1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
use super::{HasTable, Identifiable};
use crate::dsl::{Eq, EqAny, Filter, FindBy};
use crate::expression::array_comparison::AsInExpression;
use crate::expression::AsExpression;
use crate::prelude::*;
use crate::query_dsl::methods::FilterDsl;
use crate::sql_types::SqlType;

use std::borrow::Borrow;
use std::hash::Hash;

/// Indicates that a type belongs to `Parent`
///
/// Specifically, this means that this struct has fields
/// which correspond to the primary key of `Parent`.
/// This implies that a foreign key relationship exists on the tables.
///
/// This trait is not capable of supporting composite foreign keys
pub trait BelongsTo<Parent> {
    /// The foreign key of this struct
    type ForeignKey: Hash + ::std::cmp::Eq;
    /// The database column representing the foreign key
    /// of the table this struct represents
    type ForeignKeyColumn: Column;

    /// Returns the foreign key for `self`
    fn foreign_key(&self) -> Option<&Self::ForeignKey>;
    /// Returns the foreign key column of this struct's table
    fn foreign_key_column() -> Self::ForeignKeyColumn;
}

/// The `grouped_by` function groups records by their parent.
///
/// `grouped_by` is called on a `Vec<Child>` with a `&[Parent]`.
/// The return value will be `Vec<Vec<Child>>` indexed to match their parent.
/// Or to put it another way, the returned data can be passed to `zip`,
/// and it will be combined with its parent.
/// This function does not generate a `GROUP BY` SQL statement,
/// as it operates on data structures already loaded from the database
///
/// **Child** refers to the "many" part of a "one to many" relationship. It "belongs to" its parent
/// **Parent** refers to the "one" part of a "one to many" relationship and can "have many" children.
/// The child always has a foreign key, which refers to its parent's primary key.
/// In the following relationship, User has many Posts,
/// so User is the parent and Posts are children.
///
/// # Example
///
/// ```rust
/// # include!("../doctest_setup.rs");
/// # use schema::{posts, users};
/// #
/// # #[derive(Identifiable, Queryable, PartialEq, Debug)]
/// # pub struct User {
/// #     id: i32,
/// #     name: String,
/// # }
/// #
/// # #[derive(Debug, PartialEq)]
/// # #[derive(Identifiable, Queryable, Associations)]
/// # #[diesel(belongs_to(User))]
/// # pub struct Post {
/// #     id: i32,
/// #     user_id: i32,
/// #     title: String,
/// # }
/// #
/// # fn main() {
/// #     run_test();
/// # }
/// #
/// # fn run_test() -> QueryResult<()> {
/// #     let connection = &mut establish_connection();
/// let users = users::table.load::<User>(connection)?;
/// let posts = Post::belonging_to(&users)
///     .load::<Post>(connection)?
///     .grouped_by(&users);
/// let data = users.into_iter().zip(posts).collect::<Vec<_>>();
///
/// let expected_data = vec![
///     (
///         User { id: 1, name: "Sean".into() },
///         vec![
///             Post { id: 1, user_id: 1, title: "My first post".into() },
///             Post { id: 2, user_id: 1, title: "About Rust".into() },
///         ],
///     ),
///     (
///         User { id: 2, name: "Tess".into() },
///         vec![
///             Post { id: 3, user_id: 2, title: "My first post too".into() },
///         ],
///     ),
/// ];
///
/// assert_eq!(expected_data, data);
/// #     Ok(())
/// # }
/// ```
///
/// See [the module documentation] for more examples
///
/// [the module documentation]: super
pub trait GroupedBy<'a, Parent>: IntoIterator + Sized {
    /// See the trait documentation.
    fn grouped_by(self, parents: &'a [Parent]) -> Vec<Vec<Self::Item>>;
}

type Id<T> = <T as Identifiable>::Id;

impl<'a, Parent: 'a, Child, Iter> GroupedBy<'a, Parent> for Iter
where
    Iter: IntoIterator<Item = Child>,
    Child: BelongsTo<Parent>,
    &'a Parent: Identifiable,
    Id<&'a Parent>: Borrow<Child::ForeignKey>,
{
    fn grouped_by(self, parents: &'a [Parent]) -> Vec<Vec<Child>> {
        use std::collections::HashMap;

        let id_indices: HashMap<_, _> = parents
            .iter()
            .enumerate()
            .map(|(i, u)| (u.id(), i))
            .collect();
        let mut result = parents.iter().map(|_| Vec::new()).collect::<Vec<_>>();
        for child in self {
            if let Some(index) = child.foreign_key().map(|i| id_indices[i]) {
                result[index].push(child);
            }
        }
        result
    }
}

impl<'a, Parent, Child> BelongingToDsl<&'a Parent> for Child
where
    &'a Parent: Identifiable,
    Child: HasTable + BelongsTo<Parent>,
    Id<&'a Parent>: AsExpression<<Child::ForeignKeyColumn as Expression>::SqlType>,
    Child::Table: FilterDsl<Eq<Child::ForeignKeyColumn, Id<&'a Parent>>>,
    Child::ForeignKeyColumn: ExpressionMethods,
    <Child::ForeignKeyColumn as Expression>::SqlType: SqlType,
{
    type Output = FindBy<Child::Table, Child::ForeignKeyColumn, Id<&'a Parent>>;

    fn belonging_to(parent: &'a Parent) -> Self::Output {
        FilterDsl::filter(Child::table(), Child::foreign_key_column().eq(parent.id()))
    }
}

impl<'a, Parent, Child> BelongingToDsl<&'a [Parent]> for Child
where
    &'a Parent: Identifiable,
    Child: HasTable + BelongsTo<Parent>,
    Vec<Id<&'a Parent>>: AsInExpression<<Child::ForeignKeyColumn as Expression>::SqlType>,
    <Child as HasTable>::Table: FilterDsl<EqAny<Child::ForeignKeyColumn, Vec<Id<&'a Parent>>>>,
    Child::ForeignKeyColumn: ExpressionMethods,
    <Child::ForeignKeyColumn as Expression>::SqlType: SqlType,
{
    type Output = Filter<Child::Table, EqAny<Child::ForeignKeyColumn, Vec<Id<&'a Parent>>>>;

    fn belonging_to(parents: &'a [Parent]) -> Self::Output {
        let ids = parents.iter().map(Identifiable::id).collect::<Vec<_>>();
        FilterDsl::filter(Child::table(), Child::foreign_key_column().eq_any(ids))
    }
}

impl<'a, Parent, Child> BelongingToDsl<(&'a [Parent], &'a [Parent])> for Child
where
    &'a Parent: Identifiable,
    Child: HasTable + BelongsTo<Parent>,
    Vec<Id<&'a Parent>>: AsInExpression<<Child::ForeignKeyColumn as Expression>::SqlType>,
    <Child as HasTable>::Table: FilterDsl<EqAny<Child::ForeignKeyColumn, Vec<Id<&'a Parent>>>>,
    Child::ForeignKeyColumn: ExpressionMethods,
    <Child::ForeignKeyColumn as Expression>::SqlType: SqlType,
{
    type Output = Filter<Child::Table, EqAny<Child::ForeignKeyColumn, Vec<Id<&'a Parent>>>>;

    fn belonging_to(parents: (&'a [Parent], &'a [Parent])) -> Self::Output {
        let ids = parents
            .0
            .iter()
            .chain(parents.1.iter())
            .map(Identifiable::id)
            .collect::<Vec<_>>();
        FilterDsl::filter(Child::table(), Child::foreign_key_column().eq_any(ids))
    }
}

impl<'a, Parent, Child> BelongingToDsl<&'a Vec<Parent>> for Child
where
    Child: BelongingToDsl<&'a [Parent]>,
{
    type Output = Child::Output;

    fn belonging_to(parents: &'a Vec<Parent>) -> Self::Output {
        Self::belonging_to(&**parents)
    }
}