entity framework: join on the rule "A = substring(B)"?

Could I ask somebody show me the way how to declare the association between two entities 'Record' and 'DictionaryItem' if corresponded tables on the DB level are joined by such interesting rule:

FROM Records R LEFT OUTER JOIN DictionaryItems D 
  ON SUBSTRING(R.Compos开发者_Python百科iteKey,3,8) = D.DictionaryItemId

P.S. I'm now working with POCO entities.

Linq-to-entities doesn't support Substring. You must either execute SQL directly by calling context.Database.SqlQuery<> or you must use Entity SQL - that would probably require converting DbContext to ObjectContext via IObjectContextAdapter, creating ObjectSet and running ESQL query.