One to Many

A one-to-many relation is similar to a one-to-one relation. In the previous section, we gave the example of "a Cake entity has at most one Fruit topping". To make it a one-to-many relation, we remove the "at most one" constraint. So, we have a Cake entity that might have many Fruit toppings.

Defining the Relation

This is almost identical to defining a one-to-one relation; the only difference is that we use has_many annotation here.

entity/cake.rs

#[sea_orm::model]
#[derive(DeriveEntityModel, ..)]
#[sea_orm(table_name = "cake")]
pub struct Model {
    #[sea_orm(primary_key)]
    pub id: i32,
    #[sea_orm(has_many)]
    pub fruit: HasMany<super::fruit::Entity>,
}

It's expanded to:

#[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]
pub enum Relation {
    #[sea_orm(has_many = "super::fruit::Entity")]
    Fruit,
}

impl Related<super::fruit::Entity> for Entity {
    fn to() -> RelationDef {
        Relation::Fruit.def()
    }
}

Defining the Inverse Relation

It is the same as defining the one-to-one inverse relation, just without a unique key. The rule of thumb is, always define a belongs_to on the Entity with a foreign key xxx_id.

entity/fruit.rs

#[sea_orm::model]
#[derive(DeriveEntityModel, ..)]
#[sea_orm(table_name = "fruit")]
pub struct Model {
    #[sea_orm(primary_key)]
    pub id: i32,
    // without unique
    pub cake_id: Option<i32>,
    #[sea_orm(belongs_to, from = "cake_id", to = "id")]
    pub cake: HasOne<super::cake::Entity>,
}

It's expanded to:

#[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]
pub enum Relation {
    #[sea_orm(
        belongs_to = "super::cake::Entity",
        from = "Column::CakeId",
        to = "super::cake::Column::Id"
    )]
    Cake,
}

impl Related<super::cake::Entity> for Entity {
    fn to() -> RelationDef {
        Relation::Cake.def()
    }
}

Composite Foreign Key

Composite foreign key is supported using a tuple syntax.

composite_a.rs

#[sea_orm::model]
#[sea_orm(table_name = "composite_a")]
pub struct Model {
    #[sea_orm(primary_key)]
    pub id: i32,
    #[sea_orm(unique_key = "pair")]
    pub left_id: i32,
    #[sea_orm(unique_key = "pair")]
    pub right_id: i32,
    #[sea_orm(has_one)]
    pub b: Option<super::composite_b::Entity>,
}

composite_b.rs

#[sea_orm::model]
#[sea_orm(table_name = "composite_b")]
pub struct Model {
    #[sea_orm(primary_key)]
    pub id: i32,
    pub left_id: i32,
    pub right_id: i32,
    #[sea_orm(belongs_to, from = "(left_id, right_id)", to = "(left_id, right_id)")]
    pub a: Option<super::composite_a::Entity>,
}

It's expanded to:

composite_b.rs

#[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]
pub enum Relation {
    #[sea_orm(
        belongs_to = "super::composite_a::Entity",
        from = "(Column::LeftId, Column::RightId)",
        to = "(super::composite_a::Column::LeftId, super::composite_a::Column::RightId)",
    )]
    CakeFilling,
}