Derive Macro Associations 

#[derive(Associations)]
{
    // Attributes available to this derive:
    #[diesel]
    #[belongs_to]
    #[column_name]
    #[table_name]
}

Implement required traits for the associations API

This derive implements support for Diesel’s associations api. Check the module level documentation of the diesel::associations module for details.

This derive generates the following impls:

• impl BelongsTo<Parent> for YourType
• impl BelongsTo<&'a Parent> for YourType

Attributes

Required container attributes

• #[diesel(belongs_to(User))], to specify a child-to-parent relationship between the current type and the specified parent type (User). If this attribute is given multiple times, multiple relationships are generated. #[diesel(belongs_to(User, foreign_key = mykey))] variant allows us to specify the name of the foreign key. If the foreign key is not specified explicitly, the remote lower case type name with appended _id is used as a foreign key name. (user_id in this example case)

Optional container attributes

• #[diesel(table_name = path::to::table)] specifies a path to the table this type belongs to. The path is relative to the current module. If this attribute is not used, the type name converted to snake_case with an added s is used as table name.

Optional field attributes

• #[diesel(column_name = some_column_name)], overrides the column the current field maps to some_column_name. By default, the field name is used as a column name.

Expanded Code

Expanded Code

Input

#[derive(Associations)]
#[diesel(belongs_to(User))]
struct Post {
    id: i32,
    title: String,
    user_id: i32,
}

Expanded Code

Expanded code might use diesel internal API's and is only shown for educational purpose

The macro expands the input to the following Rust code:

const _: () = {
    use diesel;
    impl<__FK> diesel::associations::BelongsTo<User> for Post
    where
        __FK: std::hash::Hash + std::cmp::Eq,
        for<'__a> &'__a i32: std::convert::Into<::std::option::Option<&'__a __FK>>,
        for<'__a> &'__a User: diesel::associations::Identifiable<Id = &'__a __FK>,
    {
        type ForeignKey = __FK;
        type ForeignKeyColumn = posts::user_id;
        fn foreign_key(&self) -> std::option::Option<&Self::ForeignKey> {
            std::convert::Into::into(&self.user_id)
        }
        fn foreign_key_column() -> Self::ForeignKeyColumn {
            posts::user_id
        }
    }
    impl<__FK> diesel::associations::BelongsTo<&'_ User> for Post
    where
        __FK: std::hash::Hash + std::cmp::Eq,
        for<'__a> &'__a i32: std::convert::Into<::std::option::Option<&'__a __FK>>,
        for<'__a> &'__a User: diesel::associations::Identifiable<Id = &'__a __FK>,
    {
        type ForeignKey = __FK;
        type ForeignKeyColumn = posts::user_id;
        fn foreign_key(&self) -> std::option::Option<&Self::ForeignKey> {
            std::convert::Into::into(&self.user_id)
        }
        fn foreign_key_column() -> Self::ForeignKeyColumn {
            posts::user_id
        }
    }
};