$().SPServices.SPCascadeDropdowns

Function$().SPServices.SPCascadeDropdowns

FunctionalityThe SPCascadeDropdowns function lets you set up cascading dropdowns on SharePoint forms. What this means is that you can enforce hierarchical relationships between column values. The function uses the GetListItems operation of the Lists Web Service to refresh the allowable values based on relationships which are maintained in reference lists. By implementing this function, there are no coding requirements to manage the hierarchical relationships (once it is in place) and you can let your users manage the content in the reference lists.

This function works with any number of options in the dropdowns as well as multi-select parent and child columns, as shown in the following table. This is significant because each of the three column types are rendered significantly differently by SharePoint.

When the relationshipList contains lookup columns for both the relationshipListParentColumn and relationshipListChildColumn columns, the function uses the relationshipListParentColumn's ID rather than the relationshipList item's ID. This means that "secondary lists" are also supported.

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REF: http://spservices.codeplex.com/wikipage?title=%24%28%29.SPServices.SPCascadeDropdowns&referringTitle=Documentation