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use backend::Backend;
use expression::coerce::Coerce;
use expression::{AsExpression, Expression, NonAggregate};
use query_builder::*;
use result::QueryResult;
use sql_types::*;

/// Represents the SQL `CURRENT_TIMESTAMP` constant. This is equivalent to the
/// `NOW()` function on backends that support it.
#[allow(non_camel_case_types)]
#[derive(Debug, Copy, Clone, QueryId)]
pub struct now;

impl Expression for now {
    type SqlType = Timestamp;
}

impl NonAggregate for now {}

impl<DB: Backend> QueryFragment<DB> for now {
    fn walk_ast(&self, mut out: AstPass<DB>) -> QueryResult<()> {
        out.push_sql("CURRENT_TIMESTAMP");
        Ok(())
    }
}

impl_selectable_expression!(now);

operator_allowed!(now, Add, add);
operator_allowed!(now, Sub, sub);
sql_function! {
    /// Represents the SQL `DATE` function. The argument should be a Timestamp
    /// expression, and the return value will be an expression of type Date.

    /// # Examples

    /// ```ignore
    /// # #[macro_use] extern crate diesel;
    /// # extern crate chrono;
    /// # include!("../../doctest_setup.rs");
    /// # use diesel::dsl::*;
    /// #
    /// # fn main() {
    /// #     let connection = establish_connection();
    /// let today: chrono::NaiveDate = diesel::select(date(now)).first(&connection).unwrap();
    /// # }
    /// ```
    fn date(expr: Timestamp) -> Date;
}

impl AsExpression<Nullable<Timestamp>> for now {
    type Expression = Coerce<now, Nullable<Timestamp>>;

    fn as_expression(self) -> Self::Expression {
        Coerce::new(self)
    }
}

#[cfg(feature = "postgres")]
impl AsExpression<Timestamptz> for now {
    type Expression = Coerce<now, Timestamptz>;

    fn as_expression(self) -> Self::Expression {
        Coerce::new(self)
    }
}

#[cfg(feature = "postgres")]
impl AsExpression<Nullable<Timestamptz>> for now {
    type Expression = Coerce<now, Nullable<Timestamptz>>;

    fn as_expression(self) -> Self::Expression {
        Coerce::new(self)
    }
}