c# - Make Entity Framework Join table explicit, reduce code changes using partial classes and extension methods -

i need add additional field "implicit" entity framework join table - adding sort key. break large amount of code relies on classa.classbs , classa.add(itemb); etc.

is there route extension methods , partial classes minimise or remove other code changes ...

i've been trying figure out along lines of...

public partial class classa {         public iqueryable<classb> classbs         {                       {               return this.joinab.select(x => x.classb).orderby(x => x.sortkey).asqueryable();            }         } } 

which results in "cannot converted store expression" type problems, , i'm not sure how "lefthandside" end of

public bool icollection<classa> add(classb itemtoadd, int sortkey=0){     var jab = new joinab{sortkey=sortkey, classb=itemtoadd};     this.*lefthandside*.add(jab);     return true;  } 

so, possible @ all, if how?