1. A computer-implemented method for ranking documents that match a query, the method comprising: accessing a background ranking model stored on a computer-readable storage media, the background ranking model having been, before the method is performed, trained with background training data comprising different document-query pairings in an arbitrary search domain, each document-query pairing having a human-generated label indicated a human-perceived measure of relevance of the corresponding background document to the corresponding background query; passing the background ranking model and domain-specific labeled training data to a trainer adaptor, the domain-specific labeled training data comprising document-query pairings in a specific search domain, each domain-specific document-query pairing having a human-generated label indicating a human-perceived measure of relevance of the corresponding domain-specific document to the corresponding domain-specific query, the trainer adaptor performing an adaptation process on the background model to adapt the background model to a new ranking model comprising a domain-specific adaption model, the domain-specific adaptation model having been trained according to the domain-specific training data, the adaptation process comprising: initially set a current base model to be the background model performing a series of iterations, each iteration comprising: computing a residual of the current base model based on the labels of the domain-specific training data and rankings of respective feature vectors by the current base model, selecting an optimal basis function that is determined to fit the residual, updating the current base model by adding the selected optimal basis function to the current base model, and perform a next iteration with the updated current base model; when the iterations are finished, the domain-specific adaptation model comprises the iteratively-refined current base model; and using the adaptation model in a search engine server to rank documents for queries.