macro_rules! alias { ($($($table: ident)::+ as $alias: ident),* $(,)?) => { ... }; ($($($table: ident)::+ as $alias_name: ident: $alias_ty: ident),* $(,)?) => { ... }; ($($vis: vis const $const_name: ident: Alias<$alias_ty: ident> = $($table: ident)::+ as $alias_sql_name: ident);* $(;)?) => { ... }; (NoConst $($($table: ident)::+ as $alias_sql_name: ident: $vis: vis $alias_ty: ident),* $(,)?) => { ... }; }
Expand description
Declare a new alias for a table
This macro creates a value of the type Alias
§Example usage
fn main() {
use schema::users;
let connection = &mut establish_connection();
let (users1, users2) = diesel::alias!(schema::users as user1, schema::users as user2);
let res = users1
.inner_join(users2.on(users1.field(users::id).eq(users2.field(users::id))))
.select((users1.fields((users::id, users::name)), users2.field(users::name)))
.order_by(users2.field(users::id))
.load::<((i32, String), String)>(connection);
assert_eq!(
res,
Ok(vec![
((1, "Sean".to_owned()), "Sean".to_owned()),
((2, "Tess".to_owned()), "Tess".to_owned()),
]),
);
}
§Make type expressable
It may sometimes be useful to declare an alias at the module level, in such a way that the type of a query using it can be expressed (to not declare it anonymously).
This can be achieved in the following way
use diesel::{query_source::Alias, dsl};
diesel::alias!(schema::users as users_alias: UsersAlias);
// or
diesel::alias!{
pub const USERS_ALIAS_2: Alias<UsersAlias2> = schema::users as users_alias_2;
}
fn some_function_that_returns_a_query_fragment(
) -> dsl::InnerJoin<schema::posts::table, Alias<UsersAlias>>
{
schema::posts::table.inner_join(users_alias)
}
Note that you may also use this form within a function, in the following way:
fn main() {
diesel::alias!(schema::users as users_alias: UsersAlias);
users_alias.inner_join(schema::posts::table);
}
§Troubleshooting and limitations
If you encounter a compilation error where “the trait
AppearsInFromClause<Alias<your_alias>>
is not implemented”, when trying to use two aliases to
the same table within a single query note the following two limitations:
- You will need to declare these in a single
alias!
call. - The path to the table module will have to be expressed in the exact same
manner. (That is, you can do
alias!(schema::users as user1, schema::users as user2)
oralias!(users as user1, users as user2)
, but notalias!(schema::users as user1, users as user2)
)