1. A method for information retrieval with unified search between heterogeneous objects belonging to multiple different categories, the method comprising: defining a search index to reference multiple documents, the search index comprising one or more facets for each document, the facets corresponding to attributes of the documents; associating the categories of the objects with respective facets in the search index, including at least first and second facets associated respectively with first and second categories; indexing a first object, which belongs to the first category, as a document in the search index; identifying a second object, which belongs to the second category, as being related to the first object with a given relationship strength; referencing the second object together with the relationship strength between the first and second objects in the second facet of the document that indexes the first object in the search index; searching for objects by applying a query to the search index; retrieving document matches to the query to result in retrieved objects; and scoring the retrieved objects by a search engine scoring method, wherein applying the query comprises searching for documents by specifying a facet of the documents in the search index, and wherein retrieving the document matches comprises filtering out document matches from the query which do not include the specified facet, wherein scoring the retrieved objects comprises computing scores by the stored relationship strength of the retrieved objects to the specified facet, wherein retrieving the document matches comprises retrieving an indirect object having an indirect relation to a query object, and wherein computing the scores comprises aggregating relationship strengths between the indirect object and the query object by multiplying relationship strengths between the indirect object and another object that is directly related to the query object.