Diesel：BoxableExpressions对表及其联接具有通用性吗？

Question

我正在尝试在运行时构造一些过滤器，这些过滤器可以应用于表tunnel或tunnel LEFT OUTER JOIN connection ON (tunnel.id = connection.tunnel_id)。

表的定义如下：

// Define the tunnel table and struct
table! {
    #[allow(unused_imports)]
    use diesel::sql_types::*;
    tunnel (id) {
        id -> BigInt,
        name -> Text,
    }
}
#[derive(Queryable, Identifiable, Clone, Debug, PartialEq, Eq)]
#[table_name = "tunnel"]
pub struct Tunnel {
    pub id: i64,
    pub name: String,
}

// Define the connection table and struct
table! {
    #[allow(unused_imports)]
    use diesel::sql_types::*;
    connection(id) {
        id -> BigInt,
        tunnel_id -> BigInt,
    }
}

#[derive(Debug, Associations, Identifiable, Queryable)]
#[table_name = "connection"]
#[primary_key(id)]
#[belongs_to(Tunnel)]
pub struct Connection {
    pub id: i64,
    pub tunnel_id: i64,
}

joinable!(connection -> tunnel(tunnel_id));
allow_tables_to_appear_in_same_query!(connection, tunnel);

我可以编写一个为单个表构造动态的函数：

fn filters_t(
    name: &'static str,
) -> Vec<Box<dyn BoxableExpression<tunnel::table, Pg, SqlType = Bool>>> {
    let mut wheres: Vec<Box<dyn BoxableExpression<tunnel::table, Pg, SqlType = Bool>>> = Vec::new();
    wheres.push(Box::new(tunnel::name.eq(name)));
    wheres
}

或用于加入：

pub type TunnelJoinConnection = JoinOn<
    Join<tunnel::table, connection::table, LeftOuter>,
    Eq<Nullable<connection::columns::tunnel_id>, Nullable<tunnel::columns::id>>,
>;

fn filters_j(
    name: &'static str,
) -> Vec<Box<dyn BoxableExpression<TunnelJoinConnection, Pg, SqlType = Bool>>> {
    let mut wheres: Vec<Box<dyn BoxableExpression<TunnelJoinConnection, Pg, SqlType = Bool>>> =
        Vec::new();
    wheres.push(Box::new(tunnel::name.eq(name)));
    wheres
}

请注意，这两个过滤器函数具有完全相同的函数体，因此我应该能够制作出一个可以同时实现这两个函数的通用函数。但是，当我尝试使其通用时，我得到一个错误。

fn filters<T>(name: &'static str) -> Vec<Box<dyn BoxableExpression<T, Pg, SqlType = Bool>>>
where
    T: AppearsInFromClause<tunnel::table, Count = Once>,
{
    vec![Box::new(tunnel::name.eq(name))]
}

错误是

   |
85 |     vec![Box::new(tunnel::name.eq(name))]
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `diesel::SelectableExpression<T>` is not implemented for `tunnel::columns::name`
   |
   = note: required because of the requirements on the impl of `diesel::SelectableExpression<T>` for `diesel::expression::operators::Eq<tunnel::columns::name, diesel::expression::bound::Bound<diesel::sql_types::Text, &str>>`
   = note: required because of the requirements on the impl of `diesel::BoxableExpression<T, diesel::pg::Pg>` for `diesel::expression::operators::Eq<tunnel::columns::name, diesel::expression::bound::Bound<diesel::sql_types::Text, &str>>`
   = note: required for the cast to the object type `dyn diesel::BoxableExpression<T, diesel::pg::Pg, SqlType = diesel::sql_types::Bool>`

最小示例here，您可以克隆该最小示例并自己运行cargo check以查看错误。

Answer 1

只需少量更改即可解决：

fn filters<T>(name: &'static str) -> Vec<Box<dyn BoxableExpression<T, Pg, SqlType = Bool>>>
where
    diesel::dsl::Eq<tunnel::name, &'static str>: BoxableExpression<T, Pg, SqlType = Bool>,
{
    vec![Box::new(tunnel::name.eq(name))]
}

[基本上，您需要在编译时断言，您的盒装表达式确实为所有可能的BoxableExpression<T, Pg, SqlType = Bool>实现了T。如果只有特定的T由rustc检查，则对于一般情况，这需要明确地写出。 diesel::dsl::Eq助手类型是tunnel::name.eq(name)返回的类型的类型级别构造函数。这意味着添加到列表中的每个表达式都需要一个类似的子句。

另一个无关的注释：

pub type TunnelJoinConnection = JoinOn<
    Join<tunnel::table, connection::table, LeftOuter>,
    Eq<Nullable<connection::columns::tunnel_id>, Nullable<tunnel::columns::id>>,
>;

使用不视为柴油公共API一部分的类型。这意味着这样的表达式可以随任何更新而中断。使用公共API编写此类型的正确方法是

pub type TunnelJoinConnection = diesel::dsl::LeftJoin<tunnel::table, connection::table>;`