Fetching Data

So far we were creating tables and were generating queries from them, but wouldn't it be great if we could fetch data directly from the table object?

If you recall - our table is already associated with DataSource, but when we execute get_select_query() the resulting Query is not associated with any DataSource.

There is a type called AssociatedQuery that can be produced by a table object and it has some beautiful methods to actually fetch data.

Lets explore:

#![allow(unused)]
fn main() {
let low_cal_products = model::LowCalProduct::table();

let count = low_cal_products.count().get_one().await?;
}

Table::count() is a method that returns a query for counting rows in a table. Because our LowCalProducts contains a condition, this will return a count of rows that match the condition.

You could use count().preview() still to confirm, that DORM won't attept to fetch all records and iterate over them, but instead will use a SUM() function to count rows:

SELECT COUNT(*) FROM "product" WHERE "calories" <= 100

Calling get_one will instead allow us to fetch the value directly into a variable. We do not know what type our query would produce, so get_one() returns json::Value and you can use "as_i64" to cast it into a numeric type:

#![allow(unused)]
fn main() {
let count = model::LowCalProduct::table()
    .count()
    .get_one()
    .await?
    .as_i64()?
}

Previously I was using unwrap() and now I am using "?" to unwrap. This is because previously our code was certain that a field woudl exist, since we added it ourselves, but with .get_one() we are not sure about the response. Perhaps query execution would fail, so we need a proper error handling.

Lets explore another method:

#![allow(unused)]
fn main() {
let total_calories = model::LowCalProduct::new()
    .sum(model::Product::calories())
    .get_one()
    .await?
    .as_i64()?
}

Here we are passing a Column object to the sum() method.

Some of those methods will be useful for us later, but for now lets look at the way to fetch data from a table:

#![allow(unused)]
fn main() {
for product in model::LowCalProduct::table().get_all_data().await? {
    println!("{} ({} kcal)", product["name"]?, product["calories"]?);
}
}

Next we will look at how to use power of Deserialize trait of your custom type.