One to One

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A one-to-one relation is the most basic type of database relation. Let say a Cake entity has at most one Fruit topping.

Defining the Relation

On the Cake entity, to define the relation:

1. Add a new field fruit to the Model.
2. Annotate it with has_one.

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_one)]
    pub fruit: HasOne<super::fruit::Entity>,
}

It's expanded to:

entity/cake.rs

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

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

Defining the Inverse Relation

On the Fruit entity, its cake_id attribute is referencing the primary key of Cake entity.

tip

The rule of thumb is, always define a belongs_to on the Entity with a foreign key xxx_id.

To define the inverse relation:

1. Add a new field cake to the fruit Model.
2. Annotate the relation with belongs_to.
3. Implement the Related<cake::Entity> trait.

entity/fruit.rs

#[sea_orm::model]
#[derive(DeriveEntityModel, ..)]
#[sea_orm(table_name = "fruit")]
pub struct Model {
    #[sea_orm(primary_key)]
    pub id: i32,
    #[sea_orm(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()
    }
}