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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412
//! Traits related to relationships between multiple tables.
//!
//! Associations in Diesel are always child-to-parent.
//! You can declare an association between two records with `#[diesel(belongs_to)]`.
//! Unlike other ORMs, Diesel has no concept of `has many`
//!
//! ```rust
//! # include!("../doctest_setup.rs");
//! use schema::{posts, users};
//!
//! #[derive(Identifiable, Queryable, PartialEq, Debug)]
//! #[diesel(table_name = users)]
//! pub struct User {
//! id: i32,
//! name: String,
//! }
//!
//! #[derive(Identifiable, Queryable, Associations, PartialEq, Debug)]
//! #[diesel(belongs_to(User))]
//! #[diesel(table_name = posts)]
//! pub struct Post {
//! id: i32,
//! user_id: i32,
//! title: String,
//! }
//!
//! # fn main() {
//! # run_test().unwrap();
//! # }
//! #
//! # fn run_test() -> QueryResult<()> {
//! # let connection = &mut establish_connection();
//! # use self::users::dsl::*;
//! let user = users.find(2).get_result::<User>(connection)?;
//! let users_post = Post::belonging_to(&user)
//! .first(connection)?;
//! let expected = Post { id: 3, user_id: 2, title: "My first post too".into() };
//! assert_eq!(expected, users_post);
//! # Ok(())
//! # }
//! ```
//!
//! Note that in addition to the `#[diesel(belongs_to)]` annotation, we also need to
//! `#[derive(Associations)]`
//!
//! `#[diesel(belongs_to)]` is given the name of the struct that represents the parent.
//! Both the parent and child must implement [`Identifiable`].
//! The struct given to `#[diesel(belongs_to)]` must be in scope,
//! so you will need `use some_module::User` if `User` is defined in another module.
//!
//! If the parent record is generic over lifetimes, they can be written as `'_`.
//! You will also need to wrap the type in quotes until
//! `unrestricted_attribute_tokens` is stable.
//!
//! ```rust
//! # include!("../doctest_setup.rs");
//! # use schema::{posts, users};
//! # use std::borrow::Cow;
//! #
//! #[derive(Identifiable)]
//! #[diesel(table_name = users)]
//! pub struct User<'a> {
//! id: i32,
//! name: Cow<'a, str>,
//! }
//!
//! #[derive(Associations)]
//! #[diesel(belongs_to(User<'_>))]
//! #[diesel(table_name = posts)]
//! pub struct Post {
//! id: i32,
//! user_id: i32,
//! title: String,
//! }
//! #
//! # fn main() {}
//! ```
//!
//!
//! By default, Diesel assumes that your foreign keys will follow the convention `table_name_id`.
//! If your foreign key has a different name,
//! you can provide the `foreign_key` argument to `#[diesel(belongs_to)]`.
//! For example, `#[diesel(belongs_to(Foo, foreign_key = mykey))]`.
//!
//! Associated data is typically loaded in multiple queries (one query per table).
//! This is usually more efficient than using a join,
//! especially if 3 or more tables are involved.
//! For most datasets,
//! using a join to load in a single query transmits so much duplicate data
//! that it costs more time than the extra round trip would have.
//!
//! You can load the children for one or more parents using
//! [`belonging_to`]
//!
//! [`belonging_to`]: crate::query_dsl::BelongingToDsl::belonging_to
//!
//! ```rust
//! # include!("../doctest_setup.rs");
//! # use schema::users;
//! # use schema::posts;
//! #
//! # #[derive(Debug, PartialEq, Identifiable, Queryable)]
//! # pub struct User {
//! # id: i32,
//! # name: String,
//! # }
//! #
//! # #[derive(Debug, PartialEq, Identifiable, Queryable, Associations)]
//! # #[diesel(belongs_to(User))]
//! # pub struct Post {
//! # id: i32,
//! # user_id: i32,
//! # title: String,
//! # }
//! #
//! # fn main() {
//! # use self::users::dsl::*;
//! # let connection = &mut establish_connection();
//! #
//! let user = users.find(1).first::<User>(connection).expect("Error loading user");
//! let post_list = Post::belonging_to(&user)
//! .load::<Post>(connection)
//! .expect("Error loading posts");
//! let expected = vec![
//! Post { id: 1, user_id: 1, title: "My first post".to_string() },
//! Post { id: 2, user_id: 1, title: "About Rust".to_string() },
//! ];
//!
//! assert_eq!(post_list, expected);
//! # }
//! ```
//!
//! If you're coming from other ORMs, you'll notice that this design is quite different from most.
//! There you would have an instance method on the parent, or have the children stored somewhere on
//! the posts. This design leads to many problems, including [N+1 query
//! bugs][load-your-entire-database-into-memory-lol], and runtime errors when accessing an
//! association that isn't there.
//!
//! [load-your-entire-database-into-memory-lol]: https://stackoverflow.com/q/97197/1254484
//!
//! In Diesel, data and its associations are considered to be separate. If you want to pass around
//! a user and all of its posts, that type is `(User, Vec<Post>)`.
//!
//! Next lets look at how to load the children for more than one parent record.
//! [`belonging_to`] can be used to load the data, but we'll also need to group it
//! with its parents. For this we use an additional method [`grouped_by`].
//!
//! [`grouped_by`]: GroupedBy::grouped_by
//! [`belonging_to`]: crate::query_dsl::BelongingToDsl::belonging_to
//!
//! ```rust
//! # include!("../doctest_setup.rs");
//! # use schema::{posts, users};
//! #
//! # #[derive(Identifiable, Queryable)]
//! # 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();
//! # use self::users::dsl::*;
//! # use self::posts::dsl::{posts, title};
//! let sean = users.filter(name.eq("Sean")).first::<User>(connection)?;
//! let tess = users.filter(name.eq("Tess")).first::<User>(connection)?;
//!
//! let seans_posts = Post::belonging_to(&sean)
//! .select(title)
//! .load::<String>(connection)?;
//! assert_eq!(vec!["My first post", "About Rust"], seans_posts);
//!
//! // A vec or slice can be passed as well
//! let more_posts = Post::belonging_to(&vec![sean, tess])
//! .select(title)
//! .load::<String>(connection)?;
//! assert_eq!(vec!["My first post", "About Rust", "My first post too"], more_posts);
//! # Ok(())
//! # }
//! ```
//!
//! Typically you will want to group up the children with their parents.
//! In other ORMs, this is often called a `has_many` relationship.
//! Diesel provides support for doing this grouping, once the data has been
//! loaded.
//!
//! [`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.
//!
//! ```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(())
//! # }
//! ```
//!
//! [`grouped_by`] can be called multiple times
//! if you have multiple children or grandchildren.
//!
//! For example, this code will load some users,
//! all of their posts,
//! and all of the comments on those posts.
//! Explicit type annotations have been added
//! to make each line a bit more clear.
//!
//! ```rust
//! # include!("../doctest_setup.rs");
//! # use schema::{users, posts, comments};
//! #
//! # #[derive(Debug, PartialEq, Identifiable, Queryable)]
//! # pub struct User {
//! # id: i32,
//! # name: String,
//! # }
//! #
//! # #[derive(Debug, PartialEq, Identifiable, Queryable, Associations)]
//! # #[diesel(belongs_to(User))]
//! # pub struct Post {
//! # id: i32,
//! # user_id: i32,
//! # title: String,
//! # }
//! #
//! # #[derive(Debug, PartialEq, Identifiable, Queryable, Associations)]
//! # #[diesel(belongs_to(Post))]
//! # pub struct Comment {
//! # id: i32,
//! # post_id: i32,
//! # body: String,
//! # }
//! #
//! # fn main() {
//! # let connection = &mut establish_connection();
//! #
//! let users: Vec<User> = users::table.load::<User>(connection)
//! .expect("error loading users");
//! let posts: Vec<Post> = Post::belonging_to(&users)
//! .load::<Post>(connection)
//! .expect("error loading posts");
//! let comments: Vec<Comment> = Comment::belonging_to(&posts)
//! .load::<Comment>(connection)
//! .expect("Error loading comments");
//! let grouped_comments: Vec<Vec<Comment>> = comments.grouped_by(&posts);
//! let posts_and_comments: Vec<Vec<(Post, Vec<Comment>)>> = posts
//! .into_iter()
//! .zip(grouped_comments)
//! .grouped_by(&users);
//! let result: Vec<(User, Vec<(Post, Vec<Comment>)>)> = users
//! .into_iter()
//! .zip(posts_and_comments)
//! .collect();
//! let expected = vec![
//! (
//! User { id: 1, name: "Sean".to_string() },
//! vec![
//! (
//! Post { id: 1, user_id: 1, title: "My first post".to_string() },
//! vec![ Comment { id: 1, post_id: 1, body: "Great post".to_string() } ]
//! ),
//! (
//! Post { id: 2, user_id: 1, title: "About Rust".to_string() },
//! vec![
//! Comment { id: 2, post_id: 2, body: "Yay! I am learning Rust".to_string() }
//! ]
//!
//! )
//! ]
//! ),
//! (
//! User { id: 2, name: "Tess".to_string() },
//! vec![
//! (
//! Post { id: 3, user_id: 2, title: "My first post too".to_string() },
//! vec![ Comment { id: 3, post_id: 3, body: "I enjoyed your post".to_string() } ]
//! )
//! ]
//! )
//! ];
//!
//! assert_eq!(result, expected);
//! # }
//! ```
//!
//! And that's it.
//! It may seem odd to have load, group, and zip be explicit separate steps
//! if you are coming from another ORM.
//! However, the goal is to provide simple building blocks which can
//! be used to construct the complex behavior applications need.
mod belongs_to;
use std::hash::Hash;
use crate::query_source::Table;
pub use self::belongs_to::{BelongsTo, GroupedBy};
#[doc(inline)]
pub use diesel_derives::Associations;
/// This trait indicates that a struct is associated with a single database table.
///
/// This trait is implemented by structs which implement `Identifiable`,
/// as well as database tables themselves.
pub trait HasTable {
/// The table this type is associated with.
type Table: Table;
/// Returns the table this type is associated with.
fn table() -> Self::Table;
}
impl<'a, T: HasTable> HasTable for &'a T {
type Table = T::Table;
fn table() -> Self::Table {
T::table()
}
}
/// This trait indicates that a struct represents a single row in a database table.
///
/// This must be implemented to use associations.
/// Additionally, implementing this trait allows you to pass your struct to `update`
/// (`update(&your_struct)` is equivalent to
/// `update(YourStruct::table().find(&your_struct.primary_key())`).
///
/// This trait is usually implemented on a reference to a struct,
/// not on the struct itself. It can be [derived](derive@Identifiable).
///
pub trait Identifiable: HasTable {
/// The type of this struct's identifier.
///
/// For single-field primary keys, this is typically `&'a i32`, or `&'a String`
/// For composite primary keys, this is typically `(&'a i32, &'a i32)`
/// or `(&'a String, &'a String)`, etc.
type Id: Hash + Eq;
/// Returns the identifier for this record.
///
/// This takes `self` by value, not reference.
/// This is because composite primary keys
/// are typically stored as multiple fields.
/// We could not return `&(String, String)` if each string is a separate field.
///
/// Because of Rust's rules about specifying lifetimes,
/// this means that `Identifiable` is usually implemented on references
/// so that we have a lifetime to use for `Id`.
fn id(self) -> Self::Id;
}
#[doc(inline)]
pub use diesel_derives::Identifiable;