Diesel：BoxableExpressions 在表及其连接上通用？

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 or to tunnel LEFT OUTER JOIN connection ON (tunnel.id = connection.tunnel_id).

这些表的定义如下：

// 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
}

或者对于加入：

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，因此我应该能够创建一个实现这两个过滤器函数的通用函数。但是当我尝试使其通用时出现错误。

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>`

最小的例子here，您可以克隆该最小示例并运行cargo check自己查看错误。

这个问题可以通过一个小改动来解决：

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实现了BoxableExpression<T, Pg, SqlType = Bool>为了所有可能的T的。如果只有一个特定的T由 rustc 检查，对于一般情况，需要显式写出。这diesel::dsl::Eq辅助类型是返回类型的类型级构造函数tunnel::name.eq(name)。这意味着您添加到列表中的每个表达式都需要一个类似的子句。

另一个不相关的注释：

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

使用不被视为柴油公共 API 一部分的类型。这意味着这样的表达式可能会随着任何更新而中断。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>;`