0x00 Chapter 10 Sibling Relationships

1.Sibling relationships describe a relationship that links two models to each other
They are also known as many-to-many relationships

Unlike parent-child relationships,
there are no constraints between models in a sibling relationship

For instance, if you model the relationship between pets and toys
a pet can have one or more toys
and a toy can be used by one or more pets

In the TIL application, you’ll be able to categorize acronyms.
An acronym can be part of one or more categories
and a category can contain one or more acronyms

2.Category model
create a new file Category.swift
in Sources/App/Models

import Vapor
import Fluent

final class Category: Model, Content {
    static let schema = "categories"
    
    @ID
    var id: UUID?
    
    @Field(key: "name")
    var name: String
    
    init() {}
    
    init(id: UUID? = nil, name: String) {
        self.id = id
        self.name = name
    }
}

3.Migration
create a new file CreateCategory.swift
in Sources/App/Migrations

import Fluent

struct CreateCategory: Migration {
    func prepare(on database: Database) -> EventLoopFuture<Void> {
        database.schema("categories")
            .id()
            .field("name", .string, .required)
            .create()
    }
    
    func revert(on database: Database) -> EventLoopFuture<Void> {
        database.schema("categories").delete()
    }
}

open configure.swift and add CreateCategory to the migration list, after
app.migrations.add(CreateAcronym())

app.migrations.add(CreateCategory())

4.Category controller
create a new file called CategoriesController.swift
In Sources/App/Controllers

import Fluent
import Vapor

struct CategoriesController: RouteCollection {
    func boot(routes: RoutesBuilder) throws {
        let categoriesRoute = routes.grouped("api", "categories")
        categoriesRoute.post(use: createHandler)
        categoriesRoute.get(use: getAllHandler)
        categoriesRoute.get(":categoryID", use: getHandler)
        categoriesRoute.get(":categoryID", "acronyms", use: getAcronymsHandler)
    }
    
    func createHandler(_ req: Request) throws -> EventLoopFuture<Category> {
        let category = try req.content.decode(Category.self)
        return category.save(on: req.db).map { category }
    }
    
    func getAllHandler(_ req: Request) throws -> EventLoopFuture<[Category]> {
        Category.query(on: req.db).all()
    }
    
    //
    func getHandler(_ req: Request) throws -> EventLoopFuture<Category> {
        Category.find(req.parameters.get("categoryID"), on: req.db)
            .unwrap(or: Abort(.notFound))
    }
    
    // http://127.0.0.1:8080/api/categories/<categoryID>/acronyms
    func getAcronymsHandler(_ req: Request) throws -> EventLoopFuture<[Acronym]> {
        Category.find(req.parameters.get("categoryID"), on: req.db)
            .unwrap(or: Abort(.notFound))
            .flatMap { category in
                category.$acronyms.get(on: req.db)
            }
    }
}

open routes.swift and register the controller by adding the following to the
end of routes(_:)

app.migrations.add(CreateCategory())

通过 Rested 提交 category 数据
url：http://127.0.0.1:8080/api/categories
method：POST
parameter：{"name": "Teenager"}

5.Creating a pivot
In Chapter 9, “Parent Child Relationships”, you added a reference to the user in the acronym to create the relationship between an acronym and a user

However, you can’t model a sibling relationship like this as it would be too inefficient to query

You need a separate model to hold on to this relationship. In Fluent, this is a pivot

create file called AcronymCategoryPivot.swift
in Sources/App/Models

import Vapor
import Fluent

final class AcronymCategoryPivot: Model {
    static let schema = "acronym-category-pivot"
    
    @ID
    var id: UUID?
    
    @Parent(key: "acronymID")
    var acronym: Acronym
    
    @Parent(key: "categoryID")
    var category: Category
    
    init() {}
    
    init(id: UUID? = nil, acronym: Acronym, category: Category) throws {
        self.id = id
        self.$acronym.id = try acronym.requireID()
        self.$category.id = try category.requireID()
    }
}

Create CreateAcronymCategoryPivot.swift
in Sources/App/Migrations

import Fluent

struct CreateAcronymCategoryPivot: Migration {
    func prepare(on database: Database) -> EventLoopFuture<Void> {
        database.schema("acronym-category-pivot")
            .id()
            .field("acronymID", .uuid, .required, .references("acronyms", "id", onDelete: .cascade))
            .field("categoryID", .uuid, .required, .references("categories", "id", onDelete: .cascade))
            .create()
    }
    
    func revert(on database: Database) -> EventLoopFuture<Void> {
        database.schema("acronym-category-pivot").delete()
    }
}

As in Chapter 9, “Parent Child Relationships,” it’s good practice to use foreign key
constraints with sibling relationships

The migration also sets a cascade schema reference action when you delete the model.
This causes the database to remove the relationship automatically instead of throwing an error

open configure.swift and add CreateAcronymCategoryPivot to the
migration list, after app.migrations.add(CreateCategory())

app.migrations.add(CreateAcronymCategoryPivot())

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