Complex Relations

Linked

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, making it impossible to impl Related
2. joining across multiple entities in a relational query

Implementing Linked trait is completely optional, as there are other ways of doing relational queries in SeaORM, which will be explained in later chapters. With Linked implemented, several find_*_linked helper methods become available, and relationships can be defined in a single place.

Defining the Link

Take this as an 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 -> cake_filling
            cake_filling::Relation::Filling.def(),    // cake_filling -> filling
        ]
    }
}

Lazy Loading

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

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

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(),
    [
        "SELECT `cake`.`id` AS `A_id`, `cake`.`name` AS `A_name`,",
        "`r1`.`id` AS `B_id`, `r1`.`name` AS `B_name`, `r1`.`vendor_id` AS `B_vendor_id`",
        "FROM `cake`",
        "LEFT JOIN `cake_filling` AS `r0` ON `cake`.`id` = `r0`.`cake_id`",
        "LEFT JOIN `filling` AS `r1` ON `r0`.`filling_id` = `r1`.`id`",
    ]
    .join(" ")
);

Self Referencing Relations

staff.rs

use sea_orm::entity::prelude::*;

#[sea_orm::model]
#[derive(Clone, Debug, PartialEq, DeriveEntityModel, Eq)]
#[sea_orm(table_name = "staff")]
pub struct Model {
    #[sea_orm(primary_key)]
    pub id: i32,
    pub name: String,
    pub reports_to_id: Option<i32>,
    #[sea_orm(
        self_ref,
        relation_enum = "ReportsTo",
        from = "reports_to_id",
        to = "id"
    )]
    pub reports_to: HasOne<Entity>,
}

impl ActiveModelBehavior for ActiveModel {}

Entity Loader

let staff = staff::Entity::load()
    .with(staff::Relation::ReportsTo)
    .all(db)
    .await?;

assert_eq!(staff[0].name, "Alan");
assert_eq!(staff[0].reports_to, None);

assert_eq!(staff[1].name, "Ben");
assert_eq!(staff[1].reports_to.as_ref().unwrap().name, "Alan");

assert_eq!(staff[2].name, "Alice");
assert_eq!(staff[2].reports_to.as_ref().unwrap().name, "Alan");

assert_eq!(staff[3].name, "Elle");
assert_eq!(staff[3].reports_to, None);

Model Loader

let staff = staff::Entity::find()
    .order_by_asc(staff::Column::Id)
    .all(db)
    .await?;

let reports_to = staff
    .load_self(staff::Entity, staff::Relation::ReportsTo, db)
    .await?;

assert_eq!(staff[0].name, "Alan");
assert_eq!(reports_to[0], None);

assert_eq!(staff[1].name, "Ben");
assert_eq!(reports_to[1].unwrap().name, "Alan");

assert_eq!(staff[2].name, "Alice");
assert_eq!(reports_to[2].unwrap().name, "Alan");

assert_eq!(staff[3].name, "Elle");
assert_eq!(reports_to[3], None);

It can work in reverse too.

let manages = staff
    .load_self_rev(
        staff::Entity::find().order_by_asc(staff::Column::Id),
        staff::Relation::ReportsTo,
        db,
    )
    .await?;

assert_eq!(staff[0].name, "Alan");
assert_eq!(manages[0].len(), 2);
assert_eq!(manages[0][0].name, "Ben");
assert_eq!(manages[0][1].name, "Alice");

assert_eq!(staff[1].name, "Ben");
assert_eq!(manages[1].len(), 0);

assert_eq!(staff[2].name, "Alice");
assert_eq!(manages[2].len(), 0);

assert_eq!(staff[3].name, "Elle");
assert_eq!(manages[3].len(), 0);

Diamond Relations

Sometimes there exist multiple relations between a pair of entities. Here we take the simplest example, where Bakery can have multiple Worker.

#[sea_orm::model]
#[derive(Clone, Debug, PartialEq, DeriveEntityModel)]
#[sea_orm(table_name = "bakery")]
pub struct Model {
    #[sea_orm(primary_key)]
    pub id: i32,
    pub name: String,
    pub manager_id: i32,
    pub cashier_id: i32,
    #[sea_orm(belongs_to, relation_enum = "Manager", from = "manager_id", to = "id")]
    pub manager: HasOne<super::worker::Entity>,
    #[sea_orm(belongs_to, relation_enum = "Cashier", from = "cashier_id", to = "id")]
    pub cashier: HasOne<super::worker::Entity>,
}

How can we define the Worker Entity? By default, has_many invokes the Related trait to define the relation. As a consequence, we have to specify the Relation variant of the related entity manually with the via_rel attribute.

worker.rs

#[sea_orm::model]
#[derive(Clone, Debug, PartialEq, Eq, DeriveEntityModel)]
#[sea_orm(table_name = "worker")]
pub struct Model {
    #[sea_orm(primary_key)]
    pub id: i32,
    pub name: String,
    #[sea_orm(has_many, relation_enum = "BakeryManager", via_rel = "Manager")]
    pub manager_of: HasMany<super::bakery::Entity>,
    #[sea_orm(has_many, relation_enum = "BakeryCashier", via_rel = "Cashier")]
    pub cashier_of: HasMany<super::bakery::Entity>,
}

For compact Entities, it looks like this:

worker.rs

#[derive(Clone, Debug, PartialEq, Eq, DeriveEntityModel)]
#[sea_orm(table_name = "worker")]
pub struct Model {
    #[sea_orm(primary_key)]
    pub id: i32,
    pub name: String,
}

#[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]
pub enum Relation {
    #[sea_orm(has_many = "super::bakery::Entity", via_rel = "Relation::Manager")]
    BakeryManager,
    #[sea_orm(has_many = "super::bakery::Entity", via_rel = "Relation::Cashier")]
    BakeryCashier,
}

Then you can use the relations like:

fruit::Entity::find().join(JoinType::LeftJoin, fruit::Relation::ToppingOf.def());