RavenDB allows you to define more than one Map function for static indexes. In this case such indexes are called Multi map indexes. When using a Multi map index, we are looking to take information from different documents. A single map index has only one Map function, while a Multi map index has more than one Map function. A Multi map index will get its data from multiple sources. It may have no or only one Reduce function, and no or only one TranformResults function.
If the index has a Reduce function, then the multi map(s) function(s) output is sent to the Reduce function. The only requirement that we have is that the output of all of the Map functions should be identical (and obviously, matches the output of the Reduce function). Then we can just treat this information as normal Map/Reduce index, which means that you can use all usual index features of RavenDB.
To create a multi map index class, we have to inherit from the AbstractMultiMapIndexCreationTask class...