Many to Many

A many-to-many relation is formed by three tables, where two tables are related via a junction table. As an example, a Cake has many Filling and Filling are shared by many Cake via an intermediate entity CakeFilling.

Defining the Relation

On the Cake entity, implement the Related<filling::Entity> trait. First, join with intermediate table via the inverse of cake_filling::Relation::Cake relation, then join to Filling entity with cake_filling::Relation::Filling relation.

entity/cake.rs

impl Related<super::filling::Entity> for Entity {
    // The final relation is Cake -> CakeFilling -> Filling
    fn to() -> RelationDef {
        super::cake_filling::Relation::Filling.def()
    }

    fn via() -> Option<RelationDef> {
        // The original relation is CakeFilling -> Cake,
        // after `rev` it becomes Cake -> CakeFilling
        Some(super::cake_filling::Relation::Cake.def().rev())
    }
}

Similarly, on the Filling entity, implement the Related<cake::Entity> trait. First, join with intermediate table via the inverse of cake_filling::Relation::Filling relation, then join to Cake entity with cake_filling::Relation::Cake relation.

entity/filling.rs

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

    fn via() -> Option<RelationDef> {
        Some(super::cake_filling::Relation::Filling.def().rev())
    }
}

Defining the Inverse Relation

On the CakeFilling entity, its cake_id attribute is referencing the primary key of Cake entity, and its filling_id attribute is referencing the primary key of Filling entity.

To define the inverse relation:

1. Add two new variants Cake and Filling to the Relation enum.
2. Define both relations with Entity::belongs_to().

entity/cake_filling.rs

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

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(),
            Self::Filling => Entity::belongs_to(super::filling::Entity)
                .from(Column::FillingId)
                .to(super::filling::Column::Id)
                .into(),
        }
    }
}

Alternatively, the definition can be shortened by the DeriveRelation macro, where the following eliminates the need for the RelationTrait implementation above:

#[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,
    #[sea_orm(
        belongs_to = "super::filling::Entity",
        from = "Column::FillingId",
        to = "super::filling::Column::Id"
    )]
    Filling,
}