read_write.md

Read and write your data graph

The point of interaction with the data graph in Sworm is the PersistentContainer instance:

let pc = PersistentContainer(...)

further, all you need to do is call the pc's method:

try pc.perform { ctx in
    // read write your data here
}

or it's concurrent counterpart:

try await pc.schedule { ctx in
    // read write your data here
}

ctx is the ManagedObjectContext instance through which you read and write data:

ctx.insert(...)
ctx.fetch(...)
ctx.delete(...)

You might think these methods work with your domain models. This is true, but not directly. In fact, interaction with data occurs using proxy objects ManagedObject and ManagedObjectSet, where T is the type of the domain model. Why is this needed? ManagedObjectConvertible models define the structure of a graph, and proxy objects provide an API for working with the graph using information about that structure.

Let's take a look at how it works:

extension Author: ManagedObjectConvertible {
    ...

    struct Relations {
        let books = ToManyRelation<Book>("books")
    }

    static let relations = Relations()
}

extension Book: ManagedObjectConvertible { ... }

...

let author = Author(name: "author")
let book1 = Book(name: "book 1")
let book2 = Book(name: "book 2")

try pc.perform { ctx in
    // types can be omitted - it's only for example

    // insert author -> get an object for data access
    let authorObject: ManagedObject<Author> = try ctx.insert(author)
    // get books collection from the author's object
    let bookObjects: ManagedObjectSet<Book> = authorObject.books

    // add new book objects to this list
    try bookObjects.add(ctx.insert(book1))
    try bookObjects.add(ctx.insert(book2))
}

ManagedObject has a set of paired encode / decode methods that allow you to read / write data models or attributes separately:

try pc.perform { ctx in
    // fetch ManagedObject<Author>
    guard let authorObject = try ctx.fetchOne(Author.all) else {
        return
    }

    // get your pure swift entity
    let author = try authorObject.decode()
    // or single attribute
    let authorName = try authorObject.decode(\.name)

    // update its value
    authorObject.encode(Author(name: "foo", age: 50))
    // or update name only
    authorObject.encode(\.name, "foo")
}