Macro avg_where 

macro_rules! avg_where {
    ($data:expr, $avg_field:expr, $filter_field:expr, $pred:expr) => { ... };
    ($data:expr, $avg_field:expr) => { ... };
}

Quick average aggregation.

Example

let avg = avg_where!(products, Product::price_r(), Product::active_r(), |&a| a);