Date boosting in an eDisMax query

Let us use eDisMax to boost the results of a search based on date so that the most recent book appears on top. We will use the setBoostFunctionsMult() function to specify the boost on modified_date, which in our case stores the date when the record was last added or updated.

$query = $client->createSelect();
$query->setQuery('cat:book -author:martin');
$edismax = $query->getedismax();
$edismax->setBoostFunctionsMult('recip(ms(NOW,last_modified),1,1,1)');
$resultSet = $client->select($query);

Here we are searching for all books where the author is not named Martin (martin). The – (negative sign) is meant for not query. And we have added a multiplicative boost on the reciprocal of the date between today and last modified date. The recip function provided by Solr is defined as follows:

recip(x,m,a,b) = a/(m*x+b) which in our case becomes 1/(1*ms(NOW,last_modified)+1)

Here m, a, and b are constants, x can be any numeric value or complex function...