1. A system for training a document ranking component, comprising: a training data store that contains training data including representations of documents and, for each query of a plurality of queries, a labeling of some of the documents with relevance of the documents to the query; a propagate relevance component that propagates relevance of the labeled documents to the unlabeled documents based on similarity between documents; and a training component that trains a document ranking component to rank relevance of documents to queries based on the propagated relevance of the documents of the training data.