diesel/expression/
coerce.rs

1use crate::expression::*;
2use crate::query_builder::*;
3use crate::result::QueryResult;
4use crate::sql_types::DieselNumericOps;
5use std::marker::PhantomData;
6
7#[derive(Debug, Copy, Clone, QueryId, DieselNumericOps)]
8#[doc(hidden)]
9/// Coerces an expression to be another type. No checks are performed to ensure
10/// that the new type is valid in all positions that the previous type was.
11/// This does not perform an actual cast, it just lies to our type system.
12///
13/// This is used for a few expressions where we know that the types are actually
14/// always interchangeable. (Examples of this include `Timestamp` vs
15/// `Timestamptz`, `VarChar` vs `Text`, and `Json` vs `Jsonb`).
16///
17/// This struct should not be considered a general solution to equivalent types.
18/// It is a short term workaround for expressions which are known to be commonly
19/// used.
20pub struct Coerce<T, ST> {
21    expr: T,
22    _marker: PhantomData<ST>,
23}
24
25impl<T, ST> Coerce<T, ST> {
26    pub fn new(expr: T) -> Self {
27        Coerce {
28            expr: expr,
29            _marker: PhantomData,
30        }
31    }
32}
33
34impl<T, ST> Expression for Coerce<T, ST>
35where
36    T: Expression,
37    ST: SqlType + TypedExpressionType,
38{
39    type SqlType = ST;
40}
41
42impl<T, ST, QS> SelectableExpression<QS> for Coerce<T, ST>
43where
44    T: SelectableExpression<QS>,
45    Self: Expression,
46{
47}
48
49impl<T, ST, QS> AppearsOnTable<QS> for Coerce<T, ST>
50where
51    T: AppearsOnTable<QS>,
52    Self: Expression,
53{
54}
55
56impl<T, ST, DB> QueryFragment<DB> for Coerce<T, ST>
57where
58    T: QueryFragment<DB>,
59    DB: Backend,
60{
61    fn walk_ast<'b>(&'b self, pass: AstPass<'_, 'b, DB>) -> QueryResult<()> {
62        self.expr.walk_ast(pass)
63    }
64}
65
66impl<T, ST, GB> ValidGrouping<GB> for Coerce<T, ST>
67where
68    T: ValidGrouping<GB>,
69{
70    type IsAggregate = T::IsAggregate;
71}