How to look up the document library in its local sub site where used, rather than in the root site?

I have a content type th开发者_运维百科at has a lookup field for a document library in its sub site. The content type is declared a site collection root level but I want the look up field to look up the document library in its local sub site where used. I deploy the content types using PowerShell and have used an event receiver to add the look up field. I can't however get it to reference the correct list, it only ever references the list at the root site collection.

How can I overwrite the look up list?

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I avoid setting the List attribute in the Field element. I have had problems saving the site as a template when a custom Field has the List attribute set. But the other problem is that these properties are write-once. Better to leave it blank in the definition and then set it in the instance programmatically using SPFieldLookup.LookupWebId and SPFieldLookup.LookupList.