diesel/query_dsl/
single_value_dsl.rs

1use super::methods::LimitDsl;
2use crate::dsl::Limit;
3use crate::expression::grouped::Grouped;
4use crate::expression::subselect::Subselect;
5use crate::query_builder::SelectQuery;
6use crate::sql_types::IntoNullable;
7
8/// The `single_value` method
9///
10/// This trait should not be relied on directly by most apps. Its behavior is
11/// provided by [`QueryDsl`]. However, you may need a where clause on this trait
12/// to call `single_value` from generic code.
13///
14/// [`QueryDsl`]: crate::QueryDsl
15pub trait SingleValueDsl {
16    /// The type returned by `.single_value`.
17    type Output;
18
19    /// See the trait documentation.
20    fn single_value(self) -> Self::Output;
21}
22
23impl<T> SingleValueDsl for T
24where
25    Self: SelectQuery + LimitDsl,
26    <Self as SelectQuery>::SqlType: IntoNullable,
27{
28    type Output =
29        Grouped<Subselect<Limit<Self>, <<Self as SelectQuery>::SqlType as IntoNullable>::Nullable>>;
30
31    fn single_value(self) -> Self::Output {
32        Grouped(Subselect::new(self.limit(1)))
33    }
34}