Update and Delete Resources

#[derive(Insertable, Queryable, AsChangeset, Debug, Clone, Serialize, Deserialize)]
#[table_name="variants"]
pub struct FormVariant {
    pub id: Option<i32>,
    pub name: String
}
#[derive(Insertable, Debug, AsChangeset)]
#[table_name="products_variants"]
pub struct FormProductVariant {
    pub id: Option<i32>,
    pub variant_id: Option<i32>,
    pub product_id: i32,
    pub value: Option<String>
}
pub struct FormProductVariantComplete {
    pub variant: Option<FormVariant>,
    pub product_variant: FormProductVariant,
}
pub struct FormProduct {
    pub product: NewProduct,
    pub variants: Vec<FormProductVariantComplete>
}

#[macro_use]
extern crate diesel;
use diesel::{SqliteConnection, Connection, QueryDsl, RunQueryDsl};
use anyhow::Result;
use ::shoe_store::models::FormProduct;
no_arg_sql_function!(last_insert_rowid, diesel::sql_types::Integer);
fn update_product(product_id: i32, form_product: FormProduct, conn: &SqliteConnection) -> Result<i32> {
    use ::shoe_store::schema::products::dsl::products;
    use ::shoe_store::schema::variants;
    use ::shoe_store::schema::products_variants::dsl::products_variants;
    // We begin a transaction, just to make sure everything runs successfully
    conn.transaction(|| {
        diesel::update(products.find(product_id))
            .set(&form_product.product)
            .execute(conn)?;
        // We just loop through all variants available
        for mut form_product_variant in form_product.variants {
            // If there's no variant id, we probably need to insert a new one.
            if form_product_variant.product_variant.variant_id.is_none() {
                diesel::insert_into(variants::dsl::variants)
                    .values(form_product_variant.variant)
                    .execute(conn)?;
                let last_variant_id: i32 =
                        diesel::select(last_insert_rowid).first(conn)?;
                form_product_variant.product_variant.variant_id = Some(last_variant_id);            
            }
            // We have an id, so we should update the variant product.
            if let Some(product_variant_id) = form_product_variant.product_variant.id {
                diesel::update(products_variants.find(product_variant_id))
                    .set(&form_product_variant.product_variant)
                    .execute(conn)?;
            }
        }
        Ok(product_id)
    })
}