By default, each document found in an index is passed to the transformer once. In most cases this is a valid assumption, but there are cases when this behavior needs to be changed.
The best example is when one document produces multiple index entries which are stored and we want to extract them from an index and pass to the transformer. Default behavior would filter-out duplicates (by duplicates we mean index entries that originate from the same document) so only one result would be transformed. To address this situation the SetAllowMultipleIndexEntriesForSameDocumentToResultTransformer configuration option has been introduced.
Let us assume that we want to get all Product names for specific Order. Our index will look like this:
{CODE transformers_1@Transformers/Subcollections.cs /}
and transformer will look like this:
{CODE transformers_2@Transformers/Subcollections.cs /}
Default behavior will return 6 out of 12 results:
{CODE transformers_3@Transformers/Subcollections.cs /}
However, when SetAllowMultipleIndexEntriesForSameDocumentToResultTransformer is used, we get all results that we want:
{CODE transformers_4@Transformers/Subcollections.cs /}