Trying to access the Expression passed into an IQueryable implementor

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C#

C#4.0

I am creating a Repository that implements IQueryable. The client calls this Repository as follows:

OrderRepository.AsQueryable().Where(x => x.ID == 123);

So the point of IQueryable as I understand it is that is allows the implementor to access the expression being passed to it before fetching the data/entities.

For example, with the code above, OrderRepository would know to perform a specific fetch from it's datasource where ID=123.

Without AsQueryable the OrderRepository would need to fetch all orders and *then* the Where function would be applied by the client to filter the full Orders result set down to only those orders where ID=123.

My main problem right now is I seem to be having problems accessing the expression through the Repository.

Is my understanding of this correct? What might I be missing?