zeus
95 lines
3.5 KiB
Swift
95 lines
3.5 KiB
Swift
/// Thes `BelongsToAssociation` sets up a one-to-one connection from a record
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/// type to another record type, such as each instance of the declaring record
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/// "belongs to" an instance of the other record.
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///
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/// For example, if your application includes authors and books, and each book
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/// is assigned its author, you'd declare the association this way:
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///
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/// ```swift
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/// struct Author: TableRecord { }
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/// struct Book: TableRecord {
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/// static let author = belongsTo(Author.self)
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/// }
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/// ```
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///
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/// A `BelongsToAssociation` should be supported by an SQLite foreign key.
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///
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/// Foreign keys are the recommended way to declare relationships between
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/// database tables because not only will SQLite guarantee the integrity of your
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/// data, but GRDB will be able to use those foreign keys to automatically
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/// configure your associations.
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///
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/// You define the foreign key when you create database tables. For example:
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///
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/// ```swift
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/// try db.create(table: "author") { t in
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/// t.autoIncrementedPrimaryKey("id") // (1)
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/// t.column("name", .text)
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/// }
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/// try db.create(table: "book") { t in
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/// t.autoIncrementedPrimaryKey("id")
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/// t.belongsTo("author", onDelete: .cascade) // (2)
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/// .notNull() // (3)
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/// t.column("title", .text)
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/// }
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/// ```
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///
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/// 1. The author table has a primary key.
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/// 2. The `book.authorId` column is used to link a book to the author it
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/// belongs to. This column is indexed in order to ease the selection of
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/// an author's books. A foreign key is defined from `book.authorId`
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/// column to `authors.id`, so that SQLite guarantees that no book refers
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/// to a missing author. The `onDelete: .cascade` option has SQLite
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/// automatically delete all of an author's books when that author is
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/// deleted. See <https://sqlite.org/foreignkeys.html#fk_actions> for
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/// more information.
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/// 3. Make the `book.authorId` column not null if you want SQLite to guarantee
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/// that all books have an author.
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///
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/// The example above uses auto-incremented primary keys. But generally
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/// speaking, all primary keys are supported.
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///
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/// If the database schema does not define foreign keys between tables, you can
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/// still use `BelongsToAssociation`. But your help is needed to define the
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/// missing foreign key:
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///
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/// ```swift
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/// struct Book: TableRecord {
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/// static let author = belongsTo(Author.self, using: ForeignKey(...))
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/// }
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/// ```
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public struct BelongsToAssociation<Origin, Destination> {
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public var _sqlAssociation: _SQLAssociation
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init(
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to destinationRelation: SQLRelation,
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key: String?,
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using foreignKey: ForeignKey?)
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{
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let destinationTable = destinationRelation.source.tableName
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let foreignKeyCondition = SQLForeignKeyCondition(
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destinationTable: destinationTable,
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foreignKey: foreignKey,
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originIsLeft: true)
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let associationKey: SQLAssociationKey
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if let key {
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associationKey = .fixedSingular(key)
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} else {
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associationKey = .inflected(destinationTable)
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}
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_sqlAssociation = _SQLAssociation(
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key: associationKey,
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condition: .foreignKey(foreignKeyCondition),
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relation: destinationRelation,
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cardinality: .toOne)
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}
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}
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extension BelongsToAssociation: AssociationToOne {
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public typealias OriginRowDecoder = Origin
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public typealias RowDecoder = Destination
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}
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