1. A system that facilitates mapping arbitrary data models and is recorded on a computer-readable storage medium, comprising a mapping component that receives respective metadata from at least two arbitrary data models, and maps expressions comprising at least one of a structure, field, and relationship between the data models; wherein the mapping component performs the mapping between the data models is directional and the data models include a source domain and a target domain, wherein the directionality of the mapping influences mapping implementation and semantics for each of the arbitrary data models including data models of the source domain and the target domain such that models mapped as the source have one or more different characteristics than models mapped as the target, wherein the target domain comprises a structure and a field that can only be mapped once while the source domain comprises a structure and a field that can be mapped multiple times, and wherein the mapping between the data models is stackable, such that multiple stages of mappings occur from the source domain to the target domain, the stackable mapping allowing a user to implement data processing between any two arbitrary data models; wherein the mapping component allows a user to operate on the data models through a query language of the target domain and provides use of other languages in combination with the mapping to transform the target or source domain; wherein the mapping between the data models translates a query written in a query language of the target domain into a query language of the source domain; and wherein the mapping component facilitates automatically synchronizing updates, wherein the updates allow the user to specify different behavior for the mapping by including a user-specified structured query language statement that is to be executed when a query, update, insert or delete command is called based on the mapping, and wherein when a user creates, deletes, or modifies a structure in the target domain, these changes are automatically persisted to the source domain.