One to Many

A one-to-many relation is similar to one-to-one relation. In the previous section, we give the example of "a Cake entity has at most one Fruit topping". In one-to-many relation, we change 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 Entity::has_many() method here.

entity/cake.rs

pub enum Relation {
    Fruit,
}

impl RelationTrait for Relation {
    fn def(&self) -> RelationDef {
        match self {
            Self::Fruit => Entity::has_many(super::fruit::Entity).into(),
        }
    }
}

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.

entity/fruit.rs

#[derive(Copy, Clone, Debug, EnumIter)]
pub enum Relation {
    Cake,
}

impl RelationTrait for Relation {
    fn def(&self) -> RelationDef {
        match self {
            Self::Cake => Entity::belongs_to(super::cake::Entity)
                .from(Column::CakeId)
                .to(super::cake::Column::Id)
                .into(),
        }
    }
}

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