Module ensemble::relationships

Relationships between models.

Introduction

Database tables are often related to one another. For example, a blog post may have many comments or an order could be related to the user who placed it. Ensemble makes managing and working with these relationships easy, with native support for the three most common:

• One To One
• One To Many
• Many To Many

Defining Relationships

Ensemble relationships are defined as fields on your Ensemble model. Ensemble will automatically generate a homonymous method to resolve the relationship, but you can directly access the relationship object to chain additional query constrainsts at runtime:

use ensemble::relationships::Relationship;

let posts = user.posts().await?;

let active_posts: Vec<Post> = user.posts.query()
    .r#where_not_null("published_at")
    .get().await?;

But, before diving too deep into using relationships, let’s learn how to define each type of relationship supported by Ensemble.

One To One

A one-to-one relationship is a very basic type of database relationship. For example, a User model might be associated with one Phone model. To define this relationship, we will place a phone field on the User model. The phone field should be of type HasOne<User, Phone>, which is available under the ensemble::relationships module:

use ensemble::relationships::HasOne;

#[derive(Debug, Model)]
struct User {
    id: u64,
    name: String,

    phone: HasOne<User, Phone>
}

The HasOne type expects two generics: the type of the current model, and the type of the related model. Once the relationship is defined, we may retrieve the related record using the dynamic method Ensemble’s registers for you, which will bear the same name as the relationship field:

let mut user = User::find(1).await?;

let phone = user.phone().await?;

Ensemble determines the foreign key of the relationship based on the parent model name. In this case, the Phone model is automatically assumed to have a user_id foreign key. If you wish to override this convention, you can use the #[model(foreign_key)] attribute:

#[derive(Debug, Model)]
struct User {
    id: u64,
    name: String,

    #[model(foreign_key = "foreign_key")]
    phone: HasOne<User, Phone>
}

Additionally, Ensemble assumes that the foreign key should have a value matching the primary key column of the parent. In other words, Ensemble will look for the value of the user’s primary column in the user_id column of the Phone record. This is currently not customizable due to the intricacies of Rust’s type system. If you have a good reason to need this, open an issue!.

Defining The Inverse Of The Relationship

So, we can access the Phone model from our User model. Next, let’s define a relationship on the Phone model that will let us access the user that owns the phone. We can define the inverse of a HasOne relationship using the BelongsTo type:

use ensemble::relationships::BelongsTo;

#[derive(Debug, Model)]
struct Phone {
    id: u64,
    number: String,

    user: BelongsTo<Phone, User>
}

When invoking the user method, Ensemble will attempt to find a User model that has a primary key which matches the user_id column on the Phone model.

Ensemble determines the foreign key name by examining the name of the relationship method and suffixing the method name with _id. So, in this case, Ensemble assumes that the Phone model has a user_id column. However, if the foreign key on the Phone model is not user_id, you may provide a custom key name using the #[model(foreign_key)] attribute:

#[derive(Debug, Model)]
struct Phone {
    id: u64,
    number: String,

    #[model(foreign_key = "author_id")]
    user: BelongsTo<Phone, User>
}

Similar to the HasOne relationship, Ensemble will assume that the foreign key should have a value matching the primary key column of the parent.

One To Many

A one-to-many relationship is used to define relationships where a single model is the parent to one or more child models. For example, a blog post may have an infinite number of comments. Like all other Ensemble relationships, one-to-many relationships are defined by defining a field on your Ensemble model:

use ensemble::relationships::HasMany;
#[derive(Debug, Model)]

struct Post {
    id: u64,
    title: String,
    content: String,

    comments: HasMany<Post, Comment>
}

Remember, Ensemble will automatically determine the proper foreign key column for the Comment model. By convention, Ensemble will take the “snake case” name of the parent model and the related model’s primary key. So, in this example, Ensemble will assume the foreign key column on the Comment model is post_id.

Once the relationship method has been defined, we can access the list of related comments by calling the comments function. Remember, since Ensemble automatically registers a function for each relationship, we can access relationship methods as if they were defined as properties on the model:

let mut post = Post::find(1).await?;

for comment in post.comments().await? {
    println!("{}", comment.text);
}

Since all relationships also serve as query builders, you may add further constraints to the relationship query by calling the query method on the comments field and continuing to chain conditions onto the query:

let post = Post::find(1).await?;

let comment: Option<Comment> = post.comments.query()
    .r#where("title", '=', "foo")
    .first().await?;

Like the HasOne relationship, you may also override the foreign key with the #[model(foreign_key)] attribute:

#[derive(Debug, Model)]
struct Post {
    id: u64,
    title: String,
    content: String,

    #[model(foreign_key = "article_id")]
    comments: HasMany<Post, Comment>
}

One To Many (Inverse) / Belongs To

Now that we can access all of a post’s comments, let’s define a relationship to allow a comment to access its parent post. To define the inverse of a HasMany relationship, define a field on the child model with the BelongsTo type:

use ensemble::relationships::BelongsTo;

#[derive(Debug, Model)]
struct Comment {
    id: u64,
    content: String,

    post: BelongsTo<Comment, Post>
}

Once the relationship has been defined, we can retrieve a comment’s parent post by accessing the post function:

let mut comment = Comment::find(1).await?;

let post_title = &comment.post().await?.title;

In the example above, Ensemble will attempt to find a Post model that has an id which matches the post_id column on the Comment model.

Ensemble determines the default foreign key name by examining the name of the parent model and suffixing it with a _ followed by the name of the parent model’s primary key column. So, in this example, Ensemble will assume the Post model’s foreign key on the comments table is post_id.

However, if the foreign key for your relationship does not follow these conventions, you may provide a custom foreign key name using the #[model(foreign_key)] attribute:

#[derive(Debug, Model)]
struct Comment {
    id: u64,
    content: String,

    #[model(foreign_key = "article_id")]
    post: BelongsTo<Comment, Post>
}

Many To Many Relationships

Many-to-many relations are slightly more complicated than HasOne and HasMany relationships. An example of a many-to-many relationship is a user that has many roles and those roles are also shared by other users in the application. For example, a user may be assigned the role of “Author” and “Editor”; however, those roles may also be assigned to other users as well. So, a user has many roles and a role has many users.

Table Structure

To define this relationship, three database tables are needed: users, roles, and role_user. The role_user table is derived from the alphabetical order of the related model names and contains user_id and role_id columns. This table is used as an intermediate table linking the users and roles.

Remember, since a role can belong to many users, we cannot simply place a user_id column on the roles table. This would mean that a role could only belong to a single user. In order to provide support for roles being assigned to multiple users, the role_user table is needed. We can summarize the relationship’s table structure like so:

users
    id - integer
    name - string

roles
    id - integer
    name - string

role_user
    user_id - integer
    role_id - integer

Model Structure

Many-to-many relationships are defined by creating a field of type BelongsToMany. For example, let’s define a roles field on our User model. Like all relationships in Ensemble, the BelongsToMany type expects two generics: the type of the current model, and the type of the related model.

use ensemble::relationships::BelongsToMany;

#[derive(Debug, Model)]
struct User {
    pub id: u64,
    pub name: String,

    roles: BelongsToMany<User, Role>
}

Once the relationship is defined, you may access the user’s roles using the roles dynamic function:

let mut user = User::find(1).await?;

for role in user.roles().await? {
    println!("{}", role.name);
}

Since all relationships also serve as query builders, you may add further constraints to the relationship query by calling the query method on the roles property and continuing to chain conditions:

let user = User::find(1).await?;

let roles: Vec<Role> = user.roles.query()
    .order_by("name", "asc")
    .get().await?;

To determine the table name of the relationship’s intermediate table, Ensemble will join the two related model names in alphabetical order. However, you are free to override this convention. You may do so using the #[model(pivot_table)] attribute:

#[derive(Debug, Model)]
struct User {
    pub id: u64,
    pub name: String,

    #[model(pivot_table = "role_user")]
    roles: BelongsToMany<User, Role>
}

In addition to customizing the name of the intermediate table, you may also customize the column names of the keys on the table using the #[model(local_key)] attribute for the foreign key name of the model on which you are defining the relationship, and the #[model(foreign_key)] attribute for the foreign key name of the model that you are joining to:

#[derive(Debug, Model)]
struct User {
    pub id: u64,
    pub name: String,

    #[model(local_key = "user_id", foreign_key = "role_id")]
    roles: BelongsToMany<User, Role>
}

Defining The Inverse Of The Relationship

To define the “inverse” of a many-to-many relationship, you should define a field on the related model of type BelongsToMany as well. To complete our user / role example, let’s define the users field on the Role model:

#[derive(Debug, Model)]
struct Role {
    pub id: u64,
    pub name: String,

    users: BelongsToMany<Role, User>
}

As you can see, the relationship is defined exactly the same as its User model counterpart with the exception of referencing the User model. Since we’re reusing the BelongsToMany type, all of the usual table and key customization options are available when defining the “inverse” of many-to-many relationships.

Querying Relations

Since all Ensemble relationships are defined via fields, you may access those fields to obtain an instance of the relationship without actually executing a query to load the related models. In addition, all types of Ensemble relationships also serve as query builders, allowing you to continue to chain constraints onto the relationship query before finally executing the SQL query against your database.

For example, imagine a blog application in which a User model has many associated Post models:

#[derive(Debug, Model)]
struct User {
    pub id: u64,
    pub name: String,

    pub posts: HasMany<User, Post>
}

You may query the posts relationship and add additional constraints to the relationship like so:

let user = User::find(1).await?;

let posts: Vec<Post> = user.posts.query()
    .r#where("active", '=', 1)
    .get().await?;

Chaining or_where Clauses After Relationships

As demonstrated in the example above, you are free to add additional constraints to relationships when querying them. However, use caution when chaining or_where clauses onto a relationship, as the or_where clauses will be logically grouped at the same level as the relationship constraint:

let posts: Vec<Post> = user.posts.query()
    .r#where("active", '=', 1)
    .or_where("votes", ">=", 100)
    .get().await?;

The example above will generate the following SQL. As you can see, the or clause instructs the query to return any post with greater than 100 votes. The query is no longer constrained to a specific user:

select *
from posts
where user_id = ? and active = 1 or votes >= 100

In most situations, you should use logical groups to group the conditional checks between parentheses:

let posts: Vec<Post> = user.posts.query()
    .r#where("active", '=', 1)
    .where_group(|query| {
        query.r#where("active", '=', 1).or_where("votes", ">=", 100)
    })
    .get().await?;

The example above will produce the following SQL. Note that the logical grouping has properly grouped the constraints and the query remains constrained to a specific user:

select *
from posts
where user_id = ? and (active = 1 or votes >= 100)

Relationship Fields Vs. Dynamic Functions

If you do not need to add additional constraints to an Ensemble relationship query, you may access the relationship as if it were a method. For example, continuing to use our User and Post example models, we may access all of a user’s posts like so:

let mut user = User::find(1).await?;

for post in user.posts().await? {
    println!("{}", post.title);
}

Dynamic relationship functions perform “lazy loading”, meaning they will only load their relationship data when you actually access them. Because of this, developers often use eager loading to pre-load relationships they know will be accessed after loading the model. Eager loading provides a significant reduction in SQL queries that must be executed to load a model’s relations.

Counting Related Models

Sometimes you may want to count the number of related models for a given relationship without actually loading the models. To accomplish this, you may use the count method on the relationship’s query builder, like so:

let user = User::find(1).await?;

let posts_count = user.posts.query().count().await?;

Eager Loading

When accessing Ensemble relationships as properties, the related models are “lazy loaded”. This means the relationship data is not actually loaded until you first call the function. However, Ensemble can “eager load” relationships at the time you query the parent model. Eager loading alleviates the “N + 1” query problem. To illustrate the N + 1 query problem, consider a Book model that “belongs to” to an Author model:

#[derive(Debug, Model)]
struct Book {
    pub id: u64,
    pub title: String,

    pub author: BelongsTo<Book, Author>
}

Now, let’s retrieve all books and their authors:

for mut book in Book::all().await? {
    let author = book.author().await?;

    println!("{}", author.name);
}

This loop will execute one query to retrieve all of the books within the database table, then another query for each book in order to retrieve the book’s author. So, if we have 25 books, the code above would run 26 queries: one for the original book, and 25 additional queries to retrieve the author of each book.

Thankfully, we can use eager loading to reduce this operation to just two queries. When building a query, you may specify which relationships should be eager loaded using the with method:

for mut book in Book::with("author").get::<Book>().await? {
    let author = book.author().await?;

    println!("{}", author.name);
}

For this operation, only two queries will be executed - one query to retrieve all of the books and one query to retrieve all of the authors for all of the books:

select * from books

select * from authors where id in (1, 2, 3, 4, 5, ...)

Eager Loading Multiple Relationships

Sometimes you may need to eager load several different relationships. To do so, just pass an array of relationships to the with method:

let books: Vec<Book> = Book::with(vec!["author", "publisher"]).get().await?;

Lazy Eager Loading

Sometimes you may need to eager load a relationship after the parent model has already been retrieved. For example, this may be useful if you need to dynamically decide whether to load related models:

let mut book = Book::find(1).await?;

if someCondition {
    book.load(vec!["author", "publisher"]);
}

Structs

• BelongsTo
  A Belongs To relationship.
• BelongsToMany
  A Many to Many relationship.
• HasMany
  A One to Many relationship.
• HasOne
  A One to One relationship.

Traits

• Relationship
  A relationship between two models.