Conditions

By default, when you create a Table object, it will represent a set of all rows in the table. Sometimes you want to filter the rows. Our product SQL table contains a calories column, so lets define it and use it for filtering:

#![allow(unused)]
fn main() {
let product = Table::new("product", postgres.clone())
    .with_field("id")
    .with_field("name")
    .with_field("calories");

let low_cal_products = product
    .with_condition(
        product
            .get_field("calories")
            .unwrap()
            .lt(100)
    );

let query = low_cal_products.get_select_query();
writeln!(query.preview());

// renders into: SELECT id, name, calories FROM product WHERE (calories < 100)
}

Condition can be created from various other types, but the most convenient way is through the use of Operator.

1. get_field returns a Option<Arc> object
2. unwrap() as we know for sure field exists
3. Arc implements Operator trait
4. Operator::lt() returns a Condition object
5. Condition implements SqlChunk, so it can be part of a query

DORM capabilities allow you to be very flexible with defining your entities and we are just scratching the surface here.

Before we continue, let me address one annoying bit here. So far each chapter we have been re-creating our product table.

In the real world, you would typically have a pre-defined table structure, so that no matter how many times you need to operate with a product table, you can easily create it.

DORM has a recommended pattern for generating tables and we will explore it next.