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//! juniper-eager-loading is a library for avoiding N+1 query bugs designed to work with //! [Juniper][] and [juniper-from-schema][]. //! //! <center>🚨 **This library is still experimental and everything is subject to change** 🚨</center> //! //! It is designed to make the most common assocation setups easy to handle and while being //! flexible and allowing you to customize things as needed. It is also 100% data store agnostic. //! So regardless if your API is backed by an SQL database or another API you can still use this //! library. //! //! If you're familiar with N+1 queries in GraphQL and eager loading, feel free to skip forward to //! ["A real example"](#a-real-example). //! //! *NOTE*: Since this library requires [juniper-from-schema][] it is best if you're first familiar //! with that. //! //! # Table of contents //! //! - [What is N+1 query bugs?](#what-is-n1-query-bugs) //! - [N+1s in GraphQL](#n1s-in-graphql) //! - [How this library works at a high level](#how-this-library-works-at-a-high-level) //! - [A real example](#a-real-example) //! - [`#[derive(EagerLoading)]`](#deriveeagerloading) //! - [Attributes](#attributes) //! - [Associations](#associations) //! - [Attributes supported on all associations](#attributes-supported-on-all-associations) //! - [Eager loading interfaces or unions](#eager-loading-interfaces-or-unions) //! - [Eager loading fields that take arguments](#eager-loading-fields-that-take-arguments) //! - [Diesel helper](#diesel-helper) //! - [When your GraphQL schema doesn't match your database schema](#when-your-graphql-schema-doesnt-match-your-database-schema) //! //! # What is N+1 query bugs? //! //! Imagine you have the following GraphQL schema //! //! ```graphql //! schema { //! query: Query //! } //! //! type Query { //! allUsers: [User!]! //! } //! //! type User { //! id: Int! //! country: Country! //! } //! //! type Country { //! id: Int! //! } //! ``` //! //! And someone executes the following query: //! //! ```graphql //! query SomeQuery { //! allUsers { //! country { //! id //! } //! } //! } //! ``` //! //! If you resolve that query naively with an SQL database as you data store you will see something //! like this in your logs: //! //! ```sql //! select * from users //! select * from countries where id = ? //! select * from countries where id = ? //! select * from countries where id = ? //! select * from countries where id = ? //! ... //! ``` //! //! This happens because you first load all the users and then for each user in a loop you load //! that user's country. That is 1 query to load the users and N additional queries to load the //! countries. Therefore the name "N+1 query". These kinds of bugs can really hurt performance of //! your app since you're doing many more database calls than necessary. //! //! One possible solution to this is called "eager loading". The idea is to load all countries up //! front, before looping over the users. So instead of doing N+1 queries you do 2: //! //! ```sql //! select * from users //! select * from countries where id in (?, ?, ?, ?) //! ``` //! //! Since you're loading the countries up front, this strategy is called "eager loading". //! //! ## N+1s in GraphQL //! //! If you're not careful when implementing a GraphQL API you'll have lots of these N+1 query bugs. //! Whenever a field returns a list of types and those types perform queries in their resolvers, //! you'll have N+1 query bugs. //! //! This is also a problem in REST APIs, however because the responses are fixed we can more easily //! setup the necessary eager loads because we know the types needed to compute the response. //! //! However in GraphQL the responses are not fixed. They depend on the incoming queries, which are //! not known ahead of time. So setting up the correct amount of eager loading requires inspecting //! the queries before executing them and eager loading the types requested such that the actual //! resolvers wont need to run queries. That is exactly what this library does. //! //! # How this library works at a high level //! //! If you have a GraphQL type like this //! //! ```graphql //! type User { //! id: Int! //! country: Country! //! } //! ``` //! //! You might create the corresponding Rust model type like this: //! //! ``` //! struct User { //! id: i32, //! country_id: i32 //! } //! ``` //! //! However this approach has one big issue. How are you going to resolve the field `User.country` //! without doing a database query? All the resolver has access to is a `User` with a `country_id` //! field. It can't get the country without loading it from the database... //! //! Fundamentally these kinds of model structs don't work for eager loading with GraphQL. So //! this library takes a different approach. //! //! What if we created separate structs for the database models and the GraphQL models? Something //! like this: //! //! ``` //! # fn main() {} //! # //! mod models { //! pub struct User { //! id: i32, //! country_id: i32 //! } //! //! pub struct Country { //! id: i32, //! } //! } //! //! struct User { //! user: models::User, //! country: HasOne<Country>, //! } //! //! struct Country { //! country: models::Country //! } //! //! enum HasOne<T> { //! Loaded(T), //! NotLoaded, //! } //! ``` //! //! Now we're able to resolve the query with code like this: //! //! 1. Load all the users (first query). //! 2. Map the users to a list of country ids. //! 3. Load all the countries with those ids (second query). //! 4. Pair up the users with the country with the correct id, so change `User.country` from //! `HasOne::NotLoaded` to `HasOne::Loaded(matching_country)`. //! 5. When resolving the GraphQL field `User.country` simply return the loaded country. //! //! # A real example //! //! ``` //! use juniper::{Executor, FieldResult}; //! use juniper_eager_loading::{prelude::*, EagerLoading, HasOne}; //! use juniper_from_schema::graphql_schema; //! use std::error::Error; //! //! // Define our GraphQL schema. //! graphql_schema! { //! schema { //! query: Query //! } //! //! type Query { //! allUsers: [User!]! @juniper(ownership: "owned") //! } //! //! type User { //! id: Int! //! country: Country! //! } //! //! type Country { //! id: Int! //! } //! } //! //! // Our model types. //! mod models { //! use std::error::Error; //! use juniper_eager_loading::LoadFrom; //! //! #[derive(Clone)] //! pub struct User { //! pub id: i32, //! pub country_id: i32 //! } //! //! #[derive(Clone)] //! pub struct Country { //! pub id: i32, //! } //! //! // This trait is required for eager loading countries. //! // It defines how to load a list of countries from a list of ids. //! // Notice that `Connection` is generic and can be whatever you want. //! // This is this library can be data store agnostic. //! impl LoadFrom<i32> for Country { //! type Error = Box<dyn Error>; //! type Connection = super::DbConnection; //! //! fn load( //! employments: &[i32], //! field_args: &(), //! db: &Self::Connection, //! ) -> Result<Vec<Self>, Self::Error> { //! // ... //! # unimplemented!() //! } //! } //! } //! //! // Our sample database connection type. //! pub struct DbConnection; //! //! impl DbConnection { //! // Function that will load all the users. //! fn load_all_users(&self) -> Vec<models::User> { //! // ... //! # unimplemented!() //! } //! } //! //! // Our Juniper context type. //! pub struct Context { //! db: DbConnection, //! } //! //! impl juniper::Context for Context {} //! //! // Our GraphQL user type. //! // `#[derive(EagerLoading)]` takes care of all the heavy lifting. //! #[derive(Clone, EagerLoading)] //! // You need to set the connection and error type. //! #[eager_loading(connection = "DbConnection", error = "Box<dyn Error>")] //! pub struct User { //! // This user model is used to resolve `User.id` //! user: models::User, //! //! // Setup a "has one" association between a user and a country. //! // `default` will use all the default attribute values. //! // Exacty what they are is explained below. //! #[has_one(default)] //! country: HasOne<Country>, //! } //! //! // And the GraphQL country type. //! #[derive(Clone, EagerLoading)] //! #[eager_loading(connection = "DbConnection", error = "Box<dyn Error>")] //! pub struct Country { //! country: models::Country, //! } //! //! // The root query GraphQL type. //! pub struct Query; //! //! impl QueryFields for Query { //! // The resolver for `Query.allUsers`. //! fn field_all_users( //! &self, //! executor: &Executor<'_, Context>, //! trail: &QueryTrail<'_, User, Walked>, //! ) -> FieldResult<Vec<User>> { //! let db = &executor.context().db; //! // Load the model users. //! let user_models = db.load_all_users(); //! //! // Turn the model users into GraphQL users. //! let mut users = User::from_db_models(&user_models); //! //! // Perform the eager loading. //! // `trail` is used to only eager load the fields that are requested. Because //! // we're using `QueryTrail`s from "juniper_from_schema" it would be a compile //! // error if we eager loaded too much. //! User::eager_load_all_children_for_each(&mut users, &user_models, db, trail)?; //! //! Ok(users) //! } //! } //! //! impl UserFields for User { //! fn field_id( //! &self, //! executor: &Executor<'_, Context>, //! ) -> FieldResult<&i32> { //! Ok(&self.user.id) //! } //! //! fn field_country( //! &self, //! executor: &Executor<'_, Context>, //! trail: &QueryTrail<'_, Country, Walked>, //! ) -> FieldResult<&Country> { //! // This will unwrap the country from the `HasOne` or return an error if the //! // country wasn't loaded, or wasn't found in the database. //! Ok(self.country.try_unwrap()?) //! } //! } //! //! impl CountryFields for Country { //! fn field_id( //! &self, //! executor: &Executor<'_, Context>, //! ) -> FieldResult<&i32> { //! Ok(&self.country.id) //! } //! } //! # //! # fn main() {} //! ``` //! //! # `#[derive(EagerLoading)]` //! //! For a type to support eager loading it needs to implement the following traits: //! //! - [`GraphqlNodeForModel`][] //! - [`EagerLoadAllChildren`][] //! - Each association field must implement [`EagerLoadChildrenOfType`][] //! //! [`GraphqlNodeForModel`]: trait.GraphqlNodeForModel.html //! [`EagerLoadAllChildren`]: trait.EagerLoadAllChildren.html //! //! Implementing these traits involves lots of boilerplate, therefore you should use //! `#[derive(EagerLoading)]` to derive implementations as much as possible. //! //! Sometimes you might need customized eager loading for a specific association, in that case you //! should still have `#[derive(EagerLoading)]` on your struct but implement //! [`EagerLoadChildrenOfType`][] yourself for the field that requires a custom setup. An example //! of how to do that can be found //! [here](trait.EagerLoadChildrenOfType.html#manual-implementation). //! //! [`EagerLoadChildrenOfType`]: trait.EagerLoadChildrenOfType.html //! //! ## Attributes //! //! `#[derive(EagerLoading)]` has a few attributes you need to provide: //! //! | Name | Description | Default | Example | //! |---|---|---|---| //! | `connection` | The type of connection your app uses. This could be a database connection or a connection to another web service. | N/A | `connection = "diesel::pg::PgConnection"` | //! | `error` | The type of error eager loading might result in. | N/A | `error = "diesel::result::Error"` | //! | `model` | The model type behind your GraphQL struct | `models::{name of struct}` | `model = "crate::db::models::User"` | //! | `id` | Which id type does your app use? | `i32` | `id = "UUID"` | //! | `root_model_field` | The name of the field has holds the backing model | `{name of struct}` in snakecase. | `root_model_field = "user"` | //! //! # Associations //! //! Assocations are things like "user has one country". These are the fields that need to be eager //! loaded to avoid N+1s. Each assocation works for different kinds of foreign key setups and has //! to be eager loaded differently. They should fit most kinds of associations you have in your //! app. Click on each for more detail. //! //! The documation for each assocation assumes that you're using an SQL database, but it should be //! straight forward to adapt to other kinds of data stores. //! //! - [`HasOne`](struct.HasOne.html) //! - [`OptionHasOne`](struct.OptionHasOne.html) //! - [`HasMany`](struct.HasMany.html) //! - [`HasManyThrough`](struct.HasManyThrough.html) //! //! For each field of your GraphQL struct that is one of these four types the trait //! [`EagerLoadChildrenOfType`][] will be implemented by `#[derive(EagerLoading)]`. //! //! ## Attributes supported on all associations //! //! These are the attributes that are supported on all associations. None of these attributes take //! arguments. //! //! ### `skip` //! //! Skip implementing [`EagerLoadChildrenOfType`][] for the field. This is useful if you need to //! provide a custom implementation. //! //! ### `print` //! //! This will cause the implementation of [`EagerLoadChildrenOfType`][] for the field to be printed //! while compiling. This is useful when combined with `skip`. It will print a good starting place //! for you to customize. //! //! The resulting code wont be formatted. We recommend you do that with //! [rustfmt](https://github.com/rust-lang/rustfmt). //! //! # Eager loading interfaces or unions //! //! Eager loading interfaces or unions is possible but it will require calling `.downcast()` on the //! `QueryTrail`. See the [juniper-from-schema docs for more //! info](https://docs.rs/juniper-from-schema/0.4.0/juniper_from_schema/#downcasting-for-interface-and-union-querytrails) //! fo more info. //! //! # Eager loading fields that take arguments //! //! Eager loading fields that take arguments throws a bit of a wrench into things. Since field //! arguments can change what a GraphQL resolver method returns in arbitrary ways, you're required //! to implement [`EagerLoadChildrenOfType`][] manually for that particular field. //! //! In your implementation you have to set [`EagerLoadChildrenOfType::FieldArguments`][] to the //! type of the arguments struct generated by juniper-from-schema. You can find more info //! [here](https://docs.rs/juniper-from-schema/0.4.2/juniper_from_schema/#querytrails-for-fields-that-take-arguments). //! //! You also have to implement [`LoadFrom<T, ArgumentType>`][`LoadFrom`] for your model. You can find a complete //! example //! [here](https://github.com/davidpdrsn/juniper-eager-loading/blob/master/juniper-eager-loading/tests/fields_with_arguments.rs). //! //! If you see a type error like: //! //! ```text //! error[E0308]: mismatched types //! --> juniper-eager-loading/tests/fields_with_arguments.rs:254:56 //! | //! 254 | #[derive(Clone, Eq, PartialEq, Debug, Ord, PartialOrd, EagerLoading)] //! | ^^^^^^^^^^^^ expected (), found struct `query_trails::CountryUsersArgs` //! | //! = note: expected type `&()` //! found type `&query_trails::CountryUsersArgs<'_>` //! ``` //! //! It is because your field `Country.users` takes arguments and therefore require a manual //! implementation of [`EagerLoadChildrenOfType`][]. You can read more //! [here](trait.EagerLoadChildrenOfType.html#manual-implementation) //! about doing that. The code generation always sets [`EagerLoadChildrenOfType::FieldArguments`][] //! to `()` so therefore you get this type error. The neat bit is that the compiler wont let you //! forget to handle arguments. //! //! [`EagerLoadChildrenOfType`]: trait.EagerLoadChildrenOfType.html //! [`EagerLoadChildrenOfType::FieldArguments`]: trait.EagerLoadChildrenOfType.html#associatedtype.FieldArguments //! [`LoadFrom`]: trait.LoadFrom.html //! //! # Diesel helper //! //! Implementing [`LoadFrom`][] for lots of model types might involve lots of boilerplate. If //! you're using Diesel it is recommend that you use one of [the macros to //! generate](index.html#macros) implementations. //! //! [`LoadFrom`]: trait.LoadFrom.html //! [Diesel]: https://diesel.rs //! [`EagerLoadChildrenOfType`]: trait.EagerLoadChildrenOfType.html //! //! # When your GraphQL schema doesn't match your database schema //! //! This library supports eager loading most kinds of association setups, however it probably //! doesn't support all that might exist in your app. It also works best when your database schema //! closely matches your GraphQL schema. //! //! If you find yourself having to implement something that isn't directly supported remember that //! you're still free to implement you resolver functions exactly as you want. So if doing queries //! in a resolver is the only way to get the behaviour you need then so be it. Avoiding some N+1 //! queries is better than avoiding none. //! //! However if you have a setup that you think this library should support please don't hestitate //! to [open an issue](https://github.com/davidpdrsn/juniper-eager-loading). //! //! [Juniper]: https://github.com/graphql-rust/juniper //! [juniper-from-schema]: https://github.com/davidpdrsn/juniper-from-schema #![doc(html_root_url = "https://docs.rs/juniper-eager-loading/0.4.0")] #![allow(clippy::single_match, clippy::type_complexity)] #![deny( missing_docs, dead_code, missing_copy_implementations, missing_debug_implementations, trivial_casts, trivial_numeric_casts, unsafe_code, unstable_features, unused_import_braces, unused_imports, unused_must_use, unused_qualifications, unused_variables )] mod macros; use juniper_from_schema::{QueryTrail, Walked}; use std::{fmt, hash::Hash}; pub use juniper_eager_loading_code_gen::EagerLoading; #[doc(hidden)] pub mod proc_macros { pub use juniper_eager_loading_code_gen::{ impl_load_from_for_diesel_mysql, impl_load_from_for_diesel_pg, impl_load_from_for_diesel_sqlite, }; } /// Re-exports the traits needed for doing eager loading. Meant to be glob imported. pub mod prelude { pub use super::Association; pub use super::EagerLoadAllChildren; pub use super::EagerLoadChildrenOfType; pub use super::GraphqlNodeForModel; } /// The types of associations. /// /// This is used for [`Error`] to report which kind of association encountered an error. /// /// [`Error`]: enum.Error.html #[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)] pub enum AssociationType { /// There was an error with a [`HasOne`](struct.HasOne.html). HasOne, /// There was an error with an [`OptionHasOne`](struct.OptionHasOne.html). OptionHasOne, /// There was an error with a [`HasMany`](struct.HasMany.html). HasMany, /// There was an error with a [`HasManyThrough`](struct.HasManyThrough.html). HasManyThrough, } /// A non-optional "has one" association. /// /// Imagine you have these models: /// /// ``` /// struct User { /// id: i32, /// country_id: i32, /// } /// /// struct Country { /// id: i32, /// } /// ``` /// /// For this setup we say "a user has one country". This means that `User` has a field named /// `country_id` that references the id of another country. /// /// # Example /// /// You can find a complete example of `HasOne` [here](https://github.com/davidpdrsn/juniper-eager-loading/tree/master/juniper-eager-loading/examples/has_one.rs). /// /// # Attributes /// /// | Name | Description | Default | Example | /// |---|---|---|---| /// | `foreign_key_field` | The name of the foreign key field | `{name of field}_id` | `foreign_key_field = "country_id"` | /// | `root_model_field` | The name of the field on the associated GraphQL type that holds the database model | `{name of field}` | `root_model_field = "country"` | /// | `graphql_field` | The name of this field in your GraphQL schema | `{name of field}` | `graphql_field = "country"` | /// | `default` | Use the default value for all unspecified attributes | N/A | `default` | /// /// Additionally it also supports the attributes `print`, and `skip`. See the [root model /// docs](/#attributes-supported-on-all-associations) for more into on those. /// /// # Errors /// /// When calling [`try_unwrap`][] to get the loaded value it will return an error if the value has /// not been loaded, or if the load failed. /// /// For example if a user has a `country_id` of `10` but there is no `Country` with id `10` then /// [`try_unwrap`][] will return an error. /// /// [`try_unwrap`]: struct.HasOne.html#method.try_unwrap #[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd)] pub struct HasOne<T>(HasOneInner<T>); impl<T> Default for HasOne<T> { fn default() -> Self { HasOne(HasOneInner::default()) } } impl<T> HasOne<T> { /// Borrow the loaded value. If the value has not been loaded it will return an error. pub fn try_unwrap(&self) -> Result<&T, Error> { self.0.try_unwrap() } } #[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd)] enum HasOneInner<T> { Loaded(T), NotLoaded, LoadFailed, } impl<T> Default for HasOneInner<T> { fn default() -> Self { HasOneInner::NotLoaded } } impl<T> HasOneInner<T> { fn try_unwrap(&self) -> Result<&T, Error> { match self { HasOneInner::Loaded(inner) => Ok(inner), HasOneInner::NotLoaded => Err(Error::NotLoaded(AssociationType::HasOne)), HasOneInner::LoadFailed => Err(Error::LoadFailed(AssociationType::HasOne)), } } fn loaded(&mut self, inner: T) { std::mem::replace(self, HasOneInner::Loaded(inner)); } fn assert_loaded_otherwise_failed(&mut self) { match self { HasOneInner::NotLoaded => { std::mem::replace(self, HasOneInner::LoadFailed); } _ => {} } } } /// An optional "has-one association". /// /// It works exactly like [`HasOne`] except it doesn't error if the association doesn't get loaded. /// The value doesn't get loaded it defaults to `None`. /// /// # Example /// /// You can find a complete example of `OptionHasMany` [here](https://github.com/davidpdrsn/juniper-eager-loading/tree/master/juniper-eager-loading/examples/option_has_one.rs). /// /// # Attributes /// /// It supports the same attributes as [`HasOne`]. /// /// [`HasOne`]: struct.HasOne.html /// /// # Errors /// /// [`try_unwrap`][] will never error. If the association wasn't loaded or wasn't found it will /// return `Ok(None)`. /// /// [`try_unwrap`]: struct.OptionHasOne.html#method.try_unwrap #[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd)] pub struct OptionHasOne<T>(Option<T>); impl<T> Default for OptionHasOne<T> { fn default() -> Self { OptionHasOne(None) } } impl<T> OptionHasOne<T> { /// Borrow the loaded value. If the value has not been loaded it will return `Ok(None)`. It /// will not error. pub fn try_unwrap(&self) -> Result<&Option<T>, Error> { Ok(&self.0) } } /// A "has many" association. /// /// Imagine you have these models: /// /// ``` /// struct User { /// id: i32, /// } /// /// struct Car { /// id: i32, /// user_id: i32, /// } /// ``` /// /// For this setup we say "user has many cars" and "cars have one user". This is the inverse of a /// `HasOne` assocation because the foreign key is on `Car` instead of `User`. /// /// This means users can own many cars, but cars can only be owned by one user. /// /// # Example /// /// You can find a complete example of `HasMany` [here](https://github.com/davidpdrsn/juniper-eager-loading/tree/master/juniper-eager-loading/examples/has_many.rs). /// /// # Attributes /// /// | Name | Description | Default | Example | /// |---|---|---|---| /// | `foreign_key_field` | The name of the foreign key field | `{name of struct}_id` | `foreign_key_field = "user_id"` | /// | `foreign_key_optional` | The foreign key type is optional | Not set | `foreign_key_optional` | /// | `root_model_field` | The name of the field on the associated GraphQL type that holds the database model | N/A (unless using `skip`) | `root_model_field = "car"` | /// | `graphql_field` | The name of this field in your GraphQL schema | `{name of field}` | `graphql_field = "country"` | /// | `predicate_method` | Method used to filter child associations. This can be used if you only want to include a subset of the models | N/A (attribute is optional) | `predicate_method = "a_predicate_method"` | /// /// # Errors /// /// [`try_unwrap`][] will never error. If the association wasn't loaded or wasn't found it will /// return `Ok(vec![])`. /// /// [`try_unwrap`]: struct.HasMany.html#method.try_unwrap #[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd)] pub struct HasMany<T>(Vec<T>); impl<T> Default for HasMany<T> { fn default() -> Self { HasMany(Vec::new()) } } impl<T> HasMany<T> { /// Borrow the loaded values. If no values have been loaded it will return an empty list. /// It will not return an error. pub fn try_unwrap(&self) -> Result<&Vec<T>, Error> { Ok(&self.0) } } /// A "has many through" association. /// /// Imagine you have these models: /// /// ``` /// struct User { /// id: i32, /// } /// /// struct Company { /// id: i32, /// } /// /// struct Employments { /// id: i32, /// user_id: i32, /// company_id: i32, /// } /// ``` /// /// For this setup we say "user has many companies through employments". This means uses can work /// at many companies and companies can have many employees, provided that we join with `Employment`. /// /// This requires that we use [the `JoinModel`](trait.EagerLoadChildrenOfType.html#joinmodel) type /// on [`EagerLoadChildrenOfType`][] and is therefore a bit different from the other associations /// since it involves a third type. /// /// [`EagerLoadChildrenOfType`]: trait.EagerLoadChildrenOfType.html /// /// # Example /// /// You can find a complete example of `HasManyThrough` [here](https://github.com/davidpdrsn/juniper-eager-loading/tree/master/juniper-eager-loading/examples/has_many_through.rs). /// /// # Attributes /// /// | Name | Description | Default | Example | /// |---|---|---|---| /// | `model_field` | The field on the contained type that holds the model | `{name of contained type}` in snakecase | `model_field = "company"` | /// | `join_model` | The model we have to join with | N/A | `join_model = "models::Employment"` | /// | `join_model_field` | The field on the join model type that holds the model | `{name of join model type}` in snakecase | `join_model_field = "employment"` | /// | `foreign_key_field` | The field on the join model that contains the parent models id | `{name of parent type in lowercase}_id` | `foreign_key_field = "car_id"` | /// | `graphql_field` | The name of this field in your GraphQL schema | `{name of field}` | `graphql_field = "country"` | /// | `predicate_method` | Method used to filter child associations. This can be used if you only want to include a subset of the models. This method will be called to filter the join models. | N/A (attribute is optional) | `predicate_method = "a_predicate_method"` | /// /// # Errors /// /// [`try_unwrap`][] will never error. If the association wasn't loaded or wasn't found it will /// return `Ok(vec![])`. /// /// [`try_unwrap`]: struct.HasManyThrough.html#method.try_unwrap #[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd)] pub struct HasManyThrough<T>(Vec<T>); impl<T> Default for HasManyThrough<T> { fn default() -> Self { HasManyThrough(Vec::new()) } } impl<T> HasManyThrough<T> { /// Borrow the loaded values. If no values have been loaded it will return an empty list. /// It will not return an error. pub fn try_unwrap(&self) -> Result<&Vec<T>, Error> { Ok(&self.0) } } /// A GraphQL type backed by a model object. /// /// You shouldn't need to implement this trait yourself even when customizing eager loading. pub trait GraphqlNodeForModel: Sized { /// The model type. type Model: Clone; /// The id type the model uses. type Id: 'static + Hash + Eq; /// The connection type required to do the loading. This can be a database connection or maybe /// a connection an external web service. type Connection; /// The error type. type Error; /// Create a new GraphQL type from a model. fn new_from_model(model: &Self::Model) -> Self; /// Create a list of GraphQL types from a list of models. fn from_db_models(models: &[Self::Model]) -> Vec<Self> { models .iter() .map(|model| Self::new_from_model(model)) .collect() } } /// Perform eager loading for a single association of a GraphQL struct. /// /// `#[derive(EagerLoading)]` will implement this trait for each [association field][] your GraphQL /// struct has. /// /// [association field]: /#associations /// /// # Manual implementation /// /// Sometimes you might have a setup that `#[derive(EagerLoading)]` doesn't support. In those cases /// you have to implement this trait yourself for those struct fields. Here is an example of how to /// do that: /// /// ``` /// # use juniper::{Executor, FieldResult}; /// # use juniper_eager_loading::{prelude::*, *}; /// # use juniper_from_schema::graphql_schema; /// # use std::error::Error; /// # pub struct Query; /// # impl QueryFields for Query { /// # fn field_noop(&self, executor: &Executor<'_, Context>) -> FieldResult<bool> { /// # unimplemented!() /// # } /// # } /// # impl juniper_eager_loading::LoadFrom<i32> for models::Country { /// # type Error = Box<dyn std::error::Error>; /// # type Connection = DbConnection; /// # fn load(employments: &[i32], field_args: &(), db: &Self::Connection) -> Result<Vec<Self>, Self::Error> { /// # unimplemented!() /// # } /// # } /// # pub struct DbConnection; /// # impl DbConnection { /// # fn load_all_users(&self) -> Vec<models::User> { /// # unimplemented!() /// # } /// # } /// # pub struct Context { /// # db: DbConnection, /// # } /// # impl juniper::Context for Context {} /// # impl UserFields for User { /// # fn field_id(&self, executor: &Executor<'_, Context>) -> FieldResult<&i32> { /// # unimplemented!() /// # } /// # fn field_country( /// # &self, /// # executor: &Executor<'_, Context>, /// # trail: &QueryTrail<'_, Country, Walked>, /// # ) -> FieldResult<&Option<Country>> { /// # unimplemented!() /// # } /// # } /// # impl CountryFields for Country { /// # fn field_id(&self, executor: &Executor<'_, Context>) -> FieldResult<&i32> { /// # unimplemented!() /// # } /// # } /// # fn main() {} /// # /// # graphql_schema! { /// # schema { query: Query } /// # type Query { noop: Boolean! @juniper(ownership: "owned") } /// # type User { /// # id: Int! /// # country: Country /// # } /// # type Country { /// # id: Int! /// # } /// # } /// # mod models { /// # #[derive(Clone)] /// # pub struct User { /// # pub id: i32, /// # pub country_id: Option<i32>, /// # } /// # #[derive(Clone)] /// # pub struct Country { /// # pub id: i32, /// # } /// # } /// # /// #[derive(Clone, EagerLoading)] /// #[eager_loading(connection = "DbConnection", error = "Box<dyn std::error::Error>")] /// pub struct User { /// user: models::User, /// /// // Add `#[option_has_one(default, print)]` to get a good starting point for your /// // manual implementaion. /// #[option_has_one(skip)] /// country: OptionHasOne<Country>, /// } /// /// #[derive(Clone, EagerLoading)] /// #[eager_loading(connection = "DbConnection", error = "Box<dyn std::error::Error>")] /// pub struct Country { /// country: models::Country, /// } /// /// #[allow(missing_docs, dead_code)] /// struct EagerLoadingContextUserForCountry; /// /// impl<'a> /// EagerLoadChildrenOfType< /// 'a, /// Country, /// EagerLoadingContextUserForCountry, /// > for User /// { /// type FieldArguments = (); /// /// fn load_children( /// models: &[Self::Model], /// field_args: &Self::FieldArguments, /// db: &Self::Connection, /// ) -> Result< /// LoadChildrenOutput<<Country as juniper_eager_loading::GraphqlNodeForModel>::Model>, /// Self::Error, /// > { /// let ids = models /// .iter() /// .filter_map(|model| model.country_id) /// .map(|id| id.clone()) /// .collect::<Vec<_>>(); /// let ids = juniper_eager_loading::unique(ids); /// /// let children = < /// <Country as GraphqlNodeForModel>::Model as juniper_eager_loading::LoadFrom<Self::Id> /// >::load(&ids, field_args, db)?; /// /// Ok(juniper_eager_loading::LoadChildrenOutput::ChildModels(children)) /// } /// /// fn is_child_of( /// node: &Self, /// child: &Country, /// _join_model: &(), _field_args: &Self::FieldArguments, /// ) -> bool { /// node.user.country_id == Some(child.country.id) /// } /// /// fn association(node: &mut Self) -> &mut dyn Association<Country> { /// &mut node.country /// } /// } /// ``` /// /// # Generic parameters /// /// The number of generic parameters to this trait might look scary, but in the vast majority of /// cases you shouldn't have to worry about them. /// /// ## `Child` /// /// Is the model type of the child. If your `User` struct has a field of type `OptionHasOne<Country>`, /// this type will default to `models::Country`. /// /// ## `Context` /// /// This "context" type is needed in case your GraphQL type has multiple assocations to values /// of the same type. Could for example be something like this /// /// ```ignore /// struct User { /// home_country: HasOne<Country>, /// current_country: HasOne<Country>, /// } /// ``` /// /// If we didn't have this we wouldn't be able to implement `EagerLoadChildrenOfType<Country>` /// twice for `User`, because you cannot implement the same trait twice for the same type. /// /// ## `JoinModel` /// /// This type defaults to `()` and is only need for [`HasManyThrough`][]. In the other associations /// there are only two types involved (such as `models::User` and `models::Country`) and one of /// them will have a foreign key pointing to the other one. But consider this scenario instead /// where users can work for many companies, and companies can have many employees: /// /// ``` /// mod models { /// struct User { /// id: i32, /// } /// /// struct Company { /// id: i32, /// } /// /// struct Employment { /// id: i32, /// user_id: i32, /// company_id: i32, /// } /// } /// ``` /// /// Imagine now we need to eager load the list of companies a given user works at. That means /// [`LoadFrom`][] would return `Vec<models::Company>`. However that isn't enough information once /// we need to pair users up with the correct companies. `User` doesn't have `company_id` and /// `Company` doesn't have `user_id`. /// /// Instead we need [`LoadFrom`] to return `Vec<(models::Company, models::Employment)>`. We say /// "users have many companies through employments", because `models::Employment` is necessary for /// pairing things up at the end of [`EagerLoadChildrenOfType`][]. /// /// In this case `JoinModel` would be `models::Employment`. /// /// [`HasManyThrough`]: struct.HasManyThrough.html /// [`LoadFrom`]: trait.LoadFrom.html /// [`EagerLoadChildrenOfType`]: trait.EagerLoadChildrenOfType.html // `JoinModel` cannot be an associated type because it requires a default. pub trait EagerLoadChildrenOfType<'a, Child, Context, JoinModel = ()> where Self: GraphqlNodeForModel, Child: GraphqlNodeForModel<Connection = Self::Connection, Error = Self::Error> + EagerLoadAllChildren + Clone, JoinModel: 'static + Clone + ?Sized, { /// The types of arguments the GraphQL field takes. The code generation always sets this to `()`. type FieldArguments; /// Load the children from the data store. fn load_children( models: &[Self::Model], field_args: &Self::FieldArguments, db: &Self::Connection, ) -> Result<LoadChildrenOutput<Child::Model, JoinModel>, Self::Error>; /// Does this parent and this child belong together? /// /// The `join_model` is only used for `HasManyThrough` associations. fn is_child_of( parent: &Self, child: &Child, join_model: &JoinModel, field_args: &Self::FieldArguments, ) -> bool; /// Return the particular association type. /// /// In most cases the implementation will be something like /// /// ```ignore /// fn association(node: &mut User) -> &mut dyn Association<Country> { /// &mut node.country /// } /// ``` fn association(node: &mut Self) -> &mut dyn Association<Child>; /// Combine all the methods above to eager load the children for a list of GraphQL values and /// models. fn eager_load_children( nodes: &mut [Self], models: &[Self::Model], db: &Self::Connection, trail: &QueryTrail<'a, Child, Walked>, field_args: &Self::FieldArguments, ) -> Result<(), Self::Error> { let child_models = match Self::load_children(models, field_args, db)? { LoadChildrenOutput::ChildModels(child_models) => { assert!(same_type::<JoinModel, ()>()); child_models .into_iter() .map(|model| { #[allow(unsafe_code)] let join_model = unsafe { // This branch will only ever be called if `JoinModel` is `()`. That // happens for all the `Has*` types except `HasManyThrough`. // // `HasManyThrough` requires something to join the two types on, // therefore `child_ids` will return a variant of `LoadChildrenOutput::Models` std::mem::transmute_copy::<(), JoinModel>(&()) }; (model, join_model) }) .collect::<Vec<_>>() } LoadChildrenOutput::ChildAndJoinModels(model_and_join_pairs) => model_and_join_pairs, }; let children = child_models .iter() .map(|child_model| (Child::new_from_model(&child_model.0), child_model.1.clone())) .collect::<Vec<_>>(); let mut children_without_join_models = children.iter().map(|x| x.0.clone()).collect::<Vec<_>>(); let child_models_without_join_models = child_models.iter().map(|x| x.0.clone()).collect::<Vec<_>>(); let len_before = child_models_without_join_models.len(); Child::eager_load_all_children_for_each( &mut children_without_join_models, &child_models_without_join_models, db, trail, )?; assert_eq!(len_before, child_models_without_join_models.len()); let children = children_without_join_models .into_iter() .enumerate() .map(|(idx, child)| { let join_model = &children[idx].1; (child, join_model) }) .collect::<Vec<_>>(); for node in nodes { let matching_children = children .iter() .filter(|child_model| { Self::is_child_of(node, &child_model.0, &child_model.1, field_args) }) .cloned() .collect::<Vec<_>>(); for child in matching_children { Self::association(node).loaded_child(child.0); } Self::association(node).assert_loaded_otherwise_failed(); } Ok(()) } } /// Methods available for all association types. pub trait Association<T> { /// Store the loaded child on the association. fn loaded_child(&mut self, child: T); /// The association should have been loaded by now, if not store an error inside the /// association (if applicable for the particular association). fn assert_loaded_otherwise_failed(&mut self); } impl<T> Association<T> for HasOne<T> { fn loaded_child(&mut self, child: T) { self.0.loaded(child) } fn assert_loaded_otherwise_failed(&mut self) { self.0.assert_loaded_otherwise_failed() } } impl<T> Association<T> for OptionHasOne<T> { fn loaded_child(&mut self, child: T) { std::mem::replace(self, OptionHasOne(Some(child))); } fn assert_loaded_otherwise_failed(&mut self) { match self.0 { Some(_) => {} None => { std::mem::replace(self, OptionHasOne(None)); } } } } impl<T> Association<T> for HasMany<T> { fn loaded_child(&mut self, child: T) { self.0.push(child); } fn assert_loaded_otherwise_failed(&mut self) { // cannot fail, defaults to an empty vec } } impl<T> Association<T> for HasManyThrough<T> { fn loaded_child(&mut self, child: T) { self.0.push(child); } fn assert_loaded_otherwise_failed(&mut self) { // cannot fail, defaults to an empty vec } } /// Are two types the same? fn same_type<A: 'static, B: 'static>() -> bool { use std::any::TypeId; TypeId::of::<A>() == TypeId::of::<B>() } /// The result of loading child models. /// /// [`HasOne`][], [`OptionHasOne`][], [`HasMany`][] can return the child models directly because /// the model has the foreign key. However for [`HasManyThrough`][] neither the parent or child /// model has any of the foreign keys. Only the join model does. So we have to include those in the /// result. /// /// Unless you're customizing [`EagerLoadChildrenOfType`] you shouldn't have to worry about this. /// /// [`HasOne`]: struct.HasOne.html /// [`OptionHasOne`]: struct.OptionHasOne.html /// [`HasMany`]: struct.HasMany.html /// [`HasManyThrough`]: struct.HasManyThrough.html /// [`EagerLoadChildrenOfType`]: trait.EagerLoadChildrenOfType.html #[derive(Debug)] pub enum LoadChildrenOutput<ChildModel, JoinModel = ()> { /// Child models were loaded. ChildModels(Vec<ChildModel>), /// Child models along with the respective join model was loaded. ChildAndJoinModels(Vec<(ChildModel, JoinModel)>), } /// The main entry point trait for doing eager loading. /// /// You shouldn't need to implement this trait yourself even when customizing eager loading. pub trait EagerLoadAllChildren where Self: GraphqlNodeForModel, { /// For each field in your GraphQL type that implements [`EagerLoadChildrenOfType`][] call /// [`eager_load_children`][] to do eager loading of that field. /// /// This is the function you should call for eager loading values for a GraphQL field that returns /// a list. /// /// [`EagerLoadChildrenOfType`]: trait.EagerLoadChildrenOfType.html /// [`eager_load_children`]: trait.EagerLoadChildrenOfType.html#method.eager_load_children fn eager_load_all_children_for_each( nodes: &mut [Self], models: &[Self::Model], db: &Self::Connection, trail: &QueryTrail<'_, Self, Walked>, ) -> Result<(), Self::Error>; /// Perform eager loading for a single GraphQL value. /// /// This is the function you should call for eager loading associations of a single value. fn eager_load_all_children( node: Self, models: &[Self::Model], db: &Self::Connection, trail: &QueryTrail<'_, Self, Walked>, ) -> Result<Self, Self::Error> { let mut nodes = vec![node]; Self::eager_load_all_children_for_each(&mut nodes, models, db, trail)?; // This is safe because we just made a vec with exactly one element and // eager_load_all_children_for_each doesn't remove things from the vec Ok(nodes.remove(0)) } } /// How should associated values actually be loaded? /// /// Normally `T` will be your id type but for [`HasMany`][] and [`HasManyThrough`][] it might also /// be other values. /// /// If you're using Diesel it is recommend that you use one of [the macros to /// generate](index.html#macros) implementations. /// /// `Args` is the type of arguments your GraphQL field takes. This is how we're able to load things /// differently depending the types of arguments. You can learn more /// [here](index.html#eager-loading-fields-that-take-arguments). /// /// [`HasMany`]: struct.HasMany.html /// [`HasManyThrough`]: struct.HasManyThrough.html pub trait LoadFrom<T, Args = ()>: Sized { /// The error type. This must match the error set in `#[eager_loading(error_type = _)]`. type Error; /// The connection type required to do the loading. This can be a database connection or maybe /// a connection an external web service. type Connection; /// Perform the load. fn load(ids: &[T], args: &Args, db: &Self::Connection) -> Result<Vec<Self>, Self::Error>; } /// The kinds of errors that can happen when doing eager loading. #[derive(Debug)] #[allow(missing_copy_implementations)] pub enum Error { /// The association was not loaded. /// /// Did you forget to call /// [`eager_load_all_children_for_each`](trait.EagerLoadAllChildren.html#tymethod.eager_load_all_children_for_each)? NotLoaded(AssociationType), /// Loading the association failed. This can only happen when using /// [`HasOne`](struct.HasOne.html). All the other association types have defaults. LoadFailed(AssociationType), } impl fmt::Display for Error { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { match self { Error::NotLoaded(kind) => { write!(f, "`{:?}` should have been eager loaded, but wasn't", kind) } Error::LoadFailed(kind) => write!(f, "Failed to load `{:?}`", kind), } } } impl std::error::Error for Error {} /// Remove duplicates from a list. /// /// This function is used to remove duplicate ids from /// [`child_ids`](trait.EagerLoadChildrenOfType.html#tymethod.child_ids). pub fn unique<T: Hash + Eq>(items: Vec<T>) -> Vec<T> { use std::collections::HashSet; items .into_iter() .collect::<HashSet<_>>() .into_iter() .collect::<Vec<_>>() } #[cfg(test)] mod test { #[test] fn ui() { let t = trybuild::TestCases::new(); t.pass("tests/compile_pass/*.rs"); // We currently don't have any compile tests that should fail to build // t.compile_fail("tests/compile_fail/*.rs"); } }