1. A computer-implemented method for indexing and searching entity-relationship data performed by a computerized device having a processor, comprising: defining a logical document model for entity-relationship data including: representing an entity as a document containing the entity's searchable content and metadata; dually representing the entity as a document and as a category; and representing each relationship instance, which indicates a relationship between a plurality of entities, as a category set that contains categories of all participating entities in the relationship; translating entity-relationship data into the logical document model; and indexing the entity-relationship data of a populated logical document model as an inverted index, which indicates for each relationship attribute, the entities to which it relates, allowing lookup in the inverted index of results of queries based on relationship participation patterns.