" returnType="float"/>
            <fieldType fieldTypeName="icon|droplist|grouped droplist" returnType="string"/>
            <fieldType fieldTypeName="checklist|multilist|treelist|tree list|treelistex|tree list|multilist with search|treelist with search" returnType="stringCollection"/>
            <fieldType fieldTypeName="name lookup value list|name value list" returnType="stringCollection"/>
            <fieldType fieldTypeName="droplink|droptree|grouped droplink|tree" returnType="stringCollection"/>
        </fieldTypes>
    </fieldMap>
</defaultSolrIndexConfiguration>

Most probably your upgrade went wrong and your configs are broken.

Make sure that the index configuration used by your indexes have same <fieldTypes> as in clean Sitecore installation of the same version that you use.