需要导入什么才能在不同的 scope 中看到属于 function 的内容？

Question

I'm working through the Disel's getting started demo, but adding associations with a user who can post. When I try to use the belonging_to() function it gives the following error:

no function or associated item named belonging_to found for struct diesel_demo::models::Post in the current scope function or associated item not found in diesel_demo::models::Post

I used the Associations macro on the struct, and I thought that was enough to get the belonging_to function working, but I'm missing something.

Here is a simple query where I try to use belonging_to() , but this fails.

#[macro_use] extern crate diesel_demo;

use diesel::prelude::*;
use crate::diesel_demo::*;
use crate::diesel_demo::models::*;

fn main() {
    use diesel_demo::schema::posts::dsl::*;
    use diesel_demo::schema::users::dsl::*;

    let connection = establish_connection();
    let test_id = 2;
    let user = users.find(test_id).first::<User>(&connection).expect("error getting user");
    
    let post_list = Post::belonging_to(&user)
        .load::<Post>(&connection)
        .expect("Error loading user posts");

    println!("{:?}", post_list);

}

And here is the models.rs file where the structs live.

use super::schema::{posts, users};

#[derive(Identifiable, Queryable, PartialEq, Debug)]
#[diesel(table_name = users)]
pub struct User {
    pub id: i32,
    pub name: String,
}

#[derive(Insertable)]
#[table_name = "users"]
pub struct NewUser<'a>{
    pub name: &'a str,
}


#[derive(Identifiable, Associations, Queryable, PartialEq, Debug)]
#[diesel(belongs_to(User))]
pub struct Post {
    pub id: i32,
    pub user_id: i32,
    pub title: String,
    pub body: String,
    pub published: bool,
}

#[derive(Insertable)]
#[table_name = "posts"]
pub struct NewPost<'a>{
    pub user_id: &'a i32,
    pub title: &'a str,
    pub body: &'a str,
    pub published: &'a bool,
}

This is from cargo check :

error[E0599]: no function or associated item named `belonging_to` found for struct `diesel_demo::models::Post` in the current scope
  --> src/bin/publish_post.rs:18:27
   |
18 |     let post_list = Post::belonging_to(&user)
   |                           ^^^^^^^^^^^^ function or associated item not found in `diesel_demo::models::Post`

Answer 1

What you're looking for is the BelongingToDsl trait, which is in diesel::query_dsl or in the prelude.

---

You already have that imported into scope from use diesel::prelude::* . The reason you aren't seeing the implementation is because you've mismatched 1.0-style and 2.0-style attributes.

If you're using Diesel 1.0, then your attributes should look like this ( docs ):

#[derive(Identifiable, Associations, Queryable, PartialEq, Debug)]
#[belongs_to(User)]
#[table_name = "posts"]
pub struct Post {
    pub id: i32,
    pub user_id: i32,
    ...

But if you're using Diesel 2.0, they should look like this ( docs ):

#[derive(Identifiable, Associations, Queryable, PartialEq, Debug)]
#[diesel(belongs_to(User))]
#[diesel(table_name = posts)]
pub struct Post {
    pub id: i32,
    pub user_id: i32,
    ...

Using the correct one should resolve the problem. I'm surprised that the invalid attributes don't cause an error during code expansion, but I'll let that be a mystery for another day.