A data integration system is disclosed that incorporates one or more techniques for simplifying the design and maintenance of a mapping. As components are added or removed to an existing design, the data integration system removes the need to specify all input and output attributes. In one aspect, components types are implemented that allow assignment expressions to reference all or part of upstream components. Therefore, attributes of certain types of components can be propagated to downstream components or otherwise inherited from upstream components with minimal effort on the part of a map designer. During code generation the attributes required to be projected by any component can be derived based on the needs of the downstream components.