diesel/query_source/aliasing/
field_alias_mapper.rs

1use super::{Alias, AliasSource, AliasedField};
2
3use crate::expression;
4use crate::query_source::{Column, Table, TableNotEqual};
5
6/// Serves to map `Self` to `Alias<S>`
7///
8/// Any column `Self` that belongs to `S::Table` will be transformed into `AliasedField<S, Self>`
9///
10/// Any column `Self` that does not belong to `S::Table` will be left untouched.
11///
12/// This also works with tuples and some expressions.
13///
14// This first part is implemented by the `table!` macro.
15// The second part is useful to implement the joins, and may be useful to an end-user for
16// ergonomics.
17pub trait FieldAliasMapper<S> {
18    /// Output type when mapping `C` to `Alias<S>`
19    ///
20    /// If `C: Column<Table = S::Table>`, `Out = AliasedField<S, C>`  
21    /// Otherwise, `Out = C`
22    type Out;
23
24    /// Does the mapping
25    fn map(self, alias: &Alias<S>) -> Self::Out;
26}
27
28#[doc(hidden)]
29/// Allows implementing `FieldAliasMapper` in external crates without running into conflicting impl
30/// errors due to https://github.com/rust-lang/rust/issues/20400
31///
32/// We will always have `Self = S::Table` and `CT = C::Table`
33pub trait FieldAliasMapperAssociatedTypesDisjointnessTrick<CT, S, C> {
34    type Out;
35    fn map(column: C, alias: &Alias<S>) -> Self::Out;
36}
37impl<S, C> FieldAliasMapper<S> for C
38where
39    S: AliasSource,
40    C: Column,
41    S::Target: FieldAliasMapperAssociatedTypesDisjointnessTrick<C::Table, S, C>,
42{
43    type Out = <S::Target as FieldAliasMapperAssociatedTypesDisjointnessTrick<C::Table, S, C>>::Out;
44
45    fn map(self, alias: &Alias<S>) -> Self::Out {
46        <S::Target as FieldAliasMapperAssociatedTypesDisjointnessTrick<C::Table, S, C>>::map(
47            self, alias,
48        )
49    }
50}
51
52impl<TS, TC, S, C> FieldAliasMapperAssociatedTypesDisjointnessTrick<TC, S, C> for TS
53where
54    S: AliasSource<Target = TS>,
55    C: Column<Table = TC>,
56    TC: Table,
57    TS: TableNotEqual<TC>,
58{
59    type Out = C;
60
61    fn map(column: C, _alias: &Alias<S>) -> Self::Out {
62        // left untouched because the tables are different
63        column
64    }
65}
66
67macro_rules! field_alias_mapper {
68    ($(
69        $Tuple:tt {
70            $(($idx:tt) -> $T:ident, $ST:ident, $TT:ident,)+
71        }
72    )+) => {
73        $(
74            impl<_S, $($T,)*> FieldAliasMapper<_S> for ($($T,)*)
75            where
76                _S: AliasSource,
77                $($T: FieldAliasMapper<_S>,)*
78            {
79                type Out = ($(<$T as FieldAliasMapper<_S>>::Out,)*);
80
81                fn map(self, alias: &Alias<_S>) -> Self::Out {
82                    (
83                        $(self.$idx.map(alias),)*
84                    )
85                }
86            }
87        )*
88    }
89}
90
91diesel_derives::__diesel_for_each_tuple!(field_alias_mapper);
92
93// The following `FieldAliasMapper` impls are useful for the generic join implementations.
94// More may be added.
95impl<SPrev, SNew, F> FieldAliasMapper<SNew> for AliasedField<SPrev, F>
96where
97    SNew: AliasSource,
98{
99    type Out = Self;
100
101    fn map(self, _alias: &Alias<SNew>) -> Self::Out {
102        // left untouched because it has already been aliased
103        self
104    }
105}
106
107impl<S, F> FieldAliasMapper<S> for expression::nullable::Nullable<F>
108where
109    F: FieldAliasMapper<S>,
110{
111    type Out = expression::nullable::Nullable<<F as FieldAliasMapper<S>>::Out>;
112
113    fn map(self, alias: &Alias<S>) -> Self::Out {
114        expression::nullable::Nullable::new(self.0.map(alias))
115    }
116}
117
118impl<S, F> FieldAliasMapper<S> for expression::grouped::Grouped<F>
119where
120    F: FieldAliasMapper<S>,
121{
122    type Out = expression::grouped::Grouped<<F as FieldAliasMapper<S>>::Out>;
123
124    fn map(self, alias: &Alias<S>) -> Self::Out {
125        expression::grouped::Grouped(self.0.map(alias))
126    }
127}
diesel/query_source/aliasing/field_alias_mapper.rs

diesel/query_source/aliasing/
field_alias_mapper.rs
