How to handle both directions of one-to-many relation in federated graph

[biwecka]

Hi,
I'm just starting out with GraphQL and want to implement a basic example of two federated services. For that I defined a the following data structure:

• a Tenant has an id and a name
  • a Tenant has many Users (the ones with the tenant's id in user.tenant_id)
• a User has an id, first_name, last_name and a tenant_id
  • a User belongs to a Tenant (the tenant with an id of user.tenant_id)

I only got one direction of the relation to work (User belongs to a Tenant) and I don't know how to implement the other direction. My code looks like this

// users-service.rs

/// User
#[derive(async_graphql::SimpleObject, Clone)]
#[graphql(complex)]
pub struct User {
    pub id: i32,
    pub first_name: String,
    pub last_name: String,
    pub tenant_id: i32,
}

#[async_graphql::ComplexObject]
impl User {
    async fn tenant(&self) -> Tenant {
        Tenant { id: self.tenant_id }
    }
}

/// Tenant (partially)
#[derive(async_graphql::SimpleObject, Clone)]
pub struct Tenant {
    #[graphql(external)]
    pub id: i32,
}

/// Query
pub struct Query {
    users: Vec<User>,
}

#[async_graphql::Object]
impl Query {
    async fn users(&self) -> Vec<User> {
        self.users.clone()
    }
}

// tenants-service.rs

/// Tenant
#[derive(async_graphql::SimpleObject, Clone)]
pub struct Tenant {
    pub id: i32,
    pub name: String,
}

/// Query
pub struct Query {
    tenants: Vec<Tenant>
}

#[async_graphql::Object]
impl Query {
    async fn tenants(&self) -> Vec<Tenant> {
        self.tenants.clone()
    }

    #[graphql(entity)]
    async fn find_tenant_by_id(&self, id: async_graphql::ID) -> Tenant {
        self.tenants
            .iter()
            .find(|t| id.to_string().eq(&t.id.to_string()))
            .unwrap()
            .clone()
    }
}

As far as I understand GraphQL, for a query like this query Example { users { tenant { ... } } } all users are fetched first and then the tenant related to every user is fetched by calling the function find_tenant_by_id of the users-service.
For the other direction of this relation there must be a way of fetching all users by their tenant_id but I couldn't find documentation/example covering this case nor do I really understand what the SQL should look like for this relation.

I'm very grateful for any kind of help. Thank you very much in advance :)