zeus
51 lines
1.6 KiB
Swift
51 lines
1.6 KiB
Swift
/// DatabasePromise represents a value that can only be resolved when a
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/// database connection is available.
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///
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/// This type is important for the query interface, which lets the user define
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/// requests without any database context.
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///
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/// For example, consider those two requests:
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///
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/// let playerRequest = Player.filter(key: 1)
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/// let countryRequest = Country.filter(key: "FR")
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///
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/// Both need a database connection in order to introspect the database schema,
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/// find the primary key of both table, and generate the correct SQL:
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///
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/// try dbQueue.read { db in
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/// // SELECT * FROM player WHERE id = 1
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/// let player = try playerRequest.fetchOne(db)
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/// // SELECT * FROM country WHERE code = 'FR'
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/// let country = try countryRequest.fetchOne(db)
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/// }
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///
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/// Such late computations are backed by DatabasePromise. In our example,
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/// see SQLRelation.filterPromise.
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struct DatabasePromise<T> {
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/// Returns the resolved value.
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let resolve: (Database) throws -> T
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/// Creates a promise that resolves to a value.
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init(value: T) {
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self.resolve = { _ in value }
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}
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/// Creates a promise from a closure.
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init(_ resolve: @escaping (Database) throws -> T) {
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self.resolve = resolve
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}
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/// Returns a promise whose value is transformed by the given closure.
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func map<U>(_ transform: @escaping (T) throws -> U) -> DatabasePromise<U> {
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DatabasePromise<U> { db in
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try transform(resolve(db))
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}
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}
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}
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extension DatabasePromise: CustomStringConvertible {
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var description: String {
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"DatabasePromise<\(T.self)>"
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}
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}
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