Chained Relations

The Related trait is a representation of the arrows (1-1, 1-N, M-N) we draw on Entity Relationship Diagrams. A Linked is composed of a chain of relations, and is useful when:

1. there exist multiple join paths between a pair of entities
2. joining across multiple entities in a relational query

Take this as a simple example, where we join cake and filling via an intermediate cake_filling table.

entity/links.rs

pub struct CakeToFilling;

impl Linked for CakeToFilling {
    type FromEntity = cake::Entity;

    type ToEntity = filling::Entity;

    fn link(&self) -> Vec<RelationDef> {
        vec![
            cake_filling::Relation::Cake.def().rev(),
            cake_filling::Relation::Filling.def(),
        ]
    }
}

Alternatively, the RelationDef can be defined on the fly, where the following is equivalent to the above:

pub struct CakeToFilling;

impl Linked for CakeToFilling {
    type FromEntity = cake::Entity;

    type ToEntity = filling::Entity;

    fn link(&self) -> Vec<RelationDef> {
        vec![
            cake_filling::Relation::Cake.def().rev(),
            cake_filling::Entity::belongs_to(filling::Entity)
                .from(cake_filling::Column::FillingId)
                .to(filling::Column::Id)
                .into(),
        ]
    }
}

Lazy Loading

Find fillings that can be filled into a cake with the find_linked method.

let cake_model = cake::Model {
    id: 12,
    name: "".to_owned(),
};

assert_eq!(
    cake_model
        .find_linked(cake::CakeToFilling)
        .build(DbBackend::MySql)
        .to_string(),
    [
        "SELECT `filling`.`id`, `filling`.`name`, `filling`.`vendor_id`",
        "FROM `filling`",
        "INNER JOIN `cake_filling` AS `r0` ON `r0`.`filling_id` = `filling`.`id`",
        "INNER JOIN `cake` AS `r1` ON `r1`.`id` = `r0`.`cake_id`",
        "WHERE `r1`.`id` = 12",
    ]
    .join(" ")
);

Eager Loading

find_also_linked is a dual of find_also_related; find_with_linked is a dual of find_with_related; :

assert_eq!(
    cake::Entity::find()
        .find_also_linked(links::CakeToFilling)
        .build(DbBackend::MySql)
        .to_string(),
    [
        r#"SELECT `cake`.`id` AS `A_id`, `cake`.`name` AS `A_name`,"#,
        r#"`r1`.`id` AS `B_id`, `r1`.`name` AS `B_name`, `r1`.`vendor_id` AS `B_vendor_id`"#,
        r#"FROM `cake`"#,
        r#"LEFT JOIN `cake_filling` AS `r0` ON `cake`.`id` = `r0`.`cake_id`"#,
        r#"LEFT JOIN `filling` AS `r1` ON `r0`.`filling_id` = `r1`.`id`"#,
    ]
    .join(" ")
);