Trait diesel::associations::GroupedBy

source · [−]
pub trait GroupedBy<'a, Parent>: IntoIterator + Sized {
    fn grouped_by(self, parents: &'a [Parent]) -> Vec<Vec<Self::Item>>;
}
Expand description

The grouped_by function groups records by their parent.

grouped_by is called on a Vec<Child> with a &[Parent]. The return value will be Vec<Vec<Child>> indexed to match their parent. Or to put it another way, the returned data can be passed to zip, and it will be combined with its parent. This function does not generate a GROUP BY SQL statement, as it operates on data structures already loaded from the database

Child refers to the “many” part of a “one to many” relationship. It “belongs to” its parent Parent refers to the “one” part of a “one to many” relationship and can “have many” children. The child always has a foreign key, which refers to its parent’s primary key. In the following relationship, User has many Posts, so User is the parent and Posts are children.

Example

let users = users::table.load::<User>(connection)?;
let posts = Post::belonging_to(&users)
    .load::<Post>(connection)?
    .grouped_by(&users);
let data = users.into_iter().zip(posts).collect::<Vec<_>>();

let expected_data = vec![
    (
        User { id: 1, name: "Sean".into() },
        vec![
            Post { id: 1, user_id: 1, title: "My first post".into() },
            Post { id: 2, user_id: 1, title: "About Rust".into() },
        ],
    ),
    (
        User { id: 2, name: "Tess".into() },
        vec![
            Post { id: 3, user_id: 2, title: "My first post too".into() },
        ],
    ),
];

assert_eq!(expected_data, data);

See the module documentation for more examples

Required Methods

See the trait documentation.

Implementors