Optional join in Diesel

I want to add conditional filter on joined table:

let query = first_table::table.into_boxed();

if Some(filter_string) = filter {
  // ERROR: inner_join() generates value of new type,
  // cannot assign it to boxed query
  query = query
    .inner_join(joined_table::table.on(...))
    .filter(jointed_table::dsl::string_field.eq(filter_string))
}

// joined_table actually is not required here
let result = query.select(first_table::all_columns).load(...)?;

Is there a way to downcast type of modified query into original one's, or may be there's another way to do that without copy-pasting final chain .select().load() into condition?