diesel 2.0.2

A safe, extensible ORM and Query Builder for PostgreSQL, SQLite, and MySQL
Documentation 
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use crate::expression::SelectableExpression;
use crate::pg::Pg;
use crate::query_builder::order_clause::{NoOrderClause, OrderClause};
use crate::query_builder::{
    AstPass, FromClause, QueryFragment, QueryId, SelectQuery, SelectStatement,
};
use crate::query_dsl::methods::DistinctOnDsl;
use crate::query_dsl::order_dsl::ValidOrderingForDistinct;
use crate::result::QueryResult;
use crate::sql_types::SingleValue;
use crate::{Expression, QuerySource};

/// Represents `DISTINCT ON (...)`
#[derive(Debug, Clone, Copy, QueryId)]
#[cfg(feature = "postgres_backend")]
pub struct DistinctOnClause<T>(pub(crate) T);

impl<T> ValidOrderingForDistinct<DistinctOnClause<T>> for NoOrderClause {}
impl<T> ValidOrderingForDistinct<DistinctOnClause<T>> for OrderClause<(T,)> {}
impl<T> ValidOrderingForDistinct<DistinctOnClause<T>> for OrderClause<T>
where
    T: Expression,
    T::SqlType: SingleValue,
{
}
impl<T> ValidOrderingForDistinct<DistinctOnClause<T>> for OrderClause<crate::helper_types::Desc<T>>
where
    T: Expression,
    T::SqlType: SingleValue,
{
}
impl<T> ValidOrderingForDistinct<DistinctOnClause<T>> for OrderClause<crate::helper_types::Asc<T>>
where
    T: Expression,
    T::SqlType: SingleValue,
{
}

impl<T> ValidOrderingForDistinct<DistinctOnClause<T>>
    for OrderClause<(crate::helper_types::Desc<T>,)>
where
    T: Expression,
    T::SqlType: SingleValue,
{
}
impl<T> ValidOrderingForDistinct<DistinctOnClause<T>>
    for OrderClause<(crate::helper_types::Asc<T>,)>
where
    T: Expression,
    T::SqlType: SingleValue,
{
}

impl<T> QueryFragment<Pg> for DistinctOnClause<T>
where
    T: QueryFragment<Pg>,
{
    fn walk_ast<'b>(&'b self, mut out: AstPass<'_, 'b, Pg>) -> QueryResult<()> {
        out.push_sql("DISTINCT ON (");
        self.0.walk_ast(out.reborrow())?;
        out.push_sql(")");
        Ok(())
    }
}

impl<ST, F, S, D, W, O, LOf, G, H, Selection> DistinctOnDsl<Selection>
    for SelectStatement<FromClause<F>, S, D, W, O, LOf, G, H>
where
    F: QuerySource,
    Selection: SelectableExpression<F>,
    Selection::SqlType: SingleValue,
    Self: SelectQuery<SqlType = ST>,
    O: ValidOrderingForDistinct<DistinctOnClause<Selection>>,
    SelectStatement<FromClause<F>, S, DistinctOnClause<Selection>, W, O, LOf, G, H>:
        SelectQuery<SqlType = ST>,
{
    type Output = SelectStatement<FromClause<F>, S, DistinctOnClause<Selection>, W, O, LOf, G, H>;

    fn distinct_on(self, selection: Selection) -> Self::Output {
        SelectStatement::new(
            self.select,
            self.from,
            DistinctOnClause(selection),
            self.where_clause,
            self.order,
            self.limit_offset,
            self.group_by,
            self.having,
            self.locking,
        )
    }
}