Issue with Solr multiple sort

[Shamik Bandopadhyay]

Hi,

  I'm  facing a problem with multiple field sort in Solr. I'm using the
following fields in sort :

PublishDate asc,DocumentType asc

The sort is only happening on PublishDate, DocumentType seemsto completely
ignored. Here's my field type definition.

<field name="PublishDate" type="tdate" indexed="true" stored="true"
default="NOW"/>
<field name="DocumentType" type="string" indexed="true" stored="true"
multiValued="false" required="false" omitNorms="true"/>

Here's the sample query:

http://localhost:8983/solr/select?sort=PublishDate+desc%2CDocumentType+desc&q=cat:search&fl=PublishDate,DocumentType&debugQuery=true

Here's the output :

<result name="response" numFound="8" start="0">
<doc>
<date name="PublishDate">2015-01-17T00:00:00Z</date>
<str name="DocumentType">Hotfixes</str>
</doc>
<doc>
<date name="PublishDate">2014-11-17T00:00:00Z</date>
<str name="DocumentType">Hotfixes</str>
</doc>
<doc>
<date name="PublishDate">2013-01-17T00:00:00Z</date>
<str name="DocumentType">Tutorials</str>
</doc>
<doc>
<date name="PublishDate">2012-10-17T00:00:00Z</date>
<str name="DocumentType">Service Packs</str>
</doc>
<doc>
<date name="PublishDate">2012-01-17T00:00:00Z</date>
<str name="DocumentType">Tutorials</str>
</doc>
<doc>
<date name="PublishDate">2011-01-17T00:00:00Z</date>
<str name="DocumentType">Tutorials </str>
</doc>
<doc>
<date name="PublishDate">2006-01-17T00:00:00Z</date>
<str name="DocumentType">Object Enablers</str>
</doc>
<doc>
<date name="PublishDate">2006-01-17T00:00:00Z</date>
<str name="DocumentType">Hotfixes</str>
</doc>
</result>

As you can see, the sorting happened only on PublishDate. I'm using Solr
4.7.

Not sure what I'm missing here, any pointers will be appreciated.

Thanks,
Shamik