/// Define a new struct which derives `Model` & the serde traits.
#[derive(Model, Serialize, Deserialize)]
struct User {
    /// This renames the `id` field to match what MongoDB expects for the PK.
    #[serde(rename="_id", skip_serializing_if="Option::is_none")]
    pub id: Option<ObjectId>,

    /// This field has a unique index on it.
    #[model(index(index_type="dsc", unique="true"))]
    pub email: String,
}