"use of undeclared type or module" when using Diesel's `belongs_to` attribute

Question

I'm loosely following Diesel's getting started guide trying to set up a relational database, but getting the following error on compile:

error[E0433]: failed to resolve: use of undeclared type or module `birds`
 --> src/models.rs:9:12
  |
9 | pub struct Bird {
  |            ^^^^ use of undeclared type or module `birds`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0433`.
error: Could not compile `prrr_gql`.

Here's the binary:

extern crate prrr_gql;
extern crate diesel;

use self::prrr_gql::*;
use self::models::*;
use self::diesel::prelude::*;

fn main() {
    use prrr_gql::schema::cats::dsl::*;
    use prrr_gql::schema::birds::dsl::*;

    let connection = establish_connection();
    let results = cats.load::<Cat>(&connection)
        .expect("Error hearding cats");

    for cat in results {
        println!("{}", cat.name);
    }
}

and lib.rs (imported as prrr_gql)

#[macro_use]
extern crate diesel;
extern crate dotenv;

use diesel::prelude::*;
use diesel::pg::PgConnection;
use dotenv::dotenv;
use std::env;

pub mod schema;
pub mod models;

pub fn establish_connection() -> PgConnection {
    dotenv().ok();

    let database_url = env::var("DATABASE_URL")
        .expect("DATABASE_URL must be set");

    PgConnection::establish(&database_url)
        .expect(&format!("Error connecting to {}", database_url))
}

models.rs

#[derive(Queryable, Debug)]
pub struct Cat {
    pub id: i32,
    pub name: String,
}

#[derive(Queryable, Associations, Debug)]
#[belongs_to(Cat)]
pub struct Bird {
    pub id: i32,
    pub cat_id: i32,
    pub species: String,
    pub colors: String
}

and the schema.rs generated by Diesel

table! {
    birds (id) {
        id -> Int4,
        species -> Varchar,
        colors -> Varchar,
        cat_id -> Nullable<Int4>,
    }
}

table! {
    cats (id) {
        id -> Int4,
        name -> Varchar,
    }
}

joinable!(birds -> cats (cat_id));

allow_tables_to_appear_in_same_query!(
    birds,
    cats,
);

The only issue I could find related to this says I need to have birds in scope and references the table! macro that I have provided, so I'm not sure what's missing.

When I comment out everything related to the birds database, everything runs as expected.

Full project with Cargo.toml for reference: https://github.com/crashspringfield/prrr_gql/tree/diesel-error

Solution

As the error mentions, birds is not in scope. The table! macro creates a public module (birds), which you then need to bring into scope to be able to derive Associations (in models.rs):

use super::schema::birds;

See diesel::associations for an example, where it shows that one needs to use the schema for derive.

Comments

[Shepmaster]

That's great! That means that you are finding more details about the problem that you can update the question with. For example, you now know that belongs_to is responsible, so that can be part of your question title. I've updated that to show an example of how you can improve your question.

[Shepmaster]

Next, take smaller steps to reduce the problem. When the problem goes away, undo that change and try removing / changing other things. It's highly likely that you'll solve your own problem this way.